Re: [Qemu-devel] [PATCH] configure: Always compile with -fwrapv

[Peter Maydell <peter.maydell@linaro.org>]

On 12 September 2016 at 18:06, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
>
> On 12/09/2016 15:10, Peter Maydell wrote:
>> 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.maydell@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"
>>
>
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>

Applied to master, thanks.

-- PMM