Open inColab

Autodidax: JAX core from scratch

Ever want to learn how JAX works, but the implementation seemed impenetrable? Well, you’re in luck! By reading this tutorial, you’ll learn every big idea in JAX’s core system. You’ll even get clued into our weird jargon!

This is a work-in-progress draft. There are some important ingredients missing, still to come in parts 5 and 6 (and more?). There are also some simplifications here that we haven’t yet applied to the main system, but we will.

Part 1: Transformations as interpreters: standard evaluation, jvp, and vmap

We want to transform functions that look like this:

def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

Think of functions like sin and the arithmetic operations underlying the infix operators (mul, add, and neg) as primitive operations, meaning atomic units of processing rather than compositions.

“Transform” means “interpret differently.” Instead of standard interpretation where we apply primitive operations to numerical inputs to produce numerical outputs, we want to override primitive application and let different values flow through our program. For example, we might want to replace the application of every primitive with an application of its JVP rule, and let primal-tangent pairs flow through our program. Moreover, we want to be able to compose multiple transformations, leading to stacks of interpreters.

JAX core machinery

We can implement stacks of interpreters and even have them all discharge on the fly as we execute the Python function to be transformed. To start, let’s define these primitives so that we can intercept their application:

from typing import NamedTuple

class Primitive(NamedTuple):
  name: str

add_p = Primitive('add')
mul_p = Primitive('mul')
neg_p = Primitive("neg")
sin_p = Primitive("sin")
cos_p = Primitive("cos")
reduce_sum_p = Primitive("reduce_sum")
greater_p = Primitive("greater")
less_p = Primitive("less")
transpose_p = Primitive("transpose")
broadcast_p = Primitive("broadcast")

def add(x, y): return bind1(add_p, x, y)
def mul(x, y): return bind1(mul_p, x, y)
def neg(x): return bind1(neg_p, x)
def sin(x): return bind1(sin_p, x)
def cos(x): return bind1(cos_p, x)
def reduce_sum(x, axis=None): return bind1(reduce_sum_p, x, axis=axis)
def greater(x, y): return bind1(greater_p, x, y)
def less(x, y): return bind1(less_p, x, y)
def transpose(x, perm): return bind1(transpose_p, perm=perm)
def broadcast(x, shape, axes): return bind1(broadcast_p, x, shape=shape, axes=axes)

def bind1(prim, *args, **params):
  out, = bind(prim, *args, **params)
  return out

We’ll set up array data types and infix operator methods in a moment.

A Primitive is just an object with a name, to which we attach our interpretation rules (one for each transformation). The bind function is our interception point: it’ll figure out which transformation rule to apply, based on how the arguments are boxed in tracers and what interpreters are active.

The functions that user code calls, like add and sin, are just wrappers around calls to bind. These wrappers let us control how arguments are passed to bind, and in particular we follow a handy internal convention: when we call bind, we pass values representing array data as positional arguments, and we pass metadata like the axis argument to sum_p via keyword. This calling convention simplifies some core logic (since e.g. instances of the Tracer class to be defined below can only occur in positional arguments to bind). The wrappers can also provide docstrings!

We represent active interpreters as a stack. The stack is just a simple list, and each element is a container with an integer level (corresponding to the element’s height in the stack), an interpreter type (which we’ll call a trace_type), and an optional field for any global data the interpreter needs. We call each element a MainTrace, though maybe “Interpreter” would be more descriptive.

from contextlib import contextmanager
from typing import Type, List, Tuple, Sequence, Optional, Any

class MainTrace(NamedTuple):
  level: int
  trace_type: Type['Trace']
  global_data: Optional[Any]

trace_stack: List[MainTrace] = []
dynamic_trace: Optional[MainTrace] = None  # to be employed in Part 3

@contextmanager
def new_main(trace_type: Type['Trace'], global_data=None):
  level = len(trace_stack)
  main = MainTrace(level, trace_type, global_data)
  trace_stack.append(main)

  try:
    yield main
  finally:
    trace_stack.pop()

When we’re about to apply a transformation, we’ll push another interpreter onto the stack using new_main. Then, as we apply primitives in the function, we can think of the bind first being interpreted by the trace at the top of the stack (i.e. with the highest level). If that first interpreter itself binds other primitives in its interpretation rule for the primitive, like how the JVP rule of sin_p might bind cos_p and mul_p, then those bind calls will be handled by the interpreter at the next level down.

What goes at the bottom of the interpreter stack? At the bottom, we know all the transformation interpreters are finished, and we just want to do standard evaluation. So at the bottom we’ll put an evaluation interpreter.

Let’s sketch out the interface for interpreters, which is based on the Trace and Tracer base classes. A Tracer represents a boxed-up value, perhaps carrying some extra context data used by the interpreter. A Trace handles boxing up values into Tracers and also handles primitive application.

class Trace:
  main: MainTrace

  def __init__(self, main: MainTrace) -> None:
    self.main = main

  def pure(self, val): assert False  # must override
  def lift(self, val): assert False  # must override

  def process_primitive(self, primitive, tracers, params):
    assert False  # must override

The first two methods are about boxing up values in Tracers, which are the objects that flow through the Python programs we transform. The last method is the callback we’ll use to interpret primitive application.

The Trace itself doesn’t contain any data, other than a reference to its corresponding MainTrace instance. In fact, multiple instances of a Trace might be created and discarded during an application of a transformation, whereas only a single MainTrace instance is created per application of a transformation.

As for Tracers themselves, each one carries an abstract value (and forwards infix operators to it), and the rest is up to the transformation. (The relationship between Tracers and AbstractValues is that there’s one Tracer per transformation, and at least one AbstractValue per base type, like arrays.)

import numpy as np

class Tracer:
  _trace: Trace

  __array_priority__ = 1000

  @property
  def aval(self):
    assert False  # must override

  def full_lower(self):
    return self  # default implementation

  def __neg__(self): return self.aval._neg(self)
  def __add__(self, other): return self.aval._add(self, other)
  def __radd__(self, other): return self.aval._radd(self, other)
  def __mul__(self, other): return self.aval._mul(self, other)
  def __rmul__(self, other): return self.aval._rmul(self, other)
  def __gt__(self, other): return self.aval._gt(self, other)
  def __lt__(self, other): return self.aval._lt(self, other)
  def __bool__(self): return self.aval._bool(self)
  def __nonzero__(self): return self.aval._nonzero(self)

  def __getattr__(self, name):
    try:
      return getattr(self.aval, name)
    except AttributeError:
      raise AttributeError(f"{self.__class__.__name__} has no attribute {name}")

def swap(f): return lambda x, y: f(y, x)
class ShapedArray:
  array_abstraction_level = 1
  shape: Tuple[int]
  dtype: np.dtype

  def __init__(self, shape, dtype):
    self.shape = shape
    self.dtype = dtype

  @property
  def ndim(self):
    return len(self.shape)

  _neg = staticmethod(neg)
  _add = staticmethod(add)
  _radd = staticmethod(swap(add))
  _mul = staticmethod(mul)
  _rmul = staticmethod(swap(mul))
  _gt = staticmethod(greater)
  _lt = staticmethod(less)

  @staticmethod
  def _bool(tracer):
    raise Exception("ShapedArray can't be unambiguously converted to bool")

  @staticmethod
  def _nonzero(tracer):
    raise Exception("ShapedArray can't be unambiguously converted to bool")

  def str_short(self):
    return f'{self.dtype.name}[{",".join(str(d) for d in self.shape)}]'

  def __hash__(self):
    return hash((self.shape, self.dtype))

  def __eq__(self, other):
    return (type(self) is type(other) and
            self.shape == other.shape and self.dtype == other.dtype)

  def __repr__(self):
    return f"ShapedArray(shape={self.shape}, dtype={self.dtype})"

class ConcreteArray(ShapedArray):
  array_abstraction_level = 2
  val: np.ndarray

  def __init__(self, val):
    self.val = val
    self.shape = val.shape
    self.dtype = val.dtype

  @staticmethod
  def _bool(tracer):
    return bool(tracer.aval.val)

  @staticmethod
  def _nonzero(tracer):
    return bool(tracer.aval.val)

def get_aval(x):
  if isinstance(x, Tracer):
    return x.aval
  elif type(x) in jax_types:
    return ConcreteArray(np.asarray(x))
  else:
    raise TypeError(x)

jax_types = {bool, int, float,
             np.bool_, np.int32, np.int64, np.float32, np.float64, np.ndarray}

Notice that we actually have two AbstractValues for arrays, representing different levels of abstraction. A ShapedArray represents the set of all possible arrays with a given shape and dtype. A ConcreteArray represents a singleton set consisting of a single array value.

Now that we’ve set up the interpreter stack, the Trace/Tracer API for interpreters, and abstract values, we can come back to implement bind:

def bind(prim, *args, **params):
  top_trace = find_top_trace(args)
  tracers = [full_raise(top_trace, arg) for arg in args]
  outs = top_trace.process_primitive(prim, tracers, params)
  return [full_lower(out) for out in outs]

The main action is that we call find_top_trace to figure out which interpreter should handle this primitive application. We then call that top trace’s process_primitive so that the trace can apply its interpretation rule. The calls to full_raise just ensure that the inputs are boxed in the top trace’s Tracer instances, and the call to full_lower is an optional optimization so that we unbox values out of Tracers as much as possible.

import operator as op

def find_top_trace(xs) -> Trace:
  top_main = max((x._trace.main for x in xs if isinstance(x, Tracer)),
                 default=trace_stack[0], key=op.attrgetter('level'))
  if dynamic_trace and dynamic_trace.level > top_main.level:
    top_main = dynamic_trace
  return top_main.trace_type(top_main)

In words, ignoring the dynamic_trace step until Part 3, find_top_trace returns the highest-level interpreter associated with the Tracers on its inputs, and otherwise returns the interpreter at the bottom of the stack (which is always an evaluation trace, at least for now). This is a deviation from the description above, where we always start by running the interpreter at the top of the stack and then work our way down, applying every interpreter in the stack. Instead, we’re only applying an interpreter when the input arguments to a primitive bind are boxed in a Tracer corresponding to that interpreter. This optimization lets us skip irrelevant transformations, but bakes in an assumption that transformations mostly follow data dependence (except for the special bottom-of-the-stack interpreter, which interprets everything).

An alternative would be to have every interpreter in the stack interpret every operation. That’s worth exploring! JAX is designed around data dependence in large part because that’s so natural for automatic differentiation, and JAX’s roots are in autodiff. But it may be over-fit.

def full_lower(val: Any):
  if isinstance(val, Tracer):
    return val.full_lower()
  else:
    return val

def full_raise(trace: Trace, val: Any) -> Tracer:
  if not isinstance(val, Tracer):
    assert type(val) in jax_types
    return trace.pure(val)
  level = trace.main.level
  if val._trace.main is trace.main:
    return val
  elif val._trace.main.level < level:
    return trace.lift(val)
  elif val._trace.main.level > level:
    raise Exception(f"Can't lift level {val._trace.main.level} to {level}.")
  else:  # val._trace.level == level
    raise Exception(f"Different traces at same level: {val._trace}, {trace}.")

The logic in full_raise serves to box values into Tracers for a particular Trace, calling different methods on the Trace based on context: Trace.pure is called on non-Tracer constants, and Trace.lift is called for values that are already Tracers from a lower-level interpreter. These two methods could share the same implementation, but by distinguishing them in the core logic we can provide more information to the Trace subclass.

That’s it for the JAX core! Now we can start adding interpreters.

Evaluation interpreter

We’ll start with the simplest interpreter: the evaluation interpreter that will sit at the bottom of the interpreter stack.

class EvalTrace(Trace):
  pure = lift = lambda self, x: x  # no boxing in Tracers needed

  def process_primitive(self, primitive, tracers, params):
    return impl_rules[primitive](*tracers, **params)

trace_stack.append(MainTrace(0, EvalTrace, None))  # special bottom of the stack

# NB: in JAX, instead of a dict we attach impl rules to the Primitive instance
impl_rules = {}

impl_rules[add_p] = lambda x, y: [np.add(x, y)]
impl_rules[mul_p] = lambda x, y: [np.multiply(x, y)]
impl_rules[neg_p] = lambda x: [np.negative(x)]
impl_rules[sin_p] = lambda x: [np.sin(x)]
impl_rules[cos_p] = lambda x: [np.cos(x)]
impl_rules[reduce_sum_p] = lambda x, *, axis: [np.sum(x, axis)]
impl_rules[greater_p] = lambda x, y: [np.greater(x, y)]
impl_rules[less_p] = lambda x, y: [np.less(x, y)]
impl_rules[transpose_p] = lambda x, *, perm: [np.transpose(x, perm)]

def broadcast_impl(x, *, shape, axes):
  for axis in sorted(axes):
    x = np.expand_dims(x, axis)
  return [np.broadcast_to(x, shape)]
impl_rules[broadcast_p] = broadcast_impl

With this interpreter, we can evaluate user functions:

def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

print(f(3.0))
2.7177599838802657

Woo! Like going around in a big circle. But the point of this indirection is that now we can add some real transformations.

Forward-mode autodiff with jvp

First, a few helper functions:

def zeros_like(val):
  aval = get_aval(val)
  return np.zeros(aval.shape, aval.dtype)

def unzip2(pairs):
  lst1, lst2 = [], []
  for x1, x2 in pairs:
    lst1.append(x1)
    lst2.append(x2)
  return lst1, lst2

map_ = map
def map(f, *xs):
  return list(map_(f, *xs))

zip_ = zip
def zip(*args):
  fst, *rest = args = map(list, args)
  n = len(fst)
  for arg in rest:
    assert len(arg) == n
  return list(zip_(*args))

The Tracer for forward-mode autodiff carries a primal-tangent pair. The Trace applies JVP rules.

class JVPTracer(Tracer):
  def __init__(self, trace, primal, tangent):
    self._trace = trace
    self.primal = primal
    self.tangent = tangent

  @property
  def aval(self):
    return get_aval(self.primal)

class JVPTrace(Trace):
  pure = lift = lambda self, val: JVPTracer(self, val, zeros_like(val))

  def process_primitive(self, primitive, tracers, params):
    primals_in, tangents_in = unzip2((t.primal, t.tangent) for t in tracers)
    jvp_rule = jvp_rules[primitive]
    primal_outs, tangent_outs = jvp_rule(primals_in, tangents_in, **params)
    return [JVPTracer(self, x, t) for x, t in zip(primal_outs, tangent_outs)]

jvp_rules = {}

Notice both pure and lift package a value into a JVPTracer with the minimal amount of context, which is a zero tangent value.

