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gh-96761: Fix lto-build for clang #96762

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gh-96761: Fix lto-build for clang #96762

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aee5500
Fix lto-build for clang
matthiasgoergens Sep 12, 2022
9bd6dc3
Run with approved autoconf
matthiasgoergens Sep 12, 2022
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matthiasgoergens Sep 12, 2022
2861b12
Update configure.ac
matthiasgoergens Sep 14, 2022
8953bf7
Update configure.ac
matthiasgoergens Sep 17, 2022
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4 changes: 2 additions & 2 deletions configure
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7 changes: 4 additions & 3 deletions configure.ac
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Original file line number Diff line number Diff line change
Expand Up @@ -1774,7 +1774,7 @@ then
fi
AC_MSG_RESULT($PROFILE_TASK)

# Make llvm-relatec checks work on systems where llvm tools are not installed with their
# Make llvm-related checks work on systems where llvm tools are not installed with their
# normal names in the default $PATH (ie: Ubuntu). They exist under the
# non-suffixed name in their versioned llvm directory.

Expand Down Expand Up @@ -1828,8 +1828,9 @@ esac
if test "$Py_LTO" = 'true' ; then
case $CC in
*clang*)
dnl flag to disable lto during linking
LDFLAGS_NOLTO="-fno-lto"
dnl For clang, it seems our LTO policy for bootstrap and freezing needs
dnl to match the rest of the build.
LDFLAGS_NOLTO="-flto=${Py_LTO_POLICY}"
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LDFLAGS_NOLTO should be linker flags without LTO, hence the name no lto.

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@matthiasgoergens matthiasgoergens Sep 12, 2022 •
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I guess I'll need to make a bigger change, that changes the name to something more descriptive.

# LDFLAGS_NOLTO is an extra flag to disable lto. It is used to speed up building
# of _bootstrap_python and _freeze_module tools, which don't need LTO.

For clang that seems to work better when they are the same as for the rest of the build. The thin-lto build is already plenty fast (and even then, a slow build is better than a broken one).

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clang does not support linking with objects that have LTO information? It works fine for GCC. Could you please update the comment on line 1831?

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I am not quite sure what the exact rootcause is. I just noticed that when I tried to build some things with LTO and some things (like _bootstrap_python) without LTO, I get an error from the linker via clang.

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I updated the comment.

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Does thin LTO linking work with object files that are built with full LTO? This would speed up LTO builds a bit.

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IIUC, at this moment -flto=${Py_LTO_POLICY} could be -flto=default so it's wrong.

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Hmm, let me think about how to fix this. Or do you have a suggestion?

AC_SUBST(LLVM_AR)
AC_PATH_TOOL(LLVM_AR, llvm-ar, '', ${llvm_path})
AC_SUBST(LLVM_AR_FOUND)
Expand Down