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GeezerVison is showing github.com — gh-161821: Fall back to lld for Clang LTO builds by nascheme · Pull Request #161822 · python/cpython

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@nascheme

@nascheme nascheme commented Nov 11, 2026 •
edited by bedevere-app Bot
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Clang LTO with a GNU linker needs the LLVMgold.so plugin, which some distributions (e.g. Debian 13) don't ship. When using Clang with --with-lto on non-macOS platforms, configure now test-links an LTO program and, if that fails, retries with -fuse-ld=lld.

  • Issue: Building with LTO and Clang 20 on Debian fails to link #161821

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@nascheme
pythongh-161821: Fall back to lld for Clang LTO builds
f74b453 
Clang LTO with a GNU linker needs the LLVMgold.so plugin, which some
distributions (e.g. Debian 13) don't ship.  When using Clang with
--with-lto on non-macOS platforms, configure now test-links an LTO
program and, if that fails, retries with -fuse-ld=lld.
@nascheme nascheme added the build The build process and cross-build label Nov 11, 2026
@bedevere-app bedevere-app Bot mentioned this pull request Nov 11, 2026
Building with LTO and Clang 20 on Debian fails to link #161821
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@nascheme nascheme added the OS-linux label Nov 11, 2026
@nascheme
nascheme marked this pull request as ready for review November 11, 2026 21:56
@nascheme
nascheme requested review from AA-Turner, corona10, emmatyping, erlend-aasland and itamaro as code owners November 11, 2026 21:56
@bedevere-app bedevere-app Bot added the awaiting core review label Nov 11, 2026
emmatyping
emmatyping reviewed Nov 11, 2026
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@emmatyping emmatyping left a comment

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A couple of minor nits/suggestions but otherwise looks good!

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Comment thread Doc/using/configure.rst
than macOS, configure tries ``-fuse-ld=lld`` if the configured linker cannot
link a Clang LTO program.

.. versionchanged:: 3.16

@emmatyping emmatyping Nov 11, 2026

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Suggested change
.. versionchanged:: 3.16
.. versionchanged:: next

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Comment thread configure.ac
Comment on lines +2226 to +2227
save_LDFLAGS="$LDFLAGS"
LDFLAGS="$LDFLAGS $LDFLAGS_NODIST $LTOFLAGS"

@emmatyping emmatyping Nov 11, 2026

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I think it would be better to use the SAVE_ENV macro here

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aozmekik
aozmekik reviewed Nov 12, 2026
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Tried this on Debian trixie with clang 19 (trixie has no clang-20). LLVMgold.so comes with llvm-19-linker-tools there, so I removed it to get the failing setup:

  • main: configure passes, then the first LTO link fails with /usr/bin/ld: .../LLVMgold.so: error loading plugin.
  • this PR with lld-19 installed: configure falls back to lld, and python and the extension modules link with LLD 19.1.7. Same with --enable-shared, and test_sysconfig, test_embed and test_ctypes pass. -fuse-ld=lld stays out of sysconfig's LDFLAGS, so third-party extensions aren't affected.
  • without lld: configure stops with the new error message.
  • with LLVMgold.so present: the first check passes and GNU ld is kept.

A full --enable-optimizations --with-lto build works with the lld fallback too.

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Reviewers

@emmatyping emmatyping emmatyping 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
@AA-Turner AA-Turner Awaiting requested review from AA-Turner AA-Turner is a code owner
@itamaro itamaro Awaiting requested review from itamaro itamaro is a code owner
+1 more reviewer
@aozmekik aozmekik aozmekik left review comments
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3 participants

@nascheme @emmatyping @aozmekik
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