Let’s add some JVP rules for primitives:

def add_jvp(primals, tangents):
  (x, y), (x_dot, y_dot) = primals, tangents
  return [x + y], [x_dot + y_dot]
jvp_rules[add_p] = add_jvp

def mul_jvp(primals, tangents):
  (x, y), (x_dot, y_dot) = primals, tangents
  return [x * y], [x_dot * y + x * y_dot]
jvp_rules[mul_p] = mul_jvp

def sin_jvp(primals, tangents):
  (x,), (x_dot,) = primals, tangents
  return [sin(x)], [cos(x) * x_dot]
jvp_rules[sin_p] = sin_jvp

def cos_jvp(primals, tangents):
  (x,), (x_dot,) = primals, tangents
  return [cos(x)], [-sin(x) * x_dot]
jvp_rules[cos_p] = cos_jvp

def neg_jvp(primals, tangents):
  (x,), (x_dot,) = primals, tangents
  return [neg(x)], [neg(x_dot)]
jvp_rules[neg_p] = neg_jvp

def reduce_sum_jvp(primals, tangents, *, axis):
  (x,), (x_dot,) = primals, tangents
  return [reduce_sum(x, axis)], [reduce_sum(x_dot, axis)]
jvp_rules[reduce_sum_p] = reduce_sum_jvp

def greater_jvp(primals, tangents):
  (x, y), _ = primals, tangents
  out_primal = greater(x, y)
  return [out_primal], [zeros_like(out_primal)]
jvp_rules[greater_p] = greater_jvp

def less_jvp(primals, tangents):
  (x, y), _ = primals, tangents
  out_primal = less(x, y)
  return [out_primal], [zeros_like(out_primal)]
jvp_rules[less_p] = less_jvp

Finally, we add a transformation API to kick off the trace:

def jvp_v1(f, primals, tangents):
  with new_main(JVPTrace) as main:
    trace = JVPTrace(main)
    tracers_in = [JVPTracer(trace, x, t) for x, t in zip(primals, tangents)]
    out = f(*tracers_in)
    tracer_out = full_raise(trace, out)
    primal_out, tangent_out = tracer_out.primal, tracer_out.tangent
  return primal_out, tangent_out

And with that, we can differentiate!

x = 3.0
y, sin_deriv_at_3 = jvp_v1(sin, (x,), (1.0,))
print(sin_deriv_at_3)
print(cos(3.0))
-0.9899924966004454
-0.9899924966004454
def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

x, xdot = 3., 1.
y, ydot = jvp_v1(f, (x,), (xdot,))
print(y)
print(ydot)
2.7177599838802657
2.979984993200891
def deriv(f):
  return lambda x: jvp_v1(f, (x,), (1.,))[1]

print(deriv(sin)(3.))
print(deriv(deriv(sin))(3.))
print(deriv(deriv(deriv(sin)))(3.))
print(deriv(deriv(deriv(deriv(sin))))(3.))
-0.9899924966004454
-0.1411200080598672
0.9899924966004454
0.1411200080598672
def f(x):
  if x > 0.:  # Python control flow
    return 2. * x
  else:
    return x

print(deriv(f)(3.))
print(deriv(f)(-3.))
2.0
1.0

Pytrees and flattening user functions’ inputs and outputs

A limitation with jvp_v1 is that it assumes the user function accepts arrays as positional arguments and produces a single array as output. What if it produced a list as output? Or accepted nested containers as inputs? It would be a pain to deal with all the possible containers in inputs and outputs at every layer of the stack. Instead, we can wrap the user function so that the wrapped version accepts arrays as inputs and returns a flat list of arrays as output. The wrapper just needs to unflatten its input, call the user function, and flatten the output.

Here’s how we’d like to write jvp, assuming the user always gives us functions that take arrays as inputs and produces a flat list of arrays as outputs:

def jvp_flat(f, primals, tangents):
  with new_main(JVPTrace) as main:
    trace = JVPTrace(main)
    tracers_in = [JVPTracer(trace, x, t) for x, t in zip(primals, tangents)]
    outs = f(*tracers_in)
    tracers_out = [full_raise(trace, out) for out in outs]
    primals_out, tangents_out = unzip2((t.primal, t.tangent) for t in tracers_out)
  return primals_out, tangents_out

To support user functions that have arbitrary containers in the inputs and outputs, here’s how we’d write the user-facing jvp wrapper:

def jvp(f, primals, tangents):
  primals_flat, in_tree = tree_flatten(primals)
  tangents_flat, in_tree2 = tree_flatten(tangents)
  if in_tree != in_tree2: raise TypeError
  f, out_tree = flatten_fun(f, in_tree)
  primals_out_flat, tangents_out_flat = jvp_flat(f, primals_flat, tangents_flat)
  primals_out = tree_unflatten(out_tree(), primals_out_flat)
  tangents_out = tree_unflatten(out_tree(), tangents_out_flat)
  return primals_out, tangents_out

Notice that we had to plumb the tree structure of the user function output back to the caller of flatten_fun. That information isn’t available until we actually run the user function, so flatten_fun just returns a reference to a mutable cell, represented as a thunk. These side-effects are safe because we always run the user function exactly once. (This safe regime is the reason for the “linear” name in linear_util.py, in the sense of linear types.)

All that remains is to write tree_flatten, tree_unflatten, and flatten_fun.

def flatten_fun(f, in_tree):
  store = Store()

  def flat_fun(*args_flat):
    pytree_args = tree_unflatten(in_tree, args_flat)
    out = f(*pytree_args)
    out_flat, out_tree = tree_flatten(out)
    store.set_value(out_tree)
    return out_flat

  return flat_fun, store

class Empty: pass
empty = Empty()

class Store:
  val = empty

  def set_value(self, val):
    assert self.val is empty
    self.val = val

  def __call__(self):
    return self.val
import itertools as it
from typing import Callable, Type, Hashable, Dict, Iterable, Iterator

class NodeType(NamedTuple):
  name: str
  to_iterable: Callable
  from_iterable: Callable

def register_pytree_node(ty: Type, to_iter: Callable, from_iter: Callable
                         ) -> None:
  node_types[ty] = NodeType(str(ty), to_iter, from_iter)

node_types: Dict[Type, NodeType] = {}
register_pytree_node(tuple, lambda t: (None, t), lambda _, xs: tuple(xs))
register_pytree_node(list,  lambda l: (None, l), lambda _, xs:  list(xs))
register_pytree_node(dict,
                     lambda d: map(tuple, unzip2(sorted(d.items()))),
                     lambda keys, vals: dict(zip(keys, vals)))

class PyTreeDef(NamedTuple):
  node_type: NodeType
  node_metadata: Hashable
  child_treedefs: Tuple['PyTreeDef']

class Leaf: pass
leaf = Leaf()

def tree_flatten(x: Any) -> Tuple[List[Any], PyTreeDef]:
  children_iter, treedef = _tree_flatten(x)
  return list(children_iter), treedef

def _tree_flatten(x: Any) -> Tuple[Iterable, PyTreeDef]:
  node_type = node_types.get(type(x))
  if node_type:
    node_metadata, children = node_type.to_iterable(x)
    children_flat, child_trees = unzip2(map(_tree_flatten, children))
    flattened = it.chain.from_iterable(children_flat)
    return flattened, PyTreeDef(node_type, node_metadata, tuple(child_trees))
  else:
    return [x], leaf

def tree_unflatten(treedef: PyTreeDef, xs: List[Any]) -> Any:
  return _tree_unflatten(treedef, iter(xs))

def _tree_unflatten(treedef: PyTreeDef, xs: Iterator) -> Any:
  if treedef is leaf:
    return next(xs)
  else:
    children = (_tree_unflatten(t, xs) for t in treedef.child_treedefs)
    return treedef.node_type.from_iterable(treedef.node_metadata, children)

With this pytree-handling jvp implementation, we can now handle arbitrary input and output containers. That’ll come in handy with future transformations too!

def f(x):
  y = sin(x) * 2.
  z = - y + x
  return {'hi': z, 'there': [x, y]}

x, xdot = 3., 1.
y, ydot = jvp(f, (x,), (xdot,))
print(y)
print(ydot)
{'hi': 2.7177599838802657, 'there': [3.0, 0.2822400161197344]}
{'hi': 2.979984993200891, 'there': [1.0, -1.9799849932008908]}

Vectorized batching with vmap

First, a couple helper functions, one for producing mapped abstract values from unmapped ones (by removing an axis), and one for moving batch dimensions around:

def mapped_aval(batch_dim, aval):
  shape = list(aval.shape)
  del shape[batch_dim]
  return ShapedArray(tuple(shape), aval.dtype)

def move_batch_axis(axis_size, src, dst, x):
  if src is not_mapped:
    target_shape = list(np.shape(x))
    target_shape.insert(dst, axis_size)
    return broadcast(x, target_shape, [dst])
  elif src == dst:
    return x
  else:
    return moveaxis(x, src, dst)

def moveaxis(x, src: int, dst: int):
  perm = [i for i in range(np.ndim(x)) if i != src]
  perm.insert(dst, src)
  return transpose(x, perm)

The Tracer for vectorized batching carries a batched value and an optional integer indicating which axis (if any) is the batch axis.

from typing import Union

class NotMapped: pass
not_mapped = NotMapped()

BatchAxis = Union[NotMapped, int]

class BatchTracer(Tracer):
  def __init__(self, trace, val, batch_dim: BatchAxis):
    self._trace = trace
    self.val = val
    self.batch_dim = batch_dim

  @property
  def aval(self):
    if self.batch_dim is not_mapped:
      return get_aval(self.val)
    else:
      return mapped_aval(self.batch_dim, get_aval(self.val))

  def full_lower(self):
    if self.batch_dim is not_mapped:
      return full_lower(self.val)
    else:
      return self

class BatchTrace(Trace):
  pure = lift = lambda self, val: BatchTracer(self, val, not_mapped)

  def process_primitive(self, primitive, tracers, params):
    vals_in, bdims_in = unzip2((t.val, t.batch_dim) for t in tracers)
    vmap_rule = vmap_rules[primitive]
    val_outs, bdim_outs = vmap_rule(self.axis_size, vals_in, bdims_in, **params)
    return [BatchTracer(self, x, bd) for x, bd in zip(val_outs, bdim_outs)]

  @property
  def axis_size(self):
    return self.main.global_data

vmap_rules = {}

Here we’ve implemented the optional Tracer.full_lower method, which lets us peel off a batching tracer if it’s not needed because it doesn’t represent a batched value.

For BatchTrace, analogous to JVPTrace, the methods pure and lift just box a value in a BatchTracer with the minimal amount of context, which in this case is a batch_dim taking the sentinel value not_mapped. Notice we use the MainTrace’s interpreter-global data field to store the batch axis size.

Next we can define batching interpreter rules for each primitive:

from functools import partial

def binop_batching_rule(op, axis_size, vals_in, dims_in):
  (x, y), (x_bdim, y_bdim) = vals_in, dims_in
  if x_bdim != y_bdim:
    if x_bdim is not_mapped:
      x = move_batch_axis(axis_size, x_bdim, y_bdim, x)
      x_bdim = y_bdim
    else:
      y = move_batch_axis(axis_size, y_bdim, x_bdim, y)
  return [op(x, y)], [x_bdim]
vmap_rules[add_p] = partial(binop_batching_rule, add)
vmap_rules[mul_p] = partial(binop_batching_rule, mul)

def vectorized_unop_batching_rule(op, axis_size, vals_in, dims_in):
  (x,), (x_bdim,) = vals_in, dims_in
  return [op(x)], [x_bdim]
vmap_rules[sin_p] = partial(vectorized_unop_batching_rule, sin)
vmap_rules[cos_p] = partial(vectorized_unop_batching_rule, cos)
vmap_rules[neg_p] = partial(vectorized_unop_batching_rule, neg)

def reduce_sum_batching_rule(axis_size, vals_in, dims_in, *, axis):
  (x,), (x_bdim,) = vals_in, dims_in
  new_axis = axis + (x_bdim <= axis)
  out_bdim = x_bdim - (new_axis < x_bdim)
  return [reduce_sum(x, new_axis)], [out_bdim]
vmap_rules[reduce_sum_p] = reduce_sum_batching_rule

Finally, we add a transformation API to kick off the trace:

def vmap_flat(f, in_axes, *args):
  axis_size, = {x.shape[ax] for x, ax in zip(args, in_axes)
                if ax is not not_mapped}
  with new_main(BatchTrace, axis_size) as main:
    trace = BatchTrace(main)
    tracers_in = [BatchTracer(trace, x, ax) if ax is not None else x
                  for x, ax in zip(args, in_axes)]
    outs = f(*tracers_in)
    tracers_out = [full_raise(trace, out) for out in outs]
    vals_out, bdims_out = unzip2((t.val, t.batch_dim) for t in tracers_out)
  outs_transposed = [move_batch_axis(axis_size, bdim, 0, val_out)
                     for val_out, bdim in zip(vals_out, bdims_out)]
  return outs_transposed

def vmap(f, in_axes):
  def batched_f(*args):
    args_flat, in_tree = tree_flatten(args)
    in_axes_flat, in_tree2 = tree_flatten(in_axes)
    if in_tree != in_tree2: raise TypeError
    f_flat, out_tree = flatten_fun(f, in_tree)
    outs_flat = vmap_flat(f_flat, in_axes_flat, *args_flat)
    return tree_unflatten(out_tree(), outs_flat)
  return batched_f
def add_one_to_a_scalar(scalar):
  assert np.ndim(scalar) == 0
  return 1 + scalar

vector_in = np.arange(3.)
vector_out = vmap(add_one_to_a_scalar, (0,))(vector_in)

print(vector_in)
print(vector_out)
[0. 1. 2.]
[1. 2. 3.]
def jacfwd(f, x):
  pushfwd = lambda v: jvp(f, (x,), (v,))[1]
  vecs_in = np.eye(np.size(x)).reshape(np.shape(x) * 2)
  return vmap(pushfwd, (0,))(vecs_in)

def f(x):
  return sin(x)

jacfwd(f, np.arange(3.))
array([[ 1.        ,  0.        , -0.        ],
       [ 0.        ,  0.54030231, -0.        ],
       [ 0.        ,  0.        , -0.41614684]])

That’s it for jvp and vmap!

