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

[Laszlo Ersek <lersek@redhat.com>]

On 11/17/15 11:28, Paolo Bonzini 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.

How about adding "-fwrapv -fno-strict-overflow" right now? (Spelling out
the latter of those explicitly for pointer arithmetic.)

>> GCC is not our only supported compiler; UB is a real thing that
>> compilers in general take advantage of; we should be trying to
>> reduce our reliance on UB, not carving out extra areas where we
>> feel free to use it.
> 
> We are already feeling free to use it.
> 
> Paolo
>