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gh-101525: Check whether to skip functions with computed gotos during BOLT runs #140250

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gh-101525: Check whether to skip functions with computed gotos during BOLT runs #140250

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gh-101525: Check whether to skip functions with computed gotos during…
stratakis Oct 16, 2025
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fixup! gh-101525: Check whether to skip functions with computed gotos…
stratakis Dec 17, 2025
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stratakis Dec 17, 2025
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gh-101525: Check whether to skip functions with computed gotos during… 
… BOLT runs

With LLVM >= 21.1.0 BOLT can handle computed gotos in PIC compiled code.

However depending on the combination of --with-lto, --enable-shared,
--enable-optmizations and using packed rellocations on the linker
build configs might still need to skip functions containing computed
gotos.
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stratakis committed Nov 20, 2025
commit 33effce5e500241772fe3f815bdd62df8ba99554
94 changes: 93 additions & 1 deletion configure
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105 changes: 99 additions & 6 deletions configure.ac
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Original file line number Diff line number Diff line change
Expand Up @@ -2137,6 +2137,34 @@ if test "$Py_BOLT" = 'true' ; then
AC_MSG_ERROR([llvm-bolt is required for a --enable-bolt build but could not be found.])
fi

# Check for packed relocations (-Wl,-z,pack-relative-relocs) which use DT_RELR format.
# BOLT has issues analyzing these compact relocations.
AC_MSG_CHECKING([for packed relocations in LDFLAGS])
has_packed_relocs="no"
case "$LDFLAGS $LDFLAGS_NODIST" in
*-Wl,-z,pack-relative-relocs*)
has_packed_relocs="yes"
;;
esac
AC_MSG_RESULT([$has_packed_relocs])

# Check BOLT version to determine if we need to skip functions with computed gotos.
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AC_MSG_CHECKING([llvm-bolt version])
llvm_bolt_version=$("${LLVM_BOLT}" --version 2>/dev/null | grep -oE '[[0-9]]+\.[[0-9]]+\.[[0-9]]+' | head -1)
AC_MSG_RESULT([${llvm_bolt_version}])

# Parse version
llvm_bolt_major=$(echo "${llvm_bolt_version}" | cut -d. -f1)
llvm_bolt_minor=$(echo "${llvm_bolt_version}" | cut -d. -f2)

bolt_need_skip_computed_goto="yes"
if test -n "${llvm_bolt_major}" && test "${llvm_bolt_major}" -ge 21; then
if test "${llvm_bolt_major}" -gt 21 || test "${llvm_bolt_minor}" -ge 1; then
bolt_need_skip_computed_goto="no"
AC_MSG_RESULT([LLVM ${llvm_bolt_version} supports computed gotos])
fi
fi

AC_SUBST([MERGE_FDATA])
AC_PATH_TOOL([MERGE_FDATA], [merge-fdata], [''], [${llvm_path}])
if test -n "${MERGE_FDATA}" -a -x "${MERGE_FDATA}"
Expand Down Expand Up @@ -2166,15 +2194,80 @@ then
[BOLT_COMMON_FLAGS],
[m4_normalize("
[-update-debug-sections]

dnl At least LLVM 19.x doesn't support computed gotos in PIC compiled code.
dnl Exclude functions containing computed gotos.
dnl TODO this may be fixed in LLVM 20.x via https://github.com/llvm/llvm-project/pull/120267.
dnl GCC's LTO creates .lto_priv.0 clones of these functions.
[-skip-funcs=_PyEval_EvalFrameDefault,sre_ucs1_match/1,sre_ucs2_match/1,sre_ucs4_match/1,sre_ucs1_match.lto_priv.0/1,sre_ucs2_match.lto_priv.0/1,sre_ucs4_match.lto_priv.0/1]
")]
)

