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GeezerVison is showing github.com — [3.14] gh-148284: Block inlining of gigantic functions in ceval.c for clang 22 by Fidget-Spinner · Pull Request #148326 · python/cpython

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@Fidget-Spinner

@Fidget-Spinner Fidget-Spinner commented Apr 10, 2026 •
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It seems that on clang-22, the inliner is too aggressive on _PyEval_EvalFrameDefault when on computed goto interpreter. Together with some strange interaction with the stackref buffer, the function requires 40kB of stack space (!!!) versus the usual 1-2kB normally used.

This sets the inline limit to functions of max 512B stack space (1/4th of normal) allowed to be inlined in ceval.c. I checked the dissasembly and the new function uses about 2kB of stack.

This will need a forward-port to 3.15 too.

  • Issue: python -m test -v test_call segfault with clang 22.1.3 build #148284

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@Fidget-Spinner
Block inlining of gigantic functions
012b8a9
@Fidget-Spinner
Fidget-Spinner requested review from corona10 and erlend-aasland as code owners April 10, 2026 08:42
@bedevere-app bedevere-app Bot added the awaiting core review label Apr 10, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Apr 10, 2026
python -m test -v test_call segfault with clang 22.1.3 build #148284
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@Fidget-Spinner Fidget-Spinner changed the title [3.14] gh-148284: Block inlining of gigantic functions in ceval.c [3.14] gh-148284: Block inlining of gigantic functions in ceval.c for clang 22 Apr 10, 2026
blurb-it Bot and others added 3 commits April 10, 2026 09:21
@blurb-it
📜🤖 Added by blurb_it.
144522b
@Fidget-Spinner
remove unused function
9e14d47
@Fidget-Spinner
Merge branch 'block_inlining_of_gigantic_functions' of github.com:Fid…
ce5594f 
…get-Spinner/cpython into block_inlining_of_gigantic_functions
@Fidget-Spinner
Fidget-Spinner requested a review from vstinner April 10, 2026 12:14
@vstinner

vstinner commented Apr 10, 2026

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This will need a forward-port to 3.15 too.

If possible, I would prefer to fix the main branch first, and then backport to 3.14.
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@Fidget-Spinner

Fidget-Spinner commented Apr 10, 2026

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If possible, I would prefer to fix the main branch first, and then backport to 3.14.

OK, I need to test on main first to see if there're issues there.
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vstinner
vstinner reviewed Apr 10, 2026
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Comment thread configure.ac
else
CFLAGS_CEVAL=""
fi
AC_SUBST([CFLAGS_CEVAL])

@vstinner vstinner Apr 10, 2026

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Can you document this new variable in https://docs.python.org/dev/using/configure.html#envvar-CFLAGS_ALIASING ?

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@Fidget-Spinner Fidget-Spinner Apr 10, 2026

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Is there a way to make this a private variable? I don't feel good exposing it for now.

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Comment thread configure.ac
Comment on lines +7250 to +7253
// See gh-148284:
// Clang 22 seems to have interactions with inlining and the stackref buffer
// which cause 40kB of stack usage on x86-64 in buggy versions of _PyEval_EvalFrameDefault
// in computed goto interpreter. The normal usage seen is normally 1-2kB.

@vstinner vstinner Apr 10, 2026

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Reformat to 80 columns and minor cleanup:

Suggested change
// See gh-148284:
// Clang 22 seems to have interactions with inlining and the stackref buffer
// which cause 40kB of stack usage on x86-64 in buggy versions of _PyEval_EvalFrameDefault
// in computed goto interpreter. The normal usage seen is normally 1-2kB.
// See gh-148284: Clang 22 seems to have interactions with inlining
// and the stackref buffer which cause 40 kB of stack usage on x86-64
// in buggy versions of _PyEval_EvalFrameDefault() in computed goto
// interpreter. The normal usage seen is normally 1-2 kB.

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Comment thread configure.ac
Comment on lines +7267 to +7268
// This number should be tuned to follow the C stack consumption
// in _PyEval_EvalFrameDefault on computed goto interpreter.

@vstinner vstinner Apr 10, 2026

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I suggest adding "Suppress inlining of functions whose stack size exceeds 512 bytes." to explain the purpose of this option:

Suggested change
// This number should be tuned to follow the C stack consumption
// in _PyEval_EvalFrameDefault on computed goto interpreter.
// gh-148284: Suppress inlining of functions whose stack size exceeds
// 512 bytes. This number should be tuned to follow the C stack
// consumption in _PyEval_EvalFrameDefault() on computed goto
// interpreter.

The Clang option is documented at: https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-finline-max-stacksize

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@vstinner

vstinner commented Apr 10, 2026

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In the past, when I tried to reduce the stack memory consumption on Python function calls, I used Py_NO_INLINE with success. But it seems harder to annotate functions used by ceval.c with Py_NO_INLINE, instead of the generic -finline-max-stacksize=512. I'm just a little bit worried that a similar issue occurs with other compilers (GCC, MSVC).

In ceval.c, I see that get_exception_handler() is already annotated with Py_NO_INLINE.
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@Fidget-Spinner

Fidget-Spinner commented Apr 10, 2026

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@vstinner PR moved to retarget main at #148334
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@Fidget-Spinner Fidget-Spinner closed this Apr 10, 2026
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@vstinner vstinner vstinner left review comments
@erlend-aasland erlend-aasland Awaiting requested review from erlend-aasland erlend-aasland is a code owner
@corona10 corona10 Awaiting requested review from corona10 corona10 is a code owner

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@Fidget-Spinner @vstinner
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