jax.lax.while_loop

jax.lax.while_loop(cond_fun, body_fun, init_val)

Call body_fun repeatedly in a loop while cond_fun is True.

The type signature in brief is

while_loop :: (a -> Bool) -> (a -> a) -> a -> a

The semantics of while_loop are given by this Python implementation:

def while_loop(cond_fun, body_fun, init_val):
  val = init_val
  while cond_fun(val):
    val = body_fun(val)
  return val

Unlike that Python version, while_loop is a JAX primitive and is lowered to a single XLA While HLO. That makes it useful for reducing compilation times for jit-compiled functions, since native Python loop constructs in an @jit function are unrolled, leading to large XLA computations.

Also unlike the Python analogue, the loop-carried value val must hold a fixed shape and dtype across all iterations (and not just be consistent up to NumPy rank/shape broadcasting and dtype promotion rules, for example). In other words, the type a in the type signature above represents an array with a fixed shape and dtype (or a nested tuple/list/dict container data structure with a fixed structure and arrays with fixed shape and dtype at the leaves).

Another difference from using Python-native loop constructs is that while_loop is not reverse-mode differentiable because XLA computations require static bounds on memory requirements.

Parameters

• cond_fun (Callable[[~T], bool]) – function of type a -> Bool.

• body_fun (Callable[[~T], ~T]) – function of type a -> a.

• init_val (~T) – value of type a, a type that can be a scalar, array, or any pytree (nested Python tuple/list/dict) thereof, representing the initial loop carry value.

Return type

~T

Returns

The output from the final iteration of body_fun, of type a.