Re: [Qemu-devel] [PATCH for 2.5] QEMU does not care about left shifts of signed negative values

[Peter Maydell]

On 17 November 2015 at 10:28, Paolo Bonzini <address@hidden> wrote:
> On 17/11/2015 11:19, Peter Maydell wrote:
>> I think we should only take this patch if you can get a cast-iron
>> guarantee from both clang and gcc that they will never use this
>> UB to drive optimizations. As you say gcc already say this more or
>> less, but clang doesn't, and if they're warning about it that to
>> me suggests that they will feel freer to rely on the UB in future.
>
> If and when this happens we will add "-fno-strict-overflow" for clang,
> just like we are using "-fno-strict-aliasing" already.

-fno-strict-overflow in clang is AFAICT just an alias for -fwrapv.
These options control handling of signed overflow of addition,
subtraction and multiplication; there is nothing I can find in the
gcc or clang docs that suggests they have any effect on shift operations.

thanks
-- PMM