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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

> 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