Part 2: Jaxprs

The next transformations on the horizon are jit for just-in-time compilation and vjp for reverse-mode autodiff. (grad is just a small wrapper around vjp.) Whereas jvp and vmap only needed each Tracer to carry a little bit of extra context, for both jit and vjp we need much richer context: we need to represent programs. That is, we need jaxprs!

Jaxprs are JAX’s internal intermediate representation of programs. They are explicitly typed, functional, first-order, and in ANF form. We need a program representation for jit because the purpose of jit is to stage computation out of Python. For any computation we want to stage out, we need to be able to represent it as data, and build it up as we trace a Python function. Similarly, vjp needs a way to represent the computation for the backward pass of reverse-mode autodiff. We use the same jaxpr program representation for both needs.

(Building a program representation is the most free kind of trace-transformation, and so except for issues around handling native Python control flow, any transformation could be implemented by first tracing to a jaxpr and then interpreting the jaxpr.)

Jaxpr data structures

The jaxpr term syntax is roughly:

jaxpr ::=
  { lambda <binder> , ... .
    let <eqn>
        ...
    in ( <atom> , ... ) }

binder ::= <var>:<array_type>
var ::= a | b | c | ...
atom ::= <var> | <literal>
literal ::= <int32> | <int64> | <float32> | <float64>

eqn ::= <binder> , ... = <primitive> [ <params> ] <atom> , ...

The syntax of types is:

jaxpr_type ::= [ <array_type> , ... ] -> [ <array_type> , ... ]
array_type ::= <dtype>[<shape>]
dtype ::= f32 | f64 | i32 | i64
shape ::= <int> , ...

How do we represent these as Python data structures? We reuse ShapedArrays to represent types, and we can represent the term syntax with a few Python structs:

from typing import Set

class Var:
  aval: ShapedArray
  def __init__(self, aval): self.aval = aval

class Lit:
  val: Any
  aval: ShapedArray

  def __init__(self, val):
    self.aval = aval = raise_to_shaped(get_aval(val))
    self.val = np.array(val, aval.dtype)

Atom = Union[Var, Lit]

class JaxprEqn(NamedTuple):
  primitive: Primitive
  inputs: List[Atom]
  params: Dict[str, Any]
  out_binders: List[Var]

class Jaxpr(NamedTuple):
  in_binders: List[Var]
  eqns: List[JaxprEqn]
  outs: List[Atom]

  def __hash__(self): return id(self)
  __eq__ = op.is_

def raise_to_shaped(aval):
  return ShapedArray(aval.shape, aval.dtype)

Type-checking a jaxpr involves checking that there are no unbound variables, that variables are only bound once, and that for each equation the type of the primitive application matches the type of the output binders.

class JaxprType(NamedTuple):
  in_types:  List[ShapedArray]
  out_types: List[ShapedArray]

  def __repr__(self):
    in_types = ', '.join(aval.str_short() for aval in self.in_types)
    out_types = ', '.join(aval.str_short() for aval in self.out_types)
    return f'({in_types}) -> ({out_types})'

def typecheck_jaxpr(jaxpr: Jaxpr) -> JaxprType:
  env: Set[Var] = set()

  for v in jaxpr.in_binders:
    if v in env: raise TypeError
    env.add(v)

  for eqn in jaxpr.eqns:
    in_types = [typecheck_atom(env, x) for x in eqn.inputs]
    out_types = abstract_eval_rules[eqn.primitive](*in_types, **eqn.params)
    for out_binder, out_type in zip(eqn.out_binders, out_types):
      if not out_type == out_binder.aval: raise TypeError
    for out_binder in eqn.out_binders:
      if out_binder in env: raise TypeError
      env.add(out_binder)

  in_types = [v.aval for v in jaxpr.in_binders]
  out_types = [typecheck_atom(env, x) for x in jaxpr.outs]
  return JaxprType(in_types, out_types)

def typecheck_atom(env: Set[Var], x: Atom) -> ShapedArray:
  if isinstance(x, Var):
    if x not in env: raise TypeError("unbound variable")
    return x.aval
  elif isinstance(x, Lit):
    return raise_to_shaped(get_aval(x.val))
  else:
    assert False

We can apply the function represented by a jaxpr to arguments with a simple interpreter.

def eval_jaxpr(jaxpr: Jaxpr, args: List[Any]) -> List[Any]:
  env: Dict[Var, Any] = {}

  def read(x: Atom) -> Any:
    return env[x] if type(x) is Var else x.val

  def write(v: Var, val: Any) -> None:
    assert v not in env  # single-assignment
    env[v] = val

  map(write, jaxpr.in_binders, args)
  for eqn in jaxpr.eqns:
    in_vals = map(read, eqn.inputs)
    outs = bind(eqn.primitive, *in_vals, **eqn.params)
    map(write, eqn.out_binders, outs)
  return map(read, jaxpr.outs)

def jaxpr_as_fun(jaxpr: Jaxpr):
  return lambda *args: eval_jaxpr(jaxpr, args)

By using bind in the interpreter, this interpreter itself is traceable.

Building jaxprs with tracing

Now that we have jaxprs as a data structure, we need ways to produce these from tracing Python code. In general there are two variants of how we trace to a jaxpr; jit uses one and vjp uses the other. We’ll start with the one used by jit, which is also used by control flow primitives like lax.cond, lax.while_loop, and lax.scan.

def split_list(lst: List[Any], n: int) -> Tuple[List[Any], List[Any]]:
  assert 0 <= n <= len(lst)
  return lst[:n], lst[n:]

def partition_list(bs: List[bool], l: List[Any]) -> Tuple[List[Any], List[Any]]:
  assert len(bs) == len(l)
  lists = lst1, lst2 = [], []
  for b, x in zip(bs, l):
    lists[b].append(x)
  return lst1, lst2
# NB: the analogous class in JAX is called 'DynamicJaxprTracer'
class JaxprTracer(Tracer):
  __slots__ = ['aval']
  aval: ShapedArray

  def __init__(self, trace, aval):
    self._trace = trace
    self.aval = aval

# NB: the analogous class in JAX is called 'DynamicJaxprTrace'
class JaxprTrace(Trace):
  def new_arg(self, aval: ShapedArray) -> JaxprTracer:
    aval = raise_to_shaped(aval)
    tracer = self.builder.new_tracer(self, aval)
    self.builder.tracer_to_var[id(tracer)] = Var(aval)
    return tracer

  def get_or_make_const_tracer(self, val: Any) -> JaxprTracer:
    tracer = self.builder.const_tracers.get(id(val))
    if tracer is None:
      tracer = self.builder.new_tracer(self, raise_to_shaped(get_aval(val)))
      self.builder.add_const(tracer, val)
    return tracer
  pure = lift = get_or_make_const_tracer

  def process_primitive(self, primitive, tracers, params):
    avals_in = [t.aval for t in tracers]
    avals_out = abstract_eval_rules[primitive](*avals_in, **params)
    out_tracers = [self.builder.new_tracer(self, a) for a in avals_out]
    inputs = [self.builder.getvar(t) for t in tracers]
    outvars = [self.builder.add_var(t) for t in out_tracers]
    self.builder.add_eqn(JaxprEqn(primitive, inputs, params, outvars))
    return out_tracers

  @property
  def builder(self):
    return self.main.global_data

# NB: in JAX, we instead attach abstract eval rules to Primitive instances
abstract_eval_rules = {}

Notice that we keep as interpreter-global data a builder object, which keeps track of variables, constants, and eqns as we build up the jaxpr.

class JaxprBuilder:
  eqns: List[JaxprEqn]
  tracer_to_var: Dict[int, Var]
  const_tracers: Dict[int, JaxprTracer]
  constvals: Dict[Var, Any]
  tracers: List[JaxprTracer]

  def __init__(self):
    self.eqns = []
    self.tracer_to_var = {}
    self.const_tracers = {}
    self.constvals = {}
    self.tracers = []

  def new_tracer(self, trace: JaxprTrace, aval: ShapedArray) -> JaxprTracer:
    tracer = JaxprTracer(trace, aval)
    self.tracers.append(tracer)
    return tracer

  def add_eqn(self, eqn: JaxprEqn) -> None:
    self.eqns.append(eqn)

  def add_var(self, tracer: JaxprTracer) -> Var:
    assert id(tracer) not in self.tracer_to_var
    var = self.tracer_to_var[id(tracer)] = Var(tracer.aval)
    return var

  def getvar(self, tracer: JaxprTracer) -> Var:
    var = self.tracer_to_var.get(id(tracer))
    assert var is not None
    return var

  def add_const(self, tracer: JaxprTracer, val: Any) -> Var:
    var = self.add_var(tracer)
    self.const_tracers[id(val)] = tracer
    self.constvals[var] = val
    return var

  def build(self, in_tracers: List[JaxprTracer], out_tracers: List[JaxprTracer]
            ) -> Tuple[Jaxpr, List[Any]]:
    constvars, constvals = unzip2(self.constvals.items())
    t2v = lambda t: self.tracer_to_var[id(t)]
    in_binders = constvars + [t2v(t) for t in in_tracers]
    out_vars = [t2v(t) for t in out_tracers]
    jaxpr = Jaxpr(in_binders, self.eqns, out_vars)
    typecheck_jaxpr(jaxpr)
    jaxpr, constvals = _inline_literals(jaxpr, constvals)
    return jaxpr, constvals
def _inline_literals(jaxpr: Jaxpr, consts: List[Any]) -> Tuple[Jaxpr, List[Any]]:
  const_binders, other_binders = split_list(jaxpr.in_binders, len(consts))
  scalars = [type(x) in jax_types and not get_aval(x).shape for x in consts]
  new_const_binders, lit_binders = partition_list(scalars, const_binders)
  new_consts, lit_vals = partition_list(scalars, consts)
  literals = dict(zip(lit_binders, map(Lit, lit_vals)))
  new_eqns = [JaxprEqn(eqn.primitive, [literals.get(x, x) for x in eqn.inputs],
                       eqn.params, eqn.out_binders) for eqn in jaxpr.eqns]
  new_outs = [literals.get(x, x) for x in jaxpr.outs]
  new_jaxpr = Jaxpr(new_const_binders + other_binders, new_eqns, new_outs)
  typecheck_jaxpr(new_jaxpr)
  return new_jaxpr, new_consts

The rules we need for JaxprTrace.process_primitive are essentially typing rules for primitive applications: given the primitive, its parameters, and types for the inputs, the rule must produce a type for the output, which is then packaged with the output JaxprTracer. We can use abstract evaluation rules for this same purpose, even though they can be more general (since abstract evaluation rules must accept ConcreteArray inputs, and since they need only return an upper bound on the set of possible outputs, they can produce ConcreteArray outputs as well). We’ll reuse these abstract evaluation rules for the other jaxpr-producing trace machinery, where the potential extra generality is useful.

def binop_abstract_eval(x: ShapedArray, y: ShapedArray) -> List[ShapedArray]:
  if not isinstance(x, ShapedArray) or not isinstance(y, ShapedArray):
    raise TypeError
  if raise_to_shaped(x) != raise_to_shaped(y): raise TypeError
  return [ShapedArray(x.shape, x.dtype)]

abstract_eval_rules[add_p] = binop_abstract_eval
abstract_eval_rules[mul_p] = binop_abstract_eval

def compare_abstract_eval(x: ShapedArray, y: ShapedArray) -> List[ShapedArray]:
  if not isinstance(x, ShapedArray) or not isinstance(y, ShapedArray):
    raise TypeError
  if x.shape != y.shape: raise TypeError
  return [ShapedArray(x.shape, np.dtype('bool'))]
abstract_eval_rules[greater_p] = compare_abstract_eval
abstract_eval_rules[less_p] = compare_abstract_eval

def vectorized_unop_abstract_eval(x: ShapedArray) -> List[ShapedArray]:
  return [ShapedArray(x.shape, x.dtype)]

abstract_eval_rules[sin_p] = vectorized_unop_abstract_eval
abstract_eval_rules[cos_p] = vectorized_unop_abstract_eval
abstract_eval_rules[neg_p] = vectorized_unop_abstract_eval

def reduce_sum_abstract_eval(x: ShapedArray, *, axis: int) -> List[ShapedArray]:
  new_shape = [d for i, d in enumerate(x.shape) if i != axis]
  return [ShapedArray(tuple(new_shape), x.dtype)]
abstract_eval_rules[reduce_sum_p] = reduce_sum_abstract_eval

def broadcast_abstract_eval(x: ShapedArray, *, shape: Sequence[int],
                            axes: Sequence[int]) -> List[ShapedArray]:
  return [ShapedArray(tuple(shape), x.dtype)]
abstract_eval_rules[broadcast_p] = broadcast_abstract_eval