dnl BOLT versions before LLVM 21.1.0 don't support computed gotos in PIC compiled code.
dnl Exclude functions containing computed gotos for older versions.
dnl Fixed in LLVM 21.1.0+ via https://github.com/llvm/llvm-project/pull/120267
if test "${bolt_need_skip_computed_goto}" = "yes"; then
dnl Skip base computed goto functions
BOLT_COMMON_FLAGS="${BOLT_COMMON_FLAGS} -skip-funcs=_PyEval_EvalFrameDefault,sre_ucs1_match/1,sre_ucs2_match/1,sre_ucs4_match/1"
dnl GCC's LTO creates .lto_priv.0 clones that also need to be skipped
dnl Skip if packed relocs not present
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if test "${has_packed_relocs}" != "yes" && test "${ac_cv_cc_name}" = "gcc" && test "${with_lto}" != "no" && test -n "${with_lto}"; then
BOLT_COMMON_FLAGS="${BOLT_COMMON_FLAGS},sre_ucs1_match.lto_priv.0/1,sre_ucs2_match.lto_priv.0/1,sre_ucs4_match.lto_priv.0/1"
fi
fi

dnl When packed relocations are used, BOLT cannot properly
dnl analyze the DT_RELR format. Build skip list for functions that fail with packed relocs.
if test "${has_packed_relocs}" = "yes"; then
bolt_base="_PyEval_EvalFrameDefault,sre_ucs1_match/1,sre_ucs2_match/1,sre_ucs4_match/1"
bolt_sre_lto="sre_ucs1_match.lto_priv.0/1,sre_ucs2_match.lto_priv.0/1,sre_ucs4_match.lto_priv.0/1"
bolt_all_lto="_PyEval_EvalFrameDefault.lto_priv.0/1,${bolt_sre_lto}"

case "${ac_cv_cc_name}" in
gcc)
dnl GCC's LTO creates .lto_priv.0 variants; PGO creates .localalias variants (in shared builds)
bolt_skip_list=""

dnl Determine what to include based on build configuration
bolt_need_base=$( (test "${bolt_need_skip_computed_goto}" != "yes" && test "${enable_shared}" = "yes") && echo "yes" || echo "no")
bolt_need_lto=$( (test "${with_lto}" != "no" && test -n "${with_lto}") && echo "yes" || echo "no")
bolt_need_pgo=$( (test "${enable_optimizations}" = "yes" && test "${enable_shared}" = "yes") && echo "yes" || echo "no")

dnl Build the skip list
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test "${bolt_need_base}" = "yes" && bolt_skip_list="${bolt_base}"
if test "${bolt_need_lto}" = "yes"; then
if test "${enable_shared}" = "yes"; then
test -n "${bolt_skip_list}" && bolt_skip_list="${bolt_skip_list},"
bolt_skip_list="${bolt_skip_list}${bolt_all_lto}"
elif test "${bolt_need_skip_computed_goto}" != "yes"; then
bolt_skip_list="${bolt_all_lto}"
else
dnl BOLT < 21.1.0 + static: need to add LTO variants
test -n "${bolt_skip_list}" && bolt_skip_list="${bolt_skip_list},"
bolt_skip_list="${bolt_skip_list}${bolt_all_lto}"
fi
fi
if test "${bolt_need_pgo}" = "yes"; then
test -n "${bolt_skip_list}" && bolt_skip_list="${bolt_skip_list},"
if test "${bolt_need_lto}" = "yes"; then
bolt_skip_list="${bolt_skip_list}_PyEval_EvalFrameDefault.localalias.lto_priv.0/1"
else
bolt_skip_list="${bolt_skip_list}_PyEval_EvalFrameDefault.localalias/1"
fi
fi

dnl Apply the skip list
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if test -n "${bolt_skip_list}"; then
if test "${bolt_need_skip_computed_goto}" = "yes"; then
BOLT_COMMON_FLAGS="${BOLT_COMMON_FLAGS},${bolt_skip_list}"
else
BOLT_COMMON_FLAGS="${BOLT_COMMON_FLAGS} -skip-funcs=${bolt_skip_list}"
fi
fi
;;
clang*)
if test "${bolt_need_skip_computed_goto}" != "yes"; then
BOLT_COMMON_FLAGS="${BOLT_COMMON_FLAGS} -skip-funcs=${bolt_base}"
fi
;;
esac
fi
fi
AC_MSG_RESULT([$BOLT_COMMON_FLAGS])

AC_ARG_VAR(
[BOLT_INSTRUMENT_FLAGS],
Expand Down
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