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gh-161821: Fall back to lld for Clang LTO builds #161822

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gh-161821: Fall back to lld for Clang LTO builds #161822

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9 changes: 8 additions & 1 deletion Doc/using/configure.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -739,7 +739,14 @@ also be used to improve performance.
Enable Link Time Optimization (LTO) in any build (disabled by default).

The C compiler Clang requires ``llvm-ar`` for LTO (``ar`` on macOS), as well
as an LTO-aware linker (``ld.gold`` or ``lld``).
as an LTO-aware linker (``ld.gold`` or ``lld``). GNU linkers require the
``LLVMgold.so`` plugin for Clang LTO; ``lld`` does not. On platforms other
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

Fall back to ``lld`` when the configured linker cannot link Clang LTO
programs.

.. versionadded:: 3.6

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4 changes: 4 additions & 0 deletions Misc/NEWS.d/next/Build/2026-11-11-11-36-01.gh-issue-161821.9wjElMQP.rst
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@@ -0,0 +1,4 @@
When building with ``--with-lto`` and Clang, :program:`configure` now checks
that the linker can link LTO objects and falls back to ``lld``
(``-fuse-ld=lld``) if it cannot. This fixes LTO builds on systems where the
default GNU linker lacks the ``LLVMgold.so`` plugin, such as Debian 13.
62 changes: 62 additions & 0 deletions configure
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24 changes: 24 additions & 0 deletions configure.ac
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Expand Up @@ -2220,6 +2220,30 @@ if test "$Py_LTO" = 'true' ; then
else
LTOFLAGS="-flto=${Py_LTO_POLICY}"
fi
LTOCFLAGS="$LTOFLAGS"
# GNU ld needs LLVMgold.so for Clang LTO. If the configured linker
# cannot link LTO objects, try lld, which does not need that plugin.
save_LDFLAGS="$LDFLAGS"
LDFLAGS="$LDFLAGS $LDFLAGS_NODIST $LTOFLAGS"
Comment on lines +2226 to +2227

@emmatyping emmatyping Nov 11, 2026

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

AC_MSG_CHECKING([whether Clang LTO works with the configured linker])
AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])], [clang_lto_link=yes], [clang_lto_link=no])
AC_MSG_RESULT([$clang_lto_link])
if test "$clang_lto_link" = no; then
LDFLAGS="$LDFLAGS -fuse-ld=lld"
AC_MSG_CHECKING([whether Clang LTO works with lld])
AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])], [clang_lto_link=yes], [clang_lto_link=no])
AC_MSG_RESULT([$clang_lto_link])
if test "$clang_lto_link" = no; then
AC_MSG_ERROR([m4_normalize([
Clang LTO requires a working LTO-aware linker.
Install lld or the LLVMgold.so plugin, select a linker
with LDFLAGS, or disable LTO with --without-lto.
])])
fi
LTOFLAGS="$LTOFLAGS -fuse-ld=lld"
LDFLAGS_NOLTO="$LDFLAGS_NOLTO -fuse-ld=lld"
fi
LDFLAGS="$save_LDFLAGS"
;;
esac
;;
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