gh-144490: Fix mimalloc debug build by yoney · Pull Request #144620 · python/cpython · GitHub
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gh-144490: Fix mimalloc debug build - #144620

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vstinner merged 2 commits into
python:mainfrom
yoney:cpp_mimalloc
Feb 10, 2026
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gh-144490: Fix mimalloc debug build#144620
vstinner merged 2 commits into
python:mainfrom
yoney:cpp_mimalloc

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@yoney

@yoney yoney commented Feb 9, 2026 •
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Fix mimalloc C++ compilation error in debug builds.

The _mi_assert_fail declaration uses mi_decl_throw, which expands to __THROW (for __GNUC__ ). In C, __THROW is __attribute__((__nothrow__)) and works anywhere. In C++, __THROW is noexcept(true), which must come after the parameter list.

Before:

  // C:   __attribute__((__nothrow__)) void _mi_assert_fail(...);
  // C++: noexcept(true) void _mi_assert_fail(...);  <- Error, noexcept(...) must be after the params

After:

  // C:   void _mi_assert_fail(...) __attribute__((__nothrow__));
  // C++: void _mi_assert_fail(...) noexcept(true);

This fix also allows removing the Py_DEBUG guard in test_cppext/extension.cpp, so internal C API headers are now tested in debug builds too.

It’s possible this issue was introduced in #122587, though I haven’t verified it

Test:

$ ./configure --with-pydebug --disable-gil
$ make
$ ./python -m test test.test_cppext -u cpu=1

cc: @vstinner @colesbury @DinoV

@bedevere-app bedevere-app Bot mentioned this pull request Feb 9, 2026
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@yoney
yoney marked this pull request as ready for review February 9, 2026 07:07
@kumaraditya303

kumaraditya303 commented Feb 9, 2026

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@yoney

yoney commented Feb 9, 2026

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Does this issue exists in mimalloc upstream as well?

It looks like upstream fixed this earlier. The change is semantically the same: they just moved mi_attr_noexcept to the end. However, the naming differs, our code uses mi_decl_throw, while upstream uses mi_attr_noexcept.

microsoft/mimalloc@715acc0

vstinner
vstinner approved these changes Feb 10, 2026
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LGTM. I confirm that test_cppext.TestInteralCAPI pass on Linux with Python built in debug mode.

@vstinner

vstinner commented Feb 10, 2026

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Note: test_cppext.TestInteralCAPI fails on Windows with Python built in release mode if I remove # if !defined(MS_WINDOWS) in Lib/test/test_cppext/extension.cpp. The change is correct: we should continue skipping the two pycore includes on Windows.

@vstinner
vstinner merged commit 73fa6be into python:main Feb 10, 2026
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@vstinner vstinner added the needs backport to 3.14 bugs and security fixes label Feb 10, 2026
@miss-islington-app

miss-islington-app Bot commented Feb 10, 2026

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Thanks @yoney for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

@miss-islington-app

miss-islington-app Bot commented Feb 10, 2026

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Sorry, @yoney and @vstinner, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 73fa6be2fe6c4a17d91413e12ab6af8376767211 3.14

@vstinner vstinner removed the needs backport to 3.14 bugs and security fixes label Feb 10, 2026
@vstinner

vstinner commented Feb 10, 2026

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Ah, the 3.14 branch is not affected, since the _mi_assert_fail() declaration doesn't use mi_decl_throw:

void _mi_assert_fail(const char* assertion, const char* fname, unsigned int line, const char* func );

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vstinner commented Feb 10, 2026

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thunder-coding pushed a commit to thunder-coding/cpython that referenced this pull request Feb 15, 2026
@yoney @thunder-coding
pythongh-144490: Fix mimalloc debug build for C++ (python#144620)
8df5a84
brijkapadia pushed a commit to brijkapadia/cpython that referenced this pull request Feb 28, 2026
@yoney @brijkapadia
pythongh-144490: Fix mimalloc debug build for C++ (python#144620)
464fd67
ljfp pushed a commit to ljfp/cpython that referenced this pull request Apr 25, 2026
@yoney @ljfp
pythongh-144490: Fix mimalloc debug build for C++ (python#144620)
07a52e2
@yoney
yoney deleted the cpp_mimalloc branch September 11, 2026 04:41
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