To check our implementation of jaxprs, we can add a make_jaxpr transformation and a pretty-printer:

from functools import lru_cache

@lru_cache()  # ShapedArrays are hashable
def make_jaxpr_v1(f, *avals_in):
  avals_in, in_tree = tree_flatten(avals_in)
  f, out_tree = flatten_fun(f, in_tree)

  builder = JaxprBuilder()
  with new_main(JaxprTrace, builder) as main:
    trace = JaxprTrace(main)
    tracers_in = [trace.new_arg(aval) for aval in avals_in]
    outs = f(*tracers_in)
    tracers_out = [full_raise(trace, out) for out in outs]
    jaxpr, consts = builder.build(tracers_in, tracers_out)
  return jaxpr, consts, out_tree()
from collections import defaultdict
import string

class PPrint:
  lines: List[Tuple[int, str]]

  def __init__(self, lines):
    self.lines = lines

  def indent(self, indent: int) -> 'PPrint':
    return PPrint([(indent + orig_indent, s) for orig_indent, s in self.lines])

  def __add__(self, rhs: 'PPrint') -> 'PPrint':
    return PPrint(self.lines + rhs.lines)

  def __rshift__(self, rhs: 'PPrint') -> 'PPrint':
    if not rhs.lines: return self
    if not self.lines: return rhs
    indent, s = self.lines[-1]
    indented_block = rhs.indent(indent + len(s))
    common_line = s + ' ' * rhs.lines[0][0] + rhs.lines[0][1]
    return PPrint(self.lines[:-1]
                  + [(indent, common_line)]
                  + indented_block.lines[1:])

  def __str__(self) -> str:
    return '\n'.join(' ' * indent + s for indent, s in self.lines)

def pp(s: Any) -> PPrint:
  return PPrint([(0, line) for line in str(s).splitlines()])

def vcat(ps: List[PPrint]) -> PPrint:
  return sum(ps, pp(''))

def pp_jaxpr(jaxpr: Jaxpr):
  namegen = (''.join(s) for r in it.count(1)
             for s in it.permutations(string.ascii_lowercase, r))
  names = defaultdict(lambda: next(namegen))
  in_binders = ', '.join(var_str(names, x) for x in jaxpr.in_binders)
  eqns = vcat([pp_eqn(names, e) for e in jaxpr.eqns])
  outs = ', '.join(names[v] if isinstance(v, Var) else str(v.val)
                   for v in jaxpr.outs)
  return (pp(f'{{ lambda {in_binders} .') +
          ((pp('let ') >> eqns) + pp(f'in ( {outs} ) }}')).indent(2))

def var_str(names: Dict[Var, str], v: Var) -> str:
  return f'{names[v]}:{v.aval.str_short()}'

def pp_eqn(names: Dict[Var, str], eqn: JaxprEqn) -> PPrint:
  lhs = pp(' '.join(var_str(names, v) for v in eqn.out_binders))
  rhs = (pp(eqn.primitive.name) >> pp_params(eqn.params) >>
         pp(' '.join(names[x] if isinstance(x, Var) else str(x.val)
                     for x in eqn.inputs)))
  return lhs >> pp(' = ') >> rhs

def pp_params(params: Dict[str, Any]) -> PPrint:
  items = sorted(params.items())
  if items:
    return pp(' [ ') >> vcat([pp(f'{k}={v}') for k, v in items]) >> pp(' ] ')
  else:
    return pp(' ')

Jaxpr.__repr__ = lambda self: str(pp_jaxpr(self))
jaxpr, consts, _ = make_jaxpr_v1(lambda x: 2. * x, raise_to_shaped(get_aval(3.)))
print(jaxpr)
print(typecheck_jaxpr(jaxpr))
{ lambda a:float64[] .
  let b:float64[] = mul 2.0 a
  in ( b ) }
(float64[]) -> (float64[])

But there’s a limitation here: because of how find_top_trace operates by data dependence, make_jaxpr_v1 can’t stage out all the primitive operations performed by the Python callable it’s given. For example:

jaxpr, consts, _ = make_jaxpr_v1(lambda: mul(2., 2.))
print(jaxpr)
{ lambda  .
  let 
  in ( 4.0 ) }

This is precisely the issue that omnistaging fixed. We want to ensure that the JaxprTrace started by make_jaxpr is always applied, regardless of whether any inputs to bind are boxed in corresponding JaxprTracer instances. We can achieve this by employing the dynamic_trace global defined in Part 1:

@contextmanager
def new_dynamic(main: MainTrace):
  global dynamic_trace
  prev_dynamic_trace, dynamic_trace = dynamic_trace, main
  try:
    yield
  finally:
    dynamic_trace = prev_dynamic_trace

@lru_cache()
def make_jaxpr(f: Callable, *avals_in: ShapedArray,
               ) -> Tuple[Jaxpr, List[Any], PyTreeDef]:
  avals_in, in_tree = tree_flatten(avals_in)
  f, out_tree = flatten_fun(f, in_tree)

  builder = JaxprBuilder()
  with new_main(JaxprTrace, builder) as main:
    with new_dynamic(main):
      trace = JaxprTrace(main)
      tracers_in = [trace.new_arg(aval) for aval in avals_in]
      outs = f(*tracers_in)
      tracers_out = [full_raise(trace, out) for out in outs]
      jaxpr, consts = builder.build(tracers_in, tracers_out)
  return jaxpr, consts, out_tree()

jaxpr, consts, _ = make_jaxpr(lambda: mul(2., 2.))
print(jaxpr)
{ lambda  .
  let a:float64[] = mul 2.0 2.0
  in ( a ) }

Using dynamic_trace this way is conceptually the same as stashing the current interpreter stack and starting a new one with the JaxprTrace at the bottom. That is, no interpreters lower in the stack than the dynamic_trace are applied (since JaxprTrace.process_primitive doesn’t call bind), though if the Python callable being traced to a jaxpr itself uses transformations then those can be pushed onto the interpreter stack above the JaxprTrace. But temporarily stashing the interpreter stack would break up the system state. The dynamic_trace tag achieves the same goals while keeping the system state simpler.

That’s it for jaxprs! With jaxprs in hand, we can implement the remaining major JAX features.

Part 3: jit, simplified

While jit has a transformation-like API in that it accepts a Python callable as an argument, under the hood it’s really a higher-order primitive rather than a transformation. A primitive is higher-order when it’s parameterized by a function.

On-the-fly (“final style”) and staged (“initial style”) processing

There are two options for how to handle higher-order primitives. Each requires a different approach to tracing and engenders different tradeoffs:

  1. On-the-fly processing, where bind takes a Python callable as an argument. We defer forming a jaxpr until as late as possible, namely until we’re running the final interpreter at the bottom of the interpreter stack. That way we can swap a JaxprTrace in at the bottom of the interpreter stack and thus stage out rather than execute all primitive operations. With this approach, transformations in the stack get applied as we execute the Python callable as usual. This approach can be very tricky to implement, but it’s as general as possible because it allows higher-order primitives not to raise the abstraction level of their arguments and thus allows data-dependent Python control flow. We refer to this approach as using a “final-style higher-order primitive” employing the discharge-at-tracing-time “final-style transformations” we’ve used so far.

  2. Staged processing, where bind takes a jaxpr as an argument. Before we call bind, in the primitive wrapper we can just use make_jaxpr to form a jaxpr up-front and be done with the Python callable entirely. In this case, make_jaxpr puts its JaxprTrace at the top of the interpreter stack, and no transformations lower in the stack, which might enter via closed-over Tracers, are applied to the Python callable as we trace it. (Transformations applied within the Python callable are applied as usual, being added to the stack above the JaxprTrace.) Instead, the transformations lower in the stack are later applied to the call primitive, and the call primitive’s rules must then transform the jaxpr itself. Because we trace to a jaxpr up-front, this approach can’t support data-dependent Python control flow, but it is more straightforward to implement. We refer to this kind of higher-order primitive as an “initial-style higher-order primitive”, and say that its jaxpr-processing transformation rules are “initial-style transformation rules.”

The latter approach fits for jit because we don’t need to support data-dependent Python control flow in the user-provided Python callable, as the whole purpose of jit is to stage computation out of Python to be executed by XLA. (In contrast, custom_jvp is a higher-order primitive in which we want to support data-dependent Python control flow.)

Historically, we started using the “initial-style” and “final-style” terminology after reading the typed tagless final interpreters paper, and jokingly referring to JAX as an implementation of “untyped tagful final interpreters.” We don’t claim to carry over (or understand) any deep meaning behind these terms; we loosely use “initial style” to mean “build an AST and then transform it”, and we use “final style” to mean “transform as we trace.” But it’s just imprecise yet sticky jargon.

With the initial-style approach, here’s the user-facing jit wrapper:

def jit(f):
  def f_jitted(*args):
    avals_in = [raise_to_shaped(get_aval(x)) for x in args]
    jaxpr, consts, out_tree = make_jaxpr(f, *avals_in)
    outs = bind(xla_call_p, *consts, *args, jaxpr=jaxpr, num_consts=len(consts))
    return tree_unflatten(out_tree, outs)
  return f_jitted

xla_call_p = Primitive('xla_call')

With any new primitive, we need to give it transformation rules, starting with its evaluation rule. When we evaluate an application of the xla_call primitive, we want to stage out out the computation to XLA. That involves translating the jaxpr to an XLA HLO program, transferring the argument values to the XLA device, executing the XLA program, and transferring back the results. We’ll cache the XLA HLO compilation so that for each jitted function it only needs to be performed once per argument shape and dtype signature.

First, some utilities.

class IDHashable:
  val: Any

  def __init__(self, val):
    self.val = val

  def __hash__(self) -> int:
    return id(self.val)

  def __eq__(self, other):
    return type(other) is IDHashable and id(self.val) == id(other.val)

Next, we’ll define the evaluation rule for xla_call:

from jax.lib import xla_bridge as xb
from jax.lib import xla_client as xc
xe = xc._xla
xops = xc._xla.ops

def xla_call_impl(*args, jaxpr: Jaxpr, num_consts: int):
  consts, args = args[:num_consts], args[num_consts:]
  hashable_consts = tuple(map(IDHashable, consts))
  execute = xla_callable(IDHashable(jaxpr), hashable_consts)
  return execute(*args)
impl_rules[xla_call_p] = xla_call_impl

@lru_cache()
def xla_callable(hashable_jaxpr: IDHashable, hashable_consts: Tuple[IDHashable]):
  jaxpr: Jaxpr = hashable_jaxpr.val
  typecheck_jaxpr(jaxpr)
  consts = [x.val for x in hashable_consts]
  in_avals = [v.aval for v in jaxpr.in_binders[len(consts):]]
  c = xb.make_computation_builder('xla_call')
  xla_consts = _xla_consts(c, consts)
  xla_params = _xla_params(c, in_avals)
  outs = jaxpr_subcomp(c, jaxpr, xla_consts + xla_params)
  out = xops.Tuple(c, outs)
  compiled = xb.get_backend(None).compile(c.build(out))
  return partial(execute_compiled, compiled, [v.aval for v in jaxpr.outs])

def _xla_consts(c: xe.XlaBuilder, consts: List[Any]) -> List[xe.XlaOp]:
  unique_consts = {id(cnst): cnst for cnst in consts}
  xla_consts = {
      id_: xops.ConstantLiteral(c, cnst) for id_, cnst in unique_consts.items()}
  return [xla_consts[id(cnst)] for cnst in consts]

def _xla_params(c: xe.XlaBuilder, avals_in: List[ShapedArray]) -> List[xe.XlaOp]:
  return [xb.parameter(c, i, _xla_shape(a)) for i, a in enumerate(avals_in)]

def _xla_shape(aval: ShapedArray) -> xe.Shape:
  return xc.Shape.array_shape(xc.dtype_to_etype(aval.dtype), aval.shape)

The main action is in xla_callable, which compiles a jaxpr into an XLA HLO program using jaxpr_subcomp, then returns a callable which executes the compiled program:

def jaxpr_subcomp(c: xe.XlaBuilder, jaxpr: Jaxpr, args: List[xe.XlaOp]
                  ) -> xe.XlaOp:
  env: Dict[Var, xe.XlaOp] = {}

  def read(x: Atom) -> xe.XlaOp:
    return env[x] if type(x) is Var else xb.constant(c, x.val, False)

  def write(v: Var, val: xe.XlaOp) -> None:
    env[v] = val

  map(write, jaxpr.in_binders, args)
  for eqn in jaxpr.eqns:
    in_avals = [x.aval for x in eqn.inputs]
    in_vals = map(read, eqn.inputs)
    rule = xla_translations[eqn.primitive]
    out_vals = rule(c, in_avals, in_vals, **eqn.params)
    map(write, eqn.out_binders, out_vals)
  return map(read, jaxpr.outs)

def execute_compiled(compiled, out_avals, *args):
  input_bufs = [input_handlers[type(x)](x) for x in args]
  out_bufs = compiled.execute(input_bufs)
  return [handle_result(aval, buf) for aval, buf in zip(out_avals, out_bufs)]

default_input_handler = xb.get_backend(None).buffer_from_pyval
input_handlers = {ty: default_input_handler for ty in
                  [bool, int, float, np.ndarray, np.float64, np.float32]}

def handle_result(aval: ShapedArray, buf):
  del aval  # Unused for now.
  return buf.to_py()

xla_translations = {}
WARNING:absl:No GPU/TPU found, falling back to CPU. (Set TF_CPP_MIN_LOG_LEVEL=0 and rerun for more info.)

