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bpo-30104: Use -fno-strict-aliasing on clang (#1376) (#1377) · python/cpython@03a7ab7 · GitHub

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bpo-30104: Use -fno-strict-aliasing on clang (#1376) (#1377)
Python/dtoa.c is not compiled correctly with clang 4.0 and optimization level -O2 or higher, because of an aliasing issue on the double/ULong[2] union. Only compile dtoa.c with -fno-strict-aliasing. LLVM bug report: https://bugs.llvm.org//show_bug.cgi?id=31928 (cherry picked from commit 809101f)
1 parent e37d163 commit 03a7ab7

3 files changed

Lines changed: 64 additions & 14 deletions

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‎Makefile.pre.in‎

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -103,6 +103,8 @@ ARFLAGS= @ARFLAGS@
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CFLAGSFORSHARED=@CFLAGSFORSHARED@
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# C flags used for building the interpreter object files
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PY_CORE_CFLAGS= $(PY_CFLAGS) $(PY_CFLAGS_NODIST) $(PY_CPPFLAGS) $(CFLAGSFORSHARED) -DPy_BUILD_CORE
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# Strict or non-strict aliasing flags used to compile dtoa.c, see above
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CFLAGS_ALIASING=@CFLAGS_ALIASING@
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# Machine-dependent subdirectories
@@ -1532,6 +1534,13 @@ config.status: $(srcdir)/configure
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.c.o:
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$(CC) -c $(PY_CORE_CFLAGS) -o $@ $<
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# bpo-30104: dtoa.c uses union to cast double to unsigned long[2]. clang 4.0
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# with -O2 or higher and strict aliasing miscompiles the ratio() function
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# causing rounding issues. Compile dtoa.c using -fno-strict-aliasing on clang.
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# https://bugs.llvm.org//show_bug.cgi?id=31928
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Python/dtoa.o: Python/dtoa.c
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$(CC) -c $(PY_CORE_CFLAGS) $(CFLAGS_ALIASING) -o $@ $<
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# Run reindent on the library
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reindent:
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./$(BUILDPYTHON) $(srcdir)/Tools/scripts/reindent.py -r $(srcdir)/Lib

‎configure‎

Lines changed: 28 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -666,6 +666,7 @@ OTHER_LIBTOOL_OPT
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UNIVERSAL_ARCH_FLAGS
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CFLAGS_NODIST
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BASECFLAGS
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CFLAGS_ALIASING
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OPT
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LLVM_PROF_FOUND
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target_os
@@ -6859,6 +6860,7 @@ esac
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# tweak OPT based on compiler and platform, only if the user didn't set
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# it on the command line
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if test "${OPT-unset}" = "unset"
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then
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case $GCC in
@@ -6871,30 +6873,49 @@ then
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WRAP="-fwrapv"
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fi
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# Clang also needs -fwrapv
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case $CC in
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*clang*) WRAP="-fwrapv"
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;;
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*clang*)
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cc_is_clang=1
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;;
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*)
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if $CC --version 2>&1 | grep -q clang
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then
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cc_is_clang=1
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else
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cc_is_clang=
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fi
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esac
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if test -n "${cc_is_clang}"
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then
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# Clang also needs -fwrapv
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WRAP="-fwrapv"
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# bpo-30104: disable strict aliasing to compile correctly dtoa.c,
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# see Makefile.pre.in for more information
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CFLAGS_ALIASING="-fno-strict-aliasing"
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fi
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case $ac_cv_prog_cc_g in
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yes)
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if test "$Py_DEBUG" = 'true' ; then
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# Optimization messes up debuggers, so turn it off for
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# debug builds.
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if "$CC" -v --help 2>/dev/null |grep -- -Og > /dev/null; then
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OPT="-g -Og -Wall $STRICT_PROTO"
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OPT="-g -Og -Wall"
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else
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OPT="-g -O0 -Wall $STRICT_PROTO"
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OPT="-g -O0 -Wall"
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fi
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else
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OPT="-g $WRAP -O3 -Wall $STRICT_PROTO"
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OPT="-g $WRAP -O3 -Wall"
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fi
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;;
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*)
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OPT="-O3 -Wall $STRICT_PROTO"
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OPT="-O3 -Wall"
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;;
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esac
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OPT="$OPT $STRICT_PROTO"
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case $ac_sys_system in
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SCO_SV*) OPT="$OPT -m486 -DSCO5"
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;;

‎configure.ac‎

Lines changed: 27 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -1403,6 +1403,7 @@ esac
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# tweak OPT based on compiler and platform, only if the user didn't set
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# it on the command line
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AC_SUBST(OPT)
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AC_SUBST(CFLAGS_ALIASING)
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if test "${OPT-unset}" = "unset"
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then
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case $GCC in
@@ -1415,30 +1416,49 @@ then
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WRAP="-fwrapv"
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fi
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1418-
# Clang also needs -fwrapv
14191419
case $CC in
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*clang*) WRAP="-fwrapv"
1421-
;;
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*clang*)
1421+
cc_is_clang=1
1422+
;;
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*)
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if $CC --version 2>&1 | grep -q clang
1425+
then
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cc_is_clang=1
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else
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cc_is_clang=
1429+
fi
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esac
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1432+
if test -n "${cc_is_clang}"
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then
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# Clang also needs -fwrapv
1435+
WRAP="-fwrapv"
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# bpo-30104: disable strict aliasing to compile correctly dtoa.c,
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# see Makefile.pre.in for more information
1438+
CFLAGS_ALIASING="-fno-strict-aliasing"
1439+
fi
1440+
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case $ac_cv_prog_cc_g in
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yes)
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if test "$Py_DEBUG" = 'true' ; then
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# Optimization messes up debuggers, so turn it off for
14281445
# debug builds.
14291446
if "$CC" -v --help 2>/dev/null |grep -- -Og > /dev/null; then
1430-
OPT="-g -Og -Wall $STRICT_PROTO"
1447+
OPT="-g -Og -Wall"
14311448
else
1432-
OPT="-g -O0 -Wall $STRICT_PROTO"
1449+
OPT="-g -O0 -Wall"
14331450
fi
14341451
else
1435-
OPT="-g $WRAP -O3 -Wall $STRICT_PROTO"
1452+
OPT="-g $WRAP -O3 -Wall"
14361453
fi
14371454
;;
14381455
*)
1439-
OPT="-O3 -Wall $STRICT_PROTO"
1456+
OPT="-O3 -Wall"
14401457
;;
14411458
esac
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1460+
OPT="$OPT $STRICT_PROTO"
1461+
14421462
case $ac_sys_system in
14431463
SCO_SV*) OPT="$OPT -m486 -DSCO5"
14441464
;;

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