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configure: Add workaround for ccache and clang 

Test if ccache is interfering with semantic analysis of macros,
disable its habit of trying to compile already pre-processed
versions of code if so. ccache attempts to save time by compiling
pre-processed versions of code, but this disturbs clang's static
analysis enough to produce false positives.

ccache allows us to disable this feature, opting instead to
compile the original version instead of its preprocessed version.
This makes ccache much slower for cache misses, but at least it
becomes usable with QEMU/clang.

This workaround only activates for users using ccache AND clang,
and only if their configuration is observed to be producing warnings.
You may need to clear your ccache for builds started without -Werror,
as those may continue to produce warnings from the cache.

Thanks to Peter Eisentraut for his writeup on the issue:
http://peter.eisentraut.org/blog/2014/12/01/ccache-and-clang-part-3/

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1427324259-1481-5-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
John Snow 2015-03-25 18:57:39 -04:00 committed by Stefan Hajnoczi
parent bbbf2e04e5
commit fd0e60530f
1 changed files with 33 additions and 1 deletions
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34
configure vendored
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@ -103,7 +103,8 @@ update_cxxflags() {
}
compile_object() {
do_cc $QEMU_CFLAGS -c -o $TMPO $TMPC
local_cflags="$1"
do_cc $QEMU_CFLAGS $local_cflags -c -o $TMPO $TMPC
}
compile_prog() {
@ -4209,6 +4210,33 @@ if compile_prog "" "" ; then
getauxval=yes
fi
########################################
# check if ccache is interfering with
# semantic analysis of macros
ccache_cpp2=no
cat > $TMPC << EOF
static const int Z = 1;
#define fn() ({ Z; })
#define TAUT(X) ((X) == Z)
#define PAREN(X, Y) (X == Y)
#define ID(X) (X)
int main(int argc, char *argv[])
{
int x = 0, y = 0;
x = ID(x);
x = fn();
fn();
if (PAREN(x, y)) return 0;
if (TAUT(Z)) return 0;
return 0;
}
EOF
if ! compile_object "-Werror"; then
ccache_cpp2=yes
fi
##########################################
# End of CC checks
# After here, no more $cc or $ld runs
@ -5502,6 +5530,10 @@ if test "$numa" = "yes"; then
echo "CONFIG_NUMA=y" >> $config_host_mak
fi
if test "$ccache_cpp2" = "yes"; then
echo "export CCACHE_CPP2=y" >> $config_host_mak
fi
# build tree in object directory in case the source is not in the current directory
DIRS="tests tests/tcg tests/tcg/cris tests/tcg/lm32 tests/libqos tests/qapi-schema tests/tcg/xtensa tests/qemu-iotests"
DIRS="$DIRS fsdev"