Description

markshannon

opened on May 8, 2025

Issue body actions

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

• gh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW #133721
• gh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW #148999

Activity

1. markshannon commented on May 8, 2025

   

   markshannon

   on May 8, 2025

   MemberAuthor

   More actions

   @iritkatriel
   @mpage
2. markshannon

   added

   performancePerformance or resource usagePerformance or resource usage

   3.15bugs and security fixesbugs and security fixes

   on May 8, 2025
3. ljfp commented on May 9, 2025

   

   ljfp

   on May 9, 2025

   Contributor

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   @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?
4. ljfp

   added 2 commits that reference this issue on Apr 25, 2026

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   7beb3fa

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   bf05ce9
5. picnixz

   added

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   type-featureA feature request or enhancementA feature request or enhancement

   on Apr 25, 2026
6. ljfp

   added 4 commits that reference this issue on Apr 25, 2026

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   5fbf452

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   2bcbf29

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   ebd2c01

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   bb9166b
7. ljfp commented on Apr 26, 2026

   

   ljfp

   on Apr 26, 2026

   Contributor

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   @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.
8. ljfp

   added a commit that references this issue on May 6, 2026

   pythongh-133672: Allow LOAD_FAST to be optimized to LOAD_FAST_BORROW

   08238d2

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