Notice that jaxpr_subcomp has the structure of a simple interpreter. That’s a common pattern: the way we process jaxprs is usually with an interpreter. And as with any interpreter, we need an interpretation rule for each primitive:

def direct_translation(op, c, in_avals, in_vals):
  del c, in_avals
  return [op(*in_vals)]

xla_translations[add_p] = partial(direct_translation, xops.Add)
xla_translations[mul_p] = partial(direct_translation, xops.Mul)
xla_translations[neg_p] = partial(direct_translation, xops.Neg)
xla_translations[sin_p] = partial(direct_translation, xops.Sin)
xla_translations[cos_p] = partial(direct_translation, xops.Cos)
xla_translations[greater_p] = partial(direct_translation, xops.Gt)
xla_translations[less_p] = partial(direct_translation, xops.Lt)

def reduce_sum_translation(c, in_avals, in_vals, *, axis):
  (x_aval,), (x,) = in_avals, in_vals
  zero = xops.ConstantLiteral(c, np.array(0, x_aval.dtype))
  subc = xb.make_computation_builder('add')
  shape = _xla_shape(ShapedArray((), x_aval.dtype))
  xops.Add(xops.Parameter(subc, 0, shape), xops.Parameter(subc, 1, shape))
  return [xops.Reduce(c, [x], [zero], subc.build(), [axis])]
xla_translations[reduce_sum_p] = reduce_sum_translation

def broadcast_translation(c, in_avals, in_vals, *, shape, axes):
  x, = in_vals
  dims_complement = [i for i in range(len(shape)) if i not in axes]
  return [xops.BroadcastInDim(x, shape, dims_complement)]
xla_translations[broadcast_p] = broadcast_translation

With that, we can now use jit to stage out, compile, and execute programs with XLA!

@jit
def f(x, y):
  print('tracing!')
  return sin(x) * cos(y)
z = f(3., 4.)  # 'tracing!' prints the first time
print(z)
tracing!
-0.09224219304455371
z = f(4., 5.)  # 'tracing!' doesn't print, compilation cache hit!
print(z)
-0.21467624978306993
@jit
def f(x):
  return reduce_sum(x, axis=0)

print(f(np.array([1., 2., 3.])))
6.0
def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

def deriv(f):
  return lambda x: jvp(f, (x,), (1.,))[1]

print(    deriv(deriv(f))(3.))
print(jit(deriv(deriv(f)))(3.))
0.2822400161197344
0.2822400161197344

Instead of implementing jit to first trace to a jaxpr and then to lower the jaxpr to XLA HLO, it might appear that we could have skipped the jaxpr step and just lowered to HLO while tracing. That is, perhaps we could have instead implemented jit with a Trace and Tracer that appended to the XLA HLO graph incrementally on each primitive bind. That’s correct for now, but won’t be possible when we introduce compiled SPMD computations because there we must know the number of replicas needed before compiling the program.

We haven’t yet defined any transformation rules for xla_call_p other than its evaluation rule. That is, we can’t yet do vmap-of-jit or jvp-of-jit or even jit-of-jit. Instead jit has to be at the “top level.” Let’s fix that!

def xla_call_jvp_rule(primals, tangents, *, jaxpr, num_consts):
  del num_consts  # Unused.
  new_jaxpr, new_consts = jvp_jaxpr(jaxpr)
  outs = bind(xla_call_p, *new_consts, *primals, *tangents, jaxpr=new_jaxpr,
              num_consts=len(new_consts))
  n = len(outs) // 2
  primals_out, tangents_out = outs[:n], outs[n:]
  return primals_out, tangents_out
jvp_rules[xla_call_p] = xla_call_jvp_rule

@lru_cache()
def jvp_jaxpr(jaxpr: Jaxpr) -> Tuple[Jaxpr, List[Any]]:
  def jvp_traceable(*primals_and_tangents):
    n = len(primals_and_tangents) // 2
    primals, tangents = primals_and_tangents[:n], primals_and_tangents[n:]
    return jvp(jaxpr_as_fun(jaxpr), primals, tangents)

  in_avals = [v.aval for v in jaxpr.in_binders]
  new_jaxpr, new_consts, _ = make_jaxpr(jvp_traceable, *in_avals, *in_avals)
  return new_jaxpr, new_consts
def xla_call_vmap_rule(axis_size, vals_in, dims_in, *, jaxpr, num_consts):
  del num_consts  # Unused.
  new_jaxpr, new_consts = vmap_jaxpr(jaxpr, axis_size, tuple(dims_in))
  outs = bind(xla_call_p, *new_consts, *vals_in, jaxpr=new_jaxpr,
              num_consts=len(new_consts))
  return outs, [0] * len(outs)
vmap_rules[xla_call_p] = xla_call_vmap_rule

@lru_cache()
def vmap_jaxpr(jaxpr: Jaxpr, axis_size: int, bdims_in: Tuple[BatchAxis, ...]
               ) -> Tuple[Jaxpr, List[Any]]:
  vmap_traceable = vmap(jaxpr_as_fun(jaxpr), tuple(bdims_in))
  in_avals = [unmapped_aval(axis_size, d, v.aval)
              for v, d in zip(jaxpr.in_binders, bdims_in)]
  new_jaxpr, new_consts, _ = make_jaxpr(vmap_traceable, *in_avals)
  return new_jaxpr, new_consts

def unmapped_aval(axis_size: int, batch_dim: BatchAxis, aval: ShapedArray
                  ) -> ShapedArray:
  if batch_dim is not_mapped:
    return aval
  else:
    shape = list(aval.shape)
    shape.insert(batch_dim, axis_size)
    return ShapedArray(tuple(shape), aval.dtype)
def xla_call_abstract_eval_rule(*in_types, jaxpr, num_consts):
  del num_consts  # Unused.
  jaxpr_type = typecheck_jaxpr(jaxpr)
  if not all(t1 == t2 for t1, t2 in zip(jaxpr_type.in_types, in_types)):
    raise TypeError
  return jaxpr_type.out_types
abstract_eval_rules[xla_call_p] = xla_call_abstract_eval_rule

def xla_call_translation(c, in_avals, in_vals, *, jaxpr, num_consts):
  del num_consts  # Only used at top-level.
  # Calling jaxpr_subcomp directly would inline. We generate a Call HLO instead.
  subc = xb.make_computation_builder('inner xla_call')
  xla_params = _xla_params(subc, in_avals)
  outs = jaxpr_subcomp(subc, jaxpr, xla_params)
  subc = subc.build(xops.Tuple(subc, outs))
  return destructure_tuple(c, xops.Call(c, subc, in_vals))
xla_translations[xla_call_p] = xla_call_translation

def destructure_tuple(c, tup):
  num_elements = len(c.get_shape(tup).tuple_shapes())
  return [xops.GetTupleElement(tup, i) for i in range(num_elements)]
@jit
def f(x):
  print('tracing!')
  y = sin(x) * 2.
  z = - y + x
  return z

x, xdot = 3., 1.
y, ydot = jvp(f, (x,), (xdot,))
print(y)
print(ydot)
tracing!
2.7177599838802657
2.979984993200891
y, ydot = jvp(f, (x,), (xdot,))  # 'tracing!' not printed
ys = vmap(f, (0,))(np.arange(3.))
print(ys)
[ 0.         -0.68294197  0.18140515]

One piece missing is device memory persistence for arrays. That is, we’ve defined handle_result to transfer results back to CPU memory as NumPy arrays, but it’s often preferable to avoid transferring results just to transfer them back for the next operation. We can do that by introducing a DeviceArray class, which can wrap XLA buffers and otherwise duck-type numpy.ndarrays:

def handle_result(aval: ShapedArray, buf):  # noqa: F811
  return DeviceArray(aval, buf)

class DeviceArray:
  buf: Any
  aval: ShapedArray

  def __init__(self, aval, buf):
    self.aval = aval
    self.buf = buf

  dtype = property(lambda self: self.aval.dtype)
  shape = property(lambda self: self.aval.shape)
  ndim  = property(lambda self: self.aval.ndim)

  def __array__(self): return self.buf.to_py()
  def __repr__(self):  return repr(self.buf.to_py())
  def __str__(self):   return str(self.buf.to_py())

  _neg = staticmethod(neg)
  _add = staticmethod(add)
  _radd = staticmethod(add)
  _mul = staticmethod(mul)
  _rmul = staticmethod(mul)
  _gt = staticmethod(greater)
  _lt = staticmethod(less)
input_handlers[DeviceArray] = lambda x: x.buf

jax_types.add(DeviceArray)
@jit
def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

x, xdot = 3., 1.
y, ydot = jvp(f, (x,), (xdot,))
print(y)
print(ydot)
2.7177599838802657
2.979984993200891

Part 4: linearize and vjp (and grad!)

The linearize and vjp autodiff functions are built on jvp, but involve jaxprs as well. That’s because both involve staging out, or delaying, computation.

linearize

In the case of linearize, we want to stage out the linear part of a jvp computation. That is, if we have jvp : (a -> b) -> (a, T a) -> (b, T b), then we write linearize : (a -> b) -> a -> (b, T a -o T b), using T a to mean “the tangent type of a” and using the “lollipop” -o rather than the arrow -> to indicate a linear function. We define the semantics of linearize in terms of jvp too:

y, f_lin = linearize(f, x)
y_dot = f_lin(x_dot)

gives the same result for (y, y_dot) as

y, y_dot = jvp(f, (x,), (x_dot,))

where the application of f_lin does not redo any of the linearization work. We’ll represent the delayed linear part f_lin : T a -o T b as a jaxpr.

Tangentially, now that we have linear arrows -o, we can provide a slightly more informative type for jvp:

jvp : (a -> b) -> (UnrestrictedUse a, T a) -o (UnrestrictedUse b, T b)

Here we’re writing UnrestrictedUse just to indicate that we have a special pair where the first element can be used in an unrestricted (nonlinear) way. In conjunction with the linear arrow, this notation is just meant to express that the function jvp f uses its first input in a nonlinear way but its second input in a linear way, producing a corresponding nonlinear output (which can be used in a nonlinear way) paired with a linear output. This more refined type signature encodes the data dependencies in jvp f, which are useful for partial evaluation.

To build the f_lin jaxpr from a JVP, we need to perform partial evaluation: we evaluate all the primal values as we trace, but stage the tangent computations into a jaxpr. This is our second way to build jaxprs. But where make_jaxpr and its underlying JaxprTrace/JaxprTracer interpreters aim to stage out every primitive bind, this second approach stages out only those primitive binds with a data dependence on tangent inputs.

First, some utilities:

def split_half(lst: List[Any]) -> Tuple[List[Any], List[Any]]:
  assert not len(lst) % 2
  return split_list(lst, len(lst) // 2)

def merge_lists(which: List[bool], l1: List[Any], l2: List[Any]) -> List[Any]:
  l1, l2 = iter(l1), iter(l2)
  out = [next(l2) if b else next(l1) for b in which]
  assert next(l1, None) is next(l2, None) is None
  return out

Next, we’ll write linearize by combining jvp together with a general partial evaluation transformation, to be added next:

def linearize_flat(f, *primals_in):
  pvals_in = ([PartialVal.known(x) for x in primals_in] +
              [PartialVal.unknown(vspace(get_aval(x))) for x in primals_in])
  def f_jvp(*primals_tangents_in):
    primals_out, tangents_out = jvp(f, *split_half(primals_tangents_in))
    return [*primals_out, *tangents_out]
  jaxpr, pvals_out, consts = partial_eval_flat(f_jvp, pvals_in)
  primal_pvals, _ = split_half(pvals_out)
  assert all(pval.is_known for pval in primal_pvals)
  primals_out = [pval.const for pval in primal_pvals]
  f_lin = lambda *tangents: eval_jaxpr(jaxpr, [*consts, *tangents])
  return primals_out, f_lin

def linearize(f, *primals_in):
  primals_in_flat, in_tree = tree_flatten(primals_in)
  f, out_tree = flatten_fun(f, in_tree)
  primals_out_flat, f_lin_flat = linearize_flat(f, *primals_in_flat)
  primals_out = tree_unflatten(out_tree(), primals_out_flat)

  def f_lin(*tangents_in):
    tangents_in_flat, in_tree2 = tree_flatten(tangents_in)
    if in_tree != in_tree2: raise TypeError
    tangents_out_flat = f_lin_flat(*tangents_in_flat)
    return tree_unflatten(out_tree(), tangents_out_flat)

  return primals_out, f_lin

def vspace(aval: ShapedArray) -> ShapedArray:
  return raise_to_shaped(aval)  # TODO handle integers?

Now we turn to the general partial evaluation transformation. The goal is to accept a Python callable and a list of inputs, some known and some unknown, and to produce (1) all the outputs which can be computed from the known inputs, together with (2) a jaxpr representing the part of the Python callable’s computation which can only be performed after the remaining inputs are known.

This transformation is tricky to summarize in a type signature. If we assume the input function’s type signature is (a1, a2) -> (b1, b2), where a1 and a2 represent the known and unknown inputs, respectively, and where b1 only has a data dependency on a1 while b2 has some data dependency on a2, then we might write

partial_eval : ((a1, a2) -> (b1, b2)) -> a1 -> exists r. (b1, r, (r, a2) -> b2)

In words, given values for the inputs of type a1, partial_eval produces the outputs of type b1 along with “residual” values of existentially-quantified type r representing the intermediates required to complete the computation in the second stage. It also produces a function of type (r, a2) -> b2 which accepts the residual values as well as the remaining inputs and produces the remaining outputs.

We like to think of partial evaluation as “unzipping” one computation into two. For example, consider this jaxpr:

{ lambda a:float64[] .
  let b:float64[] = sin a
      c:float64[] = neg b
  in ( c ) }

A jaxpr for the JVP would look like:

{ lambda a:float64[] b:float64[] .
  let c:float64[] = sin a
      d:float64[] = cos a
      e:float64[] = mul d b
      f:float64[] = neg c
      g:float64[] = neg e
  in ( f, g ) }

If we imagine applying partial evaluation to this jaxpr with the first input known and the second unknown, we end up ‘unzipping’ the JVP jaxpr into primal and tangent jaxprs:

{ lambda a:float64[] .
  let c:float64[] = sin a
      d:float64[] = cos a
      f:float64[] = neg c
  in ( f, d ) }
{ lambda d:float64[] b:float64[] .
  let e:float64[] = mul d b
      g:float64[] = neg e
  in ( g ) }

This second jaxpr represents the linear computation that we want from linearize.

