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work around constant folding bug 61144 in gcc 4.9.0 and 4.9.1

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previously we detected this bug in configure and issued advice for a
workaround, but this turned out not to work. since then gcc 4.9.0 has
appeared in several distributions, and now 4.9.1 has been released
without a fix despite this being a wrong code generation bug which is
supposed to be a release-blocker, per gcc policy.

since the scope of the bug seems to affect only data objects (rather
than functions) whose definitions are overridable, and there are only
a very small number of these in musl, I am just changing them from
const to volatile for the time being. simply removing the const would
be sufficient to make gcc 4.9.1 work (the non-const case was
inadvertently fixed as part of another change in gcc), and this would
also be sufficient with 4.9.0 if we forced -O0 on the affected files
or on the whole build. however it's cleaner to just remove all the
broken compiler detection and use volatile, which will ensure that
they are never constant-folded. the quality of a non-broken compiler's
output should not be affected except for the fact that these objects
are no longer const and thus possibly add a few bytes to data/bss.

this change can be reconsidered and possibly reverted at some point in
the future when the broken gcc versions are no longer relevant.
This commit is contained in:
Rich Felker 2014-07-16 21:32:06 -04:00
commit a6adb2bcd8
8 changed files with 9 additions and 30 deletions
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21
configure vendored
View file

@ -495,27 +495,6 @@ printf "no\n"
fail "$0: error: unsupported long double type"
fi
#
# Check for known bug in GCC 4.9.0 that results in a broken libc.
#
if test "$cc_is_gcc" = yes ; then
printf "checking for gcc constant folding bug with weak aliases... "
echo 'static int x = 0;' > "$tmpc"
echo 'extern int y __attribute__((__weak__, __alias__("x")));' >> "$tmpc"
echo 'extern int should_appear;' >> "$tmpc"
echo 'int foo() { return y ? should_appear : 0; }' >> "$tmpc"
case "$($CC $CFLAGS_C99FSE -I./arch/$ARCH -I./include \
$CPPFLAGS $CFLAGS_AUTO $CFLAGS -S -o - "$tmpc" 2>/dev/null)" in
*should_appear*)
printf "no\n"
;;
*)
printf "yes\n"
fail "$0: error: broken compiler; try CFLAGS=-fno-toplevel-reorder"
;;
esac
fi
printf "creating config.mak... "
cmdline=$(quote "$0")