Re: [Qemu-devel] [PATCH 3/3] disas/arm: avoid clang shifting negative signed warning

[Peter Maydell]

On 10 November 2015 at 17:33, Paolo Bonzini <address@hidden> wrote:
>
>
> On 10/11/2015 16:57, Stefan Hajnoczi wrote:
>> clang 3.7.0 on x86_64 warns about the following:
>>
>>   disas/arm.c:1782:17: warning: shifting a negative signed value is 
>> undefined [-Wshift-negative-value]
>>     imm |= (-1 << 7);
>>             ~~ ^
>>
>> Note that this patch preserves the tab indent in this source file
>> because the surrounding code still uses tabs.
>>
>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>
> I would like to know a case where (except with ubsan) clang actually
> uses the optimization.
>
> If not, this is just error message theatre (which is not news for clang)
> and shouldn't have been part of -Wall.

It could be they're attempting to warn us now about the possibility
that in a future version of clang they will start using this UB
to optimize with.

http://stackoverflow.com/questions/22883790/left-shift-of-negative-values
reports that Intel's ICC will use this in dead-code-elimination
optimization. One day clang might do that too.

thanks
-- PMM