Re: [Qemu-devel] [PULL 0/9] Misc patches for QEMU 2.5-rc2 (2015-11-25)

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/11/2015 11:56, Peter Maydell wrote:
> On 26 November 2015 at 10:40, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 26/11/2015 10:46, Peter Maydell wrote:
>>> I definitely don't think we should apply the -fwrapv patch yet;
>>> would you mind respinning this pullrequest without it?
>>
>> In what way does that patch make that thing worse?
> 
> It makes a claim about the semantics that the compiler
> guarantees us which isn't currently valid. (Or
> alternatively, it's implicitly claiming that clang isn't
> a supported compiler, which isn't true.) I don't think
> we should document or rely on signed-shift semantics

But we are relying on them, and thus we should document them.  Witness
the number of patches fixing so called "undefined" behavior.  And those
patches are _dangerous_.

I can certainly remove the "as documented by the GCC manual" part and
the -fwrapv setting, but silencing -Wshift-negative-value and
documenting what we rely on should go in.

Paolo

> until we have the relevant documented promises from the
> compiler developers that that is what they are providing.
> (I'm happy that the gcc folks have provided those promises, they
> just need to actually document them in the