Peter Maydell <peter.mayd...@linaro.org> writes: > QEMU's code relies on left shifts of signed integers always > being defined behaviour with the obvious 2s-complement > semantics. The only way to tell the compiler (and any > associated undefined-behaviour sanitizer) that we require a > C dialect with these semantics is to use the -fwrapv option. > This is a bit of a heavy hammer for the job as it also gives > us guaranteed semantics on integer arithmetic overflow which > in theory we don't require. > > In an ideal world this would allow us to drop the warning > flag -Wno-shift-negative-value, but we must retain this to > avoid spurious warnings on clang versions predating the > fix to https://llvm.org/bugs/show_bug.cgi?id=25552. > > Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> > --- > We agreed before 2.7 release that this was the best long term > approach to our shift issues, since it's now clear that both > clang and gcc do agree that -fwrapv provides the semantics we > want. > > configure | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/configure b/configure > index 331c36f..14efce3 100755 > --- a/configure > +++ b/configure > @@ -389,7 +389,11 @@ sdl2_config="${SDL2_CONFIG-${cross_prefix}sdl2-config}" > ARFLAGS="${ARFLAGS-rv}" > > # default flags for all hosts > -QEMU_CFLAGS="-fno-strict-aliasing -fno-common $QEMU_CFLAGS" > +# We use -fwrapv to tell the compiler that we require a C dialect where > +# left shift of signed integers is well defined and has the expected > +# 2s-complement style results. (Both clang and gcc agree that it > +# provides these semantics.) > +QEMU_CFLAGS="-fno-strict-aliasing -fno-common -fwrapv $QEMU_CFLAGS" > QEMU_CFLAGS="-Wall -Wundef -Wwrite-strings -Wmissing-prototypes $QEMU_CFLAGS" > QEMU_CFLAGS="-Wstrict-prototypes -Wredundant-decls $QEMU_CFLAGS" > QEMU_CFLAGS="-D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE > $QEMU_CFLAGS"
If I remember correctly, we discussed -fno-strict-overflow (which the kernel uses), but in the end opted for the more stringent -fwrapv. Reviewed-by: Markus Armbruster <arm...@redhat.com>