However, unlike in this jaxpr example, we want the computation on known values to occur while evaluating the input Python callable. That is, rather than forming a jaxpr for the entire function (a1, a2) -> (b1, b2), staging all operations out of Python first before sorting out what can be evaluated now and what must be delayed, we want only to form a jaxpr for those operations that must be delayed due to a dependence on unknown inputs. In the context of automatic differentiation, this is the feature that ultimately enables us to handle functions like grad(lambda x: x**2 if x > 0 else 0.). Python control flow works because partial evaluation keeps the primal computation in Python. As a consequence, our Trace and Tracer subclasses must on the fly sort out what can be evaluated and what must be staged out into a jaxpr.

First, we start with a PartialVal class, which represents a value that can be either known or unknown:

class PartialVal(NamedTuple):
  aval: ShapedArray
  const: Optional[Any]

  @classmethod
  def known(cls, val: Any):
    return PartialVal(get_aval(val), val)

  @classmethod
  def unknown(cls, aval: ShapedArray):
    return PartialVal(aval, None)

  is_known   = property(lambda self: self.const is not None)
  is_unknown = property(lambda self: self.const is     None)

Partial evaluation will take a list of PartialVals representing inputs, and return a list of PartialVal outputs along with a jaxpr representing the delayed computation:

def partial_eval_flat(f: Callable, pvals_in: List[PartialVal]
                      ) -> Tuple[Jaxpr, List[PartialVal], List[Any]]:
  with new_main(PartialEvalTrace) as main:
    trace = PartialEvalTrace(main)
    tracers_in = [trace.new_arg(pval) for pval in pvals_in]
    outs = f(*tracers_in)
    tracers_out = [full_raise(trace, out) for out in outs]
    pvals_out = [t.pval for t in tracers_out]
    unk_tracers_in  = [t for t in tracers_in  if t.pval.is_unknown]
    unk_tracers_out = [t for t in tracers_out if t.pval.is_unknown]
    jaxpr, consts = tracers_to_jaxpr(unk_tracers_in, unk_tracers_out)
  return jaxpr, pvals_out, consts

Next we need to implement PartialEvalTrace and its PartialEvalTracer. This interpreter will build a jaxpr on the fly while tracking data dependencies. To do so, it builds a bipartite directed acyclic graph (DAG) between PartialEvalTracer nodes, representing staged-out values, and JaxprRecipe nodes, representing formulas for how to compute some values from others. One kind of recipe is a JaxprEqnRecipe, corresponding to a JaxprEqn’s primitive application, but we also have recipe types for constants and lambda binders:

from weakref import ref, ReferenceType

class LambdaBindingRecipe(NamedTuple):
  pass

class ConstRecipe(NamedTuple):
  val: Any

class JaxprEqnRecipe(NamedTuple):
  prim: Primitive
  tracers_in: List['PartialEvalTracer']
  params: Dict[str, Any]
  avals_out: List[ShapedArray]
  tracer_refs_out: List['ReferenceType[PartialEvalTracer]']

JaxprRecipe = Union[LambdaBindingRecipe, ConstRecipe, JaxprEqnRecipe]
class PartialEvalTracer(Tracer):
  pval: PartialVal
  recipe: Optional[JaxprRecipe]

  def __init__(self, trace, pval, recipe):
    self._trace = trace
    self.pval = pval
    self.recipe = recipe

  aval = property(lambda self: self.pval.aval)

  def full_lower(self):
    if self.pval.is_known:
      return full_lower(self.pval.const)
    return self

The PartialEvalTrace contains the logic for constructing the graph of JaxprRecipes and PartialEvalTracers. Each argument corresponds to a LambdaBindingRecipe leaf node, and each constant is a ConstRecipe leaf node holding a reference to the constant. All other tracers and recipes come from process_primitive, which forms tracers with JaxprEqnRecipes.

For most primitives, the process_primitive logic is straightforward: if all inputs are known then we can bind the primitive on the known values (evaluating it in Python) and avoid forming tracers corresponding to the output. If instead any input is unknown then we instead stage out into a JaxprEqnRecipe representing the primitive application. To build the tracers representing unknown outputs, we need avals, which we get from the abstract eval rules. (Notice that tracers reference JaxprEqnRecipes, and JaxprEqnRecipes reference tracers; we avoid circular garbage by using weakrefs.)

That process_primitive logic applies to most primitives, but xla_call_p requires recursive treatment. So we special-case its rule in a partial_eval_rules dict.

class PartialEvalTrace(Trace):
  def new_arg(self, pval: PartialVal) -> Any:
    return PartialEvalTracer(self, pval, LambdaBindingRecipe())

  def lift(self, val: Any) -> PartialEvalTracer:
    return PartialEvalTracer(self, PartialVal.known(val), None)
  pure = lift

  def instantiate_const(self, tracer: PartialEvalTracer) -> PartialEvalTracer:
    if tracer.pval.is_unknown:
      return tracer
    else:
      pval = PartialVal.unknown(raise_to_shaped(tracer.aval))
      return PartialEvalTracer(self, pval, ConstRecipe(tracer.pval.const))

  def process_primitive(self, primitive, tracers, params):
    if all(t.pval.is_known for t in tracers):
      return bind(primitive, *map(full_lower, tracers), **params)
    rule = partial_eval_rules.get(primitive)
    if rule: return rule(self, tracers, **params)
    tracers_in = [self.instantiate_const(t) for t in tracers]
    avals_in = [t.aval for t in tracers_in]
    avals_out = abstract_eval_rules[primitive](*avals_in, **params)
    tracers_out = [PartialEvalTracer(self, PartialVal.unknown(aval), None)
                   for aval in avals_out]
    eqn = JaxprEqnRecipe(primitive, tracers_in, params, avals_out,
                         map(ref, tracers_out))
    for t in tracers_out: t.recipe = eqn
    return tracers_out

partial_eval_rules = {}

Now that we can build graph representations of jaxprs with PartialEvalTrace, we need a mechanism to convert the graph representation to a standard jaxpr. The jaxpr corresponds to a topological sort of the graph.

def tracers_to_jaxpr(tracers_in: List[PartialEvalTracer],
                     tracers_out: List[PartialEvalTracer]):
  tracer_to_var = {id(t): Var(raise_to_shaped(t.aval)) for t in tracers_in}
  constvar_to_val = {}
  constid_to_var = {}
  processed_eqns = set()
  eqns = []
  for t in toposort(tracers_out, tracer_parents):
    if isinstance(t.recipe, LambdaBindingRecipe):
      assert id(t) in set(map(id, tracers_in))
    elif isinstance(t.recipe, ConstRecipe):
      val = t.recipe.val
      var = constid_to_var.get(id(val))
      if var is None:
        aval = raise_to_shaped(get_aval(val))
        var = tracer_to_var[id(t)] = constid_to_var[id(val)] = Var(aval)
        constvar_to_val[var] = val
    elif isinstance(t.recipe, JaxprEqnRecipe):
      if id(t.recipe) not in processed_eqns:
        eqns.append(recipe_to_eqn(tracer_to_var, t.recipe))
        processed_eqns.add(id(t.recipe))
    else:
      raise TypeError(t.recipe)

  constvars, constvals = unzip2(constvar_to_val.items())
  in_binders = constvars + [tracer_to_var[id(t)] for t in tracers_in]
  out_vars = [tracer_to_var[id(t)] for t in tracers_out]
  jaxpr = Jaxpr(in_binders, eqns, out_vars)
  typecheck_jaxpr(jaxpr)
  return jaxpr, constvals

def recipe_to_eqn(tracer_to_var: Dict[int, Var], recipe: JaxprEqnRecipe
                  ) -> JaxprEqn:
  inputs = [tracer_to_var[id(t)] for t in recipe.tracers_in]
  out_binders = [Var(aval) for aval in recipe.avals_out]
  for t_ref, var in zip(recipe.tracer_refs_out, out_binders):
    if t_ref() is not None: tracer_to_var[id(t_ref())] = var
  return JaxprEqn(recipe.prim, inputs, recipe.params, out_binders)

def tracer_parents(t: PartialEvalTracer) -> List[PartialEvalTracer]:
  return t.recipe.tracers_in if isinstance(t.recipe, JaxprEqnRecipe) else []
def toposort(out_nodes: List[Any], parents: Callable[[Any], List[Any]]):
  if not out_nodes: return []
  out_nodes = remove_duplicates(out_nodes)

  child_counts = {}
  stack = list(out_nodes)
  while stack:
    node = stack.pop()
    if id(node) in child_counts:
      child_counts[id(node)] += 1
    else:
      child_counts[id(node)] = 1
      stack.extend(parents(node))
  for node in out_nodes:
    child_counts[id(node)] -= 1

  sorted_nodes = []
  childless_nodes = [node for node in out_nodes if not child_counts[id(node)]]
  while childless_nodes:
    node = childless_nodes.pop()
    sorted_nodes.append(node)
    for parent in parents(node):
      if child_counts[id(parent)] == 1:
        childless_nodes.append(parent)
      else:
        child_counts[id(parent)] -= 1

  sorted_nodes = sorted_nodes[::-1]
  check_toposort(sorted_nodes, parents)
  return sorted_nodes

def remove_duplicates(lst):
  seen = set()
  return [x for x in lst if id(x) not in seen and not seen.add(id(x))]

def check_toposort(nodes: List[Any], parents: Callable[[Any], List[Any]]):
  seen = set()
  for node in nodes:
    assert all(id(parent) in seen for parent in parents(node))
    seen.add(id(node))

Now we can linearize!

y, sin_lin = linearize(sin, 3.)
print(y, sin(3.))
print(sin_lin(1.), cos(3.))
0.1411200080598672 0.1411200080598672
-0.9899924966004454 -0.9899924966004454

To handle linearize-of-jit, we still need to write a partial evaluation rule for xla_call_p. Other than tracer bookkeeping, the main task is to perform partial evaluation of a jaxpr, ‘unzipping’ it into two jaxprs.

There are actually two rules to write: one for trace-time partial evaluation, which we’ll call xla_call_partial_eval, and one for partial evaluation of jaxprs, which we’ll call xla_call_peval_eqn.

def xla_call_partial_eval(trace, tracers, *, jaxpr, num_consts):
  del num_consts  # Unused.
  in_unknowns = [not t.pval.is_known for t in tracers]
  jaxpr1, jaxpr2, out_unknowns, num_res = partial_eval_jaxpr(jaxpr, in_unknowns)
  known_tracers, unknown_tracers = partition_list(in_unknowns, tracers)
  known_vals = [t.pval.const for t in known_tracers]
  outs1_res = bind(xla_call_p, *known_vals, jaxpr=jaxpr1, num_consts=0)
  outs1, res = split_list(outs1_res, len(jaxpr1.outs) - num_res)
  res_tracers = [trace.instantiate_const(full_raise(trace, x)) for x in res]
  outs2 = [PartialEvalTracer(trace, PartialVal.unknown(v.aval), None)
           for v in jaxpr2.outs]
  eqn = JaxprEqnRecipe(xla_call_p, res_tracers + unknown_tracers,
                       dict(jaxpr=jaxpr2, num_consts=0),
                       [v.aval for v in jaxpr2.outs], map(ref, outs2))
  for t in outs2: t.recipe = eqn
  return merge_lists(out_unknowns, outs1, outs2)
partial_eval_rules[xla_call_p] = xla_call_partial_eval

def partial_eval_jaxpr(jaxpr: Jaxpr, in_unknowns: List[bool],
                       instantiate: Optional[List[bool]] = None,
                       ) -> Tuple[Jaxpr, Jaxpr, List[bool], int]:
  env: Dict[Var, bool] = {}
  residuals: Set[Var] = set()

  def read(v: Atom) -> bool:
    return type(v) is Var and env[v]

  def write(unk: bool, v: Var) -> None:
    env[v] = unk

  def new_res(x: Atom) -> Atom:
    if type(x) is Var: residuals.add(x)
    return x

  eqns1, eqns2 = [], []
  map(write, in_unknowns, jaxpr.in_binders)
  for eqn in jaxpr.eqns:
    unks_in = map(read, eqn.inputs)
    rule = partial_eval_jaxpr_rules.get(eqn.primitive)
    if rule:
      eqn1, eqn2, unks_out, res = rule(unks_in, eqn)
      eqns1.append(eqn1); eqns2.append(eqn2); residuals.update(res)
      map(write, unks_out, eqn.out_binders)
    elif any(unks_in):
      inputs = [v if unk else new_res(v) for unk, v in zip(unks_in, eqn.inputs)]
      eqns2.append(JaxprEqn(eqn.primitive, inputs, eqn.params, eqn.out_binders))
      map(partial(write, True), eqn.out_binders)
    else:
      eqns1.append(eqn)
      map(partial(write, False), eqn.out_binders)
  out_unknowns = map(read, jaxpr.outs)
  if instantiate is not None:
    for v, uk, inst in zip(jaxpr.outs, out_unknowns, instantiate):
      if inst and not uk: new_res(v)
    out_unknowns = map(op.or_, out_unknowns, instantiate)

  residuals, num_res = list(residuals), len(residuals)

  ins1, ins2 = partition_list(in_unknowns, jaxpr.in_binders)
  outs1, outs2 = partition_list(out_unknowns, jaxpr.outs)

  jaxpr1 = Jaxpr(ins1, eqns1, outs1 + residuals)
  jaxpr2 = Jaxpr(residuals + ins2, eqns2, outs2)
  typecheck_partial_eval_jaxpr(jaxpr, in_unknowns, out_unknowns, jaxpr1, jaxpr2)

  return jaxpr1, jaxpr2, out_unknowns, num_res

def typecheck_partial_eval_jaxpr(jaxpr, unks_in, unks_out, jaxpr1, jaxpr2):
  jaxprty = typecheck_jaxpr(jaxpr)    # (a1,  a2) -> (b1, b2 )
  jaxpr1ty = typecheck_jaxpr(jaxpr1)  #  a1       -> (b1, res)
  jaxpr2ty = typecheck_jaxpr(jaxpr2)  # (res, a2) -> b2

  a1, a2 = partition_list(unks_in, jaxprty.in_types)
  b1, b2 = partition_list(unks_out, jaxprty.out_types)
  b1_, res = split_list(jaxpr1ty.out_types, len(b1))
  res_, a2_ = split_list(jaxpr2ty.in_types, len(res))
  b2_ = jaxpr2ty.out_types

  if jaxpr1ty.in_types != a1: raise TypeError
  if jaxpr2ty.out_types != b2: raise TypeError
  if b1 != b1_: raise TypeError
  if res != res_: raise TypeError
  if a2 != a2_: raise TypeError
  if b2 != b2_: raise TypeError

partial_eval_jaxpr_rules = {}

def xla_call_peval_eqn(unks_in: List[bool], eqn: JaxprEqn,
                       ) -> Tuple[JaxprEqn, JaxprEqn, List[bool], List[Atom]]:
  jaxpr = eqn.params['jaxpr']
  jaxpr1, jaxpr2, unks_out, num_res = partial_eval_jaxpr(jaxpr, unks_in)
  ins1, ins2 = partition_list(unks_in, eqn.inputs)
  outs1, outs2 = partition_list(unks_out, eqn.out_binders)
  residuals, _ = split_list(jaxpr2.in_binders, num_res)
  eqn1 = JaxprEqn(xla_call_p, ins1, dict(jaxpr=jaxpr1, num_consts=0),
                  outs1 + residuals)
  eqn2 = JaxprEqn(xla_call_p, residuals + ins2,
                  dict(jaxpr=jaxpr2, num_consts=0), outs2)
  return eqn1, eqn2, unks_out, residuals
partial_eval_jaxpr_rules[xla_call_p] = xla_call_peval_eqn

