ponder-lab / Hybridize-Functions-Refactoring / #3467

90%

push

github

Refuse a returned variable or tensor array, or a passed tensor spec, at the tf.function boundary (#1080)

* Refuse values tf.function cannot carry across its boundary

A function was converted when a value crossing its tf.function
boundary would not cross it as written, so the converted function, or
its caller, raised where the eager one ran. Measured on TensorFlow
2.9.3:

- A returned tf.Variable comes back as a tensor holding its value, so a
  caller that assigns to it or compares its ref() fails.
- A returned tf.TensorArray comes back as a tensor, so a caller that
  writes to it fails.
- A tf.TensorSpec passed as an argument makes the call raise ValueError.

A new analysis decides these from the points-to sets, by the type of
the allocation reached: a returned value, or an element of a returned
list, tuple, or dictionary, that is a variable or a tensor array, or a
tensor spec that a call site passes, also inside a container. A
function that has one fails a new precondition,
HAS_UNCROSSABLE_BOUNDARY_VALUES (36). A variable only read in the body
and a tensor array stacked before it is returned are left alone.

Three cases are pinned as not yet detected, with TODOs citing the
engine gaps: a tensor array (wala/ML#1035, modeled from Ariadne
0.52.105), a list built with += (wala/ML#1069), and a Keras weight
from add_weight (wala/ML#1068).

Part of #1075.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NTnpjPjJj2aZy1ZkUD1eAn

* Say the boundary converts a returned variable, not that it cannot carry one

Measured on TensorFlow 2.9.3, a tf.function that returns a tf.Variable
or a tf.TensorArray raises nothing: the boundary converts the value to
a tensor, so the eager object's identity and mutability are lost, and
a caller that assigns to the variable, compares its reference, or
writes to the array fails or means something else. Only a passed
tf.TensorSpec makes the call itself raise. The failure mess... (continued)

75 of 83 new or added lines in 4 files covered. (90.36%)

5136 of 5812 relevant lines covered (88.37%)

0.88 hits per line