LOAD_FAST_BORROW not being used even when safe to do so, if value is live at BB end.

This function

def f(x, y, c):
    return 1 + (x if c else y)

compiles to

  1           RESUME                   0

  2           LOAD_SMALL_INT           1
              LOAD_FAST_BORROW         2 (c)
              TO_BOOL
              POP_JUMP_IF_FALSE        9 (to L1)
              NOT_TAKEN
              LOAD_FAST_BORROW         0 (x)
              BINARY_OP                0 (+)
              RETURN_VALUE
      L1:     LOAD_FAST                1 (y)
              BINARY_OP                0 (+)
              RETURN_VALUE

Note that the load of y uses LOAD_FAST even though LOAD_FAST_BORROW is safe.

This becomes important with virtual iterators as the iterable for the loop is live at BB end.

Linked PRs

@iritkatriel

@iritkatriel
@mpage

@markshannon

@markshannon I've created this PR to modify the optimization condition in Python/flowgraph.c. The goal is to allow LOAD_FAST to be optimized to LOAD_FAST_BORROW even when the reference is still on the stack at the end of a basic block (i.e., the REF_UNCONSUMED flag is set).

If I got this right, this should be safe as long as the reference is not killed (SUPPORT_KILLED flag isn't set) and not stored as a local (STORED_AS_LOCAL flag isn't set).

With these changes, y in your original example now correctly uses LOAD_FAST_BORROW:

Python 3.15.0a0 (heads/fix-issue-133672:f038e7f472e, May  9 2025, 01:25:23) [GCC 15.1.1 20250425] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def f(x, y, c):
...     return 1 + (x if c else y)
... 
>>> import dis
>>> dis.dis(f)
  1           RESUME                   0

  2           LOAD_SMALL_INT           1
              LOAD_FAST_BORROW         2 (c)
              TO_BOOL
              POP_JUMP_IF_FALSE        9 (to L1)
              NOT_TAKEN
              LOAD_FAST_BORROW         0 (x)
              BINARY_OP                0 (+)
              RETURN_VALUE
      L1:     LOAD_FAST_BORROW         1 (y)
              BINARY_OP                0 (+)
              RETURN_VALUE

Could you take a look when you have a moment and confirm if this aligns with what you had in mind?

pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

@markshannon

@markshannon I opened a new PR for this in #148999.

I made a few mistakes with the previous PR, so I cleaned up the branch history and opened a replacement PR instead. The new PR should also address the failing tests from the earlier version.

pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

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`LOAD_FAST_BORROW` not being used even when safe to do so, if value is live at BB end. #133672

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LOAD_FAST_BORROW not being used even when safe to do so, if value is live at BB end. #133672

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markshannon commented on May 8, 2025

ljfp commented on May 9, 2025

ljfp commented on Apr 26, 2026

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`LOAD_FAST_BORROW` not being used even when safe to do so, if value is live at BB end. #133672