With that, we can compose linearize and jit however we like:

@jit
def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

y, f_lin = linearize(f, 3.)
y_dot = f_lin(1.)
print(y, y_dot)
2.7177599838802657 2.979984993200891
@jit
def f(x):
  y = sin(x) * 2.
  z = g(x, y)
  return z

@jit
def g(x, y):
  return cos(x) + y

y, f_lin = linearize(f, 3.)
y_dot = f_lin(1.)
print(y, y_dot)
-0.7077524804807109 -2.121105001260758

vjp and grad

The vjp transformation works a lot like linearize. Its type signature is analogous:

linearize : (a -> b) -> a -> (b, T a -o T b)
vjp       : (a -> b) -> a -> (b, T b -o T a)

The only difference is that we transpose the linear part of the computation before returning it, so that it goes from type T a -o T b to type T b -o T a. That is, we’ll implement vjp as, essentially,

def vjp(f, x):
  y, f_lin = linearize(f, x)
  f_vjp = lambda y_bar: transpose(f_lin)(y_bar)
  return y, f_vjp

Since we have the linear computation as a jaxpr, not just a Python callable, we can implement the transpose transformation as a jaxpr interpreter.

def vjp_flat(f, *primals_in):
  pvals_in = ([PartialVal.known(x) for x in primals_in] +
              [PartialVal.unknown(vspace(get_aval(x))) for x in primals_in])
  primal_pvals_in, tangent_pvals_in = split_half(pvals_in)
  def f_jvp(*primals_tangents_in):
    primals_out, tangents_out = jvp(f, *split_half(primals_tangents_in))
    return [*primals_out, *tangents_out]
  jaxpr, pvals_out, consts = partial_eval_flat(f_jvp, pvals_in)  # linearize
  primal_pvals, _ = split_half(pvals_out)
  assert all(pval.is_known for pval in primal_pvals)
  primals_out = [pval.const for pval in primal_pvals]
  transpose_inputs = consts + [UndefPrimal(p.aval) for p in tangent_pvals_in]
  f_vjp = lambda *cts: eval_jaxpr_transposed(jaxpr, transpose_inputs, cts)
  return primals_out, f_vjp

def vjp(f, *primals_in):
  primals_in_flat, in_tree = tree_flatten(primals_in)
  f, out_tree = flatten_fun(f, in_tree)
  primals_out_flat, f_vjp_flat = vjp_flat(f, *primals_in_flat)
  primals_out = tree_unflatten(out_tree(), primals_out_flat)

  def f_vjp(*cotangents_out):
    cotangents_out_flat, _ = tree_flatten(cotangents_out)
    cotangents_in_flat = f_vjp_flat(*cotangents_out_flat)
    return tree_unflatten(in_tree, cotangents_in_flat)

  return primals_out, f_vjp

class UndefPrimal(NamedTuple):
  aval: ShapedArray

register_pytree_node(UndefPrimal,
                     lambda u: (u.aval, ()),
                     lambda aval, _: UndefPrimal(aval))

We use UndefPrimal instances to indicate which arguments with respect to which we want to transpose. These arise because in general, being explicit about closed-over values, we want to transpose functions of type a -> b -o c to functions of type a -> c -o b. Even more generally, the inputs with respect to which the function is linear could be scattered through the argument list. So we indicate the linear positions using UndefPrimal. We register UndefPrimal as a pytree node because the pytree mechanism gives a handy way to prune these placeholders out of argument lists.

Next, we can write eval_jaxpr_transposed, along with transpose rules for all primitives which can be linear in at least one argument:

# NB: the analogous function in JAX is called 'backward_pass'
def eval_jaxpr_transposed(jaxpr: Jaxpr, args: List[Any], cotangents: List[Any]
                          ) -> List[Any]:
  primal_env: Dict[Var, Any] = {}
  ct_env: Dict[Var, Any] = {}

  def read_primal(x: Atom) -> Any:
    return primal_env.get(x, UndefPrimal(x.aval)) if type(x) is Var else x.val

  def write_primal(v: Var, val: Any) -> None:
    if type(val) is not UndefPrimal:
      primal_env[v] = val

  def read_cotangent(v: Var) -> Any:
    return ct_env.pop(v, np.zeros(v.aval.shape, v.aval.dtype))

  def write_cotangent(x: Atom, val: Any):
    if type(x) is Var and val is not None:
      ct_env[x] = add(ct_env[x], val) if x in ct_env else val

  map(write_primal, jaxpr.in_binders, args)
  map(write_cotangent, jaxpr.outs, cotangents)
  for eqn in jaxpr.eqns[::-1]:
    primals_in = map(read_primal, eqn.inputs)
    cts_in = map(read_cotangent, eqn.out_binders)
    rule = transpose_rules[eqn.primitive]
    cts_out = rule(cts_in, *primals_in, **eqn.params)
    map(write_cotangent, eqn.inputs, cts_out)

  return [read_cotangent(v) for v, x in zip(jaxpr.in_binders, args)
          if type(x) is UndefPrimal]

transpose_rules = {}
def mul_transpose_rule(cts, x, y):
  z_bar, = cts
  assert (type(x) is UndefPrimal) ^ (type(y) is UndefPrimal)
  return [mul(z_bar, y), None] if type(x) is UndefPrimal else [None, mul(x, z_bar)]
transpose_rules[mul_p] = mul_transpose_rule

def neg_transpose_rule(cts, x):
  ybar, = cts
  assert type(x) is UndefPrimal
  return [neg(ybar)]
transpose_rules[neg_p] = neg_transpose_rule

def add_transpose_rule(cts, x, y):
  z_bar, = cts
  return [z_bar, z_bar]
transpose_rules[add_p] = add_transpose_rule

def xla_call_transpose_rule(cts, *invals, jaxpr, num_consts):
  del num_consts  # Unused.
  undef_primals = [type(x) is UndefPrimal for x in invals]
  transposed_jaxpr, new_consts = transpose_jaxpr(jaxpr, tuple(undef_primals))
  residuals, _ = partition_list(undef_primals, invals)
  outs = bind(xla_call_p, *new_consts, *residuals, *cts,
              jaxpr=transposed_jaxpr, num_consts=len(new_consts))
  outs = iter(outs)
  return [next(outs) if undef else None for undef in undef_primals]
transpose_rules[xla_call_p] = xla_call_transpose_rule

@lru_cache()
def transpose_jaxpr(jaxpr: Jaxpr, undef_primals: Tuple[bool, ...]
                    ) -> Tuple[Jaxpr, List[Any]]:
  avals_in, avals_out = typecheck_jaxpr(jaxpr)
  traceable = partial(eval_jaxpr_transposed, jaxpr)
  args = [UndefPrimal(a) if u else a for a, u in zip(avals_in, undef_primals)]
  trans_jaxpr, consts, _ = make_jaxpr(traceable, tuple(args), tuple(avals_out))
  typecheck_jaxpr(trans_jaxpr)
  return trans_jaxpr, consts

Now that we can linearize and transpose, we can finally write grad:

def grad(f):
  def gradfun(x, *xs):
    y, f_vjp = vjp(f, x, *xs)
    if np.shape(y) != (): raise TypeError
    x_bar, *_ = f_vjp(np.ones(np.shape(y), np.result_type(y)))
    return x_bar
  return gradfun
y, f_vjp = vjp(sin, 3.)
print(f_vjp(1.), cos(3.))
(-0.9899924966004454,) -0.9899924966004454
def f(x):
  y = sin(x) * 2.
  z = - y + x
  return z

print(grad(f)(3.))
2.979984993200891
@jit
def f(x):
  y = x * 2.
  z = g(y)
  return z

@jit
def g(x):
  return cos(x) * 2.

print(grad(f)(3.))
1.1176619927957034

Here’s something of a compositionality stress test:

# from core_test.py fun_with_nested_calls_2
def foo(x):
  @jit
  def bar(y):
    def baz(w):
      q = jit(lambda x: y)(x)
      q = q + jit(lambda: y)()
      q = q + jit(lambda y: w + y)(y)
      q = jit(lambda w: jit(sin)(x) * y)(1.0) + q
      return q
    p, t = jvp(baz, (x + 1.0,), (y,))
    return t + (x * p)
  return bar(x)

def assert_allclose(*vals):
  for v1, v2 in zip(vals[:-1], vals[1:]):
    np.testing.assert_allclose(v1, v2)

ans1 = f(3.)
ans2 = jit(f)(3.)
ans3, _ = jvp(f, (3.,), (5.,))
ans4, _ = jvp(jit(f), (3.,), (5.,))
assert_allclose(ans1, ans2, ans3, ans4)

deriv1 = grad(f)(3.)
deriv2 = grad(jit(f))(3.)
deriv3 = jit(grad(jit(f)))(3.)
_, deriv4 = jvp(f, (3.,), (1.,))
_, deriv5 = jvp(jit(f), (3.,), (1.,))
assert_allclose(deriv1, deriv2, deriv3, deriv4, deriv5)

hess1 = grad(grad(f))(3.)
hess2 = grad(grad(jit(f)))(3.)
hess3 = grad(jit(grad(f)))(3.)
hess4 = jit(grad(grad(f)))(3.)
_, hess5 = jvp(grad(f), (3.,), (1.,))
_, hess6 = jvp(jit(grad(f)), (3.,), (1.,))
_, hess7 = jvp(jit(grad(f)), (3.,), (1.,))
assert_allclose(hess1, hess2, hess3, hess4, hess5, hess6, hess7)

Part 5: the control flow primitives cond

Next we’ll add higher-order primitives for staged-out control flow. These resemble jit from Part 3, another higher-order primitive, but differ in that they are parameterized by multiple callables rather than just one.

Adding cond

We introduce a cond primitive to represent conditional application of one function or another inside a jaxpr. We write the type of cond as Bool -> (a -> b) -> (a -> b) -> a -> b. In words, cond takes a boolean representing the predicate and two functions of equal types. Depending on the value of the predicate, it applies one function or the other to its final argument.

In Python, we represent it as a function which itself takes two functions as arguments. As with jit, the first step is to call make_jaxpr on its callable arguments to turn them into jaxprs:

def cond(pred, true_fn, false_fn, *operands):
  avals_in = [raise_to_shaped(get_aval(x)) for x in operands]
  true_jaxpr, true_consts, out_tree = make_jaxpr(true_fn, *avals_in)
  false_jaxpr, false_consts, out_tree_ = make_jaxpr(false_fn, *avals_in)
  if out_tree != out_tree_: raise TypeError
  true_jaxpr, false_jaxpr = _join_jaxpr_consts(
      true_jaxpr, false_jaxpr, len(true_consts), len(false_consts))
  if typecheck_jaxpr(true_jaxpr) != typecheck_jaxpr(false_jaxpr):
    raise TypeError
  outs = bind_cond(pred, *true_consts, *false_consts, *operands,
                   true_jaxpr=true_jaxpr, false_jaxpr=false_jaxpr)
  return tree_unflatten(out_tree, outs)
cond_p = Primitive('cond')

def _join_jaxpr_consts(jaxpr1: Jaxpr, jaxpr2: Jaxpr, n1: int, n2: int
                       ) -> Tuple[Jaxpr, Jaxpr]:
  jaxpr1_type, jaxpr2_type = typecheck_jaxpr(jaxpr1), typecheck_jaxpr(jaxpr2)
  assert jaxpr1_type.in_types[n1:] == jaxpr2_type.in_types[n2:]
  consts1, rest1 = split_list(jaxpr1.in_binders, n1)
  consts2, rest2 = split_list(jaxpr2.in_binders, n2)
  new_jaxpr1 = Jaxpr(consts1 + consts2 + rest1, jaxpr1.eqns, jaxpr1.outs)
  new_jaxpr2 = Jaxpr(consts1 + consts2 + rest2, jaxpr2.eqns, jaxpr2.outs)
  return new_jaxpr1, new_jaxpr2

def bind_cond(pred, *args, true_jaxpr, false_jaxpr):
  assert len(args) == len(true_jaxpr.in_binders) == len(false_jaxpr.in_binders)
  return bind(cond_p, pred, *args, true_jaxpr=true_jaxpr, false_jaxpr=false_jaxpr)

We require true_jaxpr and false_jaxpr to have the same type, but because they might close over different constants (and because jaxprs can only represent closed terms, i.e. can’t have free variables and are instead closure-converted) we need to use the helper _join_jaxpr_consts to make consistent the input binder lists of the two jaxprs. (To be more economical we could try to identify pairs of constants with the same shapes, but instead we just concatenate the lists of constants.)

Next we can turn to adding interpreter rules for cond. Its evaluation rule is simple:

def cond_impl(pred, *operands, true_jaxpr, false_jaxpr):
  if pred:
    return eval_jaxpr(true_jaxpr, operands)
  else:
    return eval_jaxpr(false_jaxpr, operands)
impl_rules[cond_p] = cond_impl
out = cond(True, lambda: 3, lambda: 4)
print(out)
3

For its JVP and vmap rules, we only need to call the same jvp_jaxpr and vmap_jaxpr utilities we created for jit, followed by another pass of _join_jaxpr_consts:

def cond_jvp_rule(primals, tangents, *, true_jaxpr, false_jaxpr):
  pred, *primals = primals
  _   , *tangents = tangents
  true_jaxpr , true_consts  = jvp_jaxpr(true_jaxpr)
  false_jaxpr, false_consts = jvp_jaxpr(false_jaxpr)
  true_jaxpr, false_jaxpr = _join_jaxpr_consts(
      true_jaxpr, false_jaxpr, len(true_consts), len(false_consts))
  assert typecheck_jaxpr(true_jaxpr) == typecheck_jaxpr(false_jaxpr)
  outs = bind_cond(pred, *true_consts, *false_consts, *primals, *tangents,
                   true_jaxpr=true_jaxpr, false_jaxpr=false_jaxpr)
  primals_out, tangents_out = split_half(outs)
  return primals_out, tangents_out
jvp_rules[cond_p] = cond_jvp_rule
out, out_tan = jvp(lambda x: cond(True, lambda: x * x, lambda: 0.), (1.,), (1.,))
print(out_tan)
2.0
def cond_vmap_rule(axis_size, vals_in, dims_in, *, true_jaxpr, false_jaxpr):
  pred    , *vals_in = vals_in
  pred_dim, *dims_in = dims_in
  if pred_dim is not not_mapped: raise NotImplementedError  # TODO
  true_jaxpr, true_consts = vmap_jaxpr(true_jaxpr, axis_size, tuple(dims_in))
  false_jaxpr, false_consts = vmap_jaxpr(false_jaxpr, axis_size, tuple(dims_in))
  true_jaxpr, false_jaxpr = _join_jaxpr_consts(
      true_jaxpr, false_jaxpr, len(true_consts), len(false_consts))
  assert typecheck_jaxpr(true_jaxpr) == typecheck_jaxpr(false_jaxpr)
  outs = bind_cond(pred, *true_consts, *false_consts, *vals_in,
                   true_jaxpr=true_jaxpr, false_jaxpr=false_jaxpr)
  return outs, [0] * len(outs)
vmap_rules[cond_p] = cond_vmap_rule
xs = np.array([1., 2., 3])
out = vmap(lambda x: cond(True, lambda: x + 1., lambda: 0.), (0,))(xs)
print(out)
[2. 3. 4.]

Notice that we’re not currently supporting the case where the predicate value itself is batched. In mainline JAX, we handle this case by transforming the conditional to a select primitive. That transformation is semantically correct so long as true_fun and false_fun do not involve any side-effecting primitives.

Another thing not represented here, but present in the mainline JAX, is that applying transformations to two jaxprs of equal type might result in jaxprs of different types. For example, applying the mainline JAX version of vmap_jaxpr to the identity-function jaxpr

{ lambda a:float32[] .
  let
  in ( a ) }

would result in a jaxpr with a batched output, of type [float32[10]] -> [float32[10]] if the batch size were 10, while applying it to the zero-function jaxpr

{ lambda a:float32[] .
  let
  in ( 0. ) }

would result in a jaxpr with an unbatched output, of type [float32[10]] -> [float32[]]. This is an optimization, aimed at not batching values unnecessarily. But it means that in cond we’d need an extra step of joining the two transformed jaxprs to have consistent output types. We don’t need this step here because we chose vmap_jaxpr always to batch all outputs over the leading axis.

Next we can turn to abstract evaluation and XLA lowering rules:

def cond_abstract_eval(pred_type, *in_types, true_jaxpr, false_jaxpr):
  if pred_type != ShapedArray((), np.dtype('bool')): raise TypeError
  jaxpr_type = typecheck_jaxpr(true_jaxpr)
  if jaxpr_type != typecheck_jaxpr(false_jaxpr):
    raise TypeError
  if not all(t1 == t2 for t1, t2 in zip(jaxpr_type.in_types, in_types)):
    raise TypeError
  return jaxpr_type.out_types
abstract_eval_rules[cond_p] = cond_abstract_eval

def cond_translation(c, in_avals, in_vals, *, true_jaxpr, false_jaxpr):
  del in_avals  # Unused.
  pred, *in_vals = in_vals
  flat_vals, in_tree = tree_flatten(in_vals)
  operand = xops.Tuple(c, flat_vals)
  operand_shape = c.get_shape(operand)

  def make_comp(name: str, jaxpr: Jaxpr) -> xe.XlaComputation:
    c = xb.make_computation_builder(name)
    operand = xb.parameter(c, 0, operand_shape)
    operands = tree_unflatten(in_tree, destructure_tuple(c, operand))
    outs = jaxpr_subcomp(c, jaxpr, operands)
    return c.build(xops.Tuple(c, outs))

  true_comp = make_comp('true_fn', true_jaxpr)
  false_comp = make_comp('false_fn', false_jaxpr)

  int_etype = xc.dtype_to_etype(np.dtype('int32'))
  out = xops.Conditional(xops.ConvertElementType(pred, int_etype),
                         [false_comp, true_comp], [operand] * 2)
  return destructure_tuple(c, out)
xla_translations[cond_p] = cond_translation
out = jit(lambda: cond(False, lambda: 1, lambda: 2))()
print(out)
2

Finally, to support reverse-mode automatic differentiation, we need partial evaluation and transposition rules. For partial evaluation, we need to introduce another jaxpr-munging utility, _join_jaxpr_res, to handle the fact that applying partial evaluation to true_fun and false_fun will in general result in distinct residuals. We use _join_jaxpr_res to make the output types of the transformed jaxprs consistent (while _join_jaxpr_consts dealt with input types).

def cond_partial_eval(trace, tracers, *, true_jaxpr, false_jaxpr):
  pred_tracer, *tracers = tracers
  assert pred_tracer.pval.is_known
  pred = pred_tracer.pval.const
  in_uks = [not t.pval.is_known for t in tracers]

  *jaxprs, out_uks, num_res = _cond_partial_eval(true_jaxpr, false_jaxpr, in_uks)
  t_jaxpr1, f_jaxpr1, t_jaxpr2, f_jaxpr2 = jaxprs

  known_tracers, unknown_tracers = partition_list(in_uks, tracers)
  known_vals = [t.pval.const for t in known_tracers]
  outs1_res = bind_cond(pred, *known_vals,
                        true_jaxpr=t_jaxpr1, false_jaxpr=f_jaxpr1)
  outs1, res = split_list(outs1_res, len(outs1_res) - num_res)
  pred_tracer_ = trace.instantiate_const(full_raise(trace, pred_tracer))
  res_tracers = [trace.instantiate_const(full_raise(trace, x)) for x in res]
  outs2 = [PartialEvalTracer(trace, PartialVal.unknown(v.aval), None)
           for v in t_jaxpr2.outs]
  eqn = JaxprEqnRecipe(cond_p, [pred_tracer_, *res_tracers, *unknown_tracers],
                       dict(true_jaxpr=t_jaxpr2, false_jaxpr=f_jaxpr2),
                       [v.aval for v in t_jaxpr2.outs], map(ref, outs2))
  for t in outs2: t.recipe = eqn
  return merge_lists(out_uks, outs1, outs2)
partial_eval_rules[cond_p] = cond_partial_eval

def _cond_partial_eval(true_jaxpr: Jaxpr, false_jaxpr: Jaxpr, in_uks: List[bool]
                       ) -> Tuple[Jaxpr, Jaxpr, Jaxpr, Jaxpr, List[bool], int]:
  _, _, t_out_uks, _ = partial_eval_jaxpr(true_jaxpr , in_uks)
  _, _, f_out_uks, _ = partial_eval_jaxpr(false_jaxpr, in_uks)
  out_uks = map(op.or_, t_out_uks, f_out_uks)

  t_jaxpr1, t_jaxpr2, _, t_nres = partial_eval_jaxpr(true_jaxpr , in_uks, out_uks)
  f_jaxpr1, f_jaxpr2, _, f_nres = partial_eval_jaxpr(false_jaxpr, in_uks, out_uks)

  t_jaxpr1, f_jaxpr1 = _join_jaxpr_res(t_jaxpr1, f_jaxpr1, t_nres, f_nres)
  t_jaxpr2, f_jaxpr2 = _join_jaxpr_consts(t_jaxpr2, f_jaxpr2, t_nres, f_nres)
  assert typecheck_jaxpr(t_jaxpr1) == typecheck_jaxpr(f_jaxpr1)
  assert typecheck_jaxpr(t_jaxpr2) == typecheck_jaxpr(f_jaxpr2)
  num_res = t_nres + f_nres

  return t_jaxpr1, f_jaxpr1, t_jaxpr2, f_jaxpr2, out_uks, num_res

def _join_jaxpr_res(jaxpr1: Jaxpr, jaxpr2: Jaxpr, n1: int, n2: int
                    ) -> Tuple[Jaxpr, Jaxpr]:
  jaxpr1_type, jaxpr2_type = typecheck_jaxpr(jaxpr1), typecheck_jaxpr(jaxpr2)
  out_types1, _ = split_list(jaxpr1_type.out_types, len(jaxpr1.outs) - n1)
  out_types2, _ = split_list(jaxpr2_type.out_types, len(jaxpr2.outs) - n2)
  assert out_types1 == out_types2
  outs1, res1 = split_list(jaxpr1.outs, len(jaxpr1.outs) - n1)
  outs2, res2 = split_list(jaxpr2.outs, len(jaxpr2.outs) - n2)
  zeros_like1 = [Lit(np.zeros(v.aval.shape, v.aval.dtype)) for v in res1]
  zeros_like2 = [Lit(np.zeros(v.aval.shape, v.aval.dtype)) for v in res2]
  new_jaxpr1 = Jaxpr(jaxpr1.in_binders, jaxpr1.eqns, outs1 + res1 + zeros_like2)
  new_jaxpr2 = Jaxpr(jaxpr2.in_binders, jaxpr2.eqns, outs2 + zeros_like1 + res2)
  return new_jaxpr1, new_jaxpr2
_, f_lin = linearize(lambda x: cond(True, lambda: x, lambda: 0.), 1.)
out = f_lin(3.14)
print(out)
3.14
def cond_peval_eqn(unks_in: List[bool], eqn: JaxprEqn,
                   ) -> Tuple[JaxprEqn, JaxprEqn, List[bool], List[Atom]]:
  pred_unk, *unks_in = unks_in
  assert not pred_unk
  true_jaxpr, false_jaxpr = eqn.params['true_jaxpr'], eqn.params['false_jaxpr']
  *jaxprs, unks_out, num_res = _cond_partial_eval(true_jaxpr, false_jaxpr, unks_in)
  t_jaxpr1, f_jaxpr1, t_jaxpr2, f_jaxpr2 = jaxprs
  ins1, ins2 = partition_list(unks_in, eqn.inputs[1:])
  outs1, outs2 = partition_list(unks_out, eqn.out_binders)
  residuals, _ = split_list(t_jaxpr2.in_binders, num_res)
  eqn1 = JaxprEqn(cond_p, [eqn.inputs[0], *ins1],
                  dict(true_jaxpr=t_jaxpr1, false_jaxpr=f_jaxpr1),
                  outs1 + residuals)
  eqn2 = JaxprEqn(cond_p, [eqn.inputs[0], *residuals, *ins2],
                  dict(true_jaxpr=t_jaxpr2, false_jaxpr=f_jaxpr2),
                  outs2)
  return eqn1, eqn2, unks_out, [eqn.inputs[0], *residuals]
partial_eval_jaxpr_rules[cond_p] = cond_peval_eqn
_, f_lin = linearize(jit(lambda x: cond(True, lambda: x, lambda: 0.)), 1.)
out = f_lin(3.14)
print(out)
3.14

Transposition is a fairly straightforward application of transpose_jaxpr:

def cond_transpose_rule(cts, pred, *invals, true_jaxpr, false_jaxpr):
  undef_primals = tuple([type(x) is UndefPrimal for x in invals])
  true_jaxpr, true_consts = transpose_jaxpr(true_jaxpr, undef_primals)
  false_jaxpr, false_consts = transpose_jaxpr(false_jaxpr, undef_primals)
  true_jaxpr, false_jaxpr = _join_jaxpr_consts(
      true_jaxpr, false_jaxpr, len(true_consts), len(false_consts))
  res = [x for x in invals if type(x) is not UndefPrimal]
  outs = bind_cond(pred, *true_consts, *false_consts, *res, *cts,
                   true_jaxpr=true_jaxpr, false_jaxpr=false_jaxpr)
  outs = iter(outs)
  return [None] + [next(outs) if type(x) is UndefPrimal else None for x in invals]
transpose_rules[cond_p] = cond_transpose_rule
out = grad(lambda x: cond(True, lambda: x * x, lambda: 0.))(1.)
print(out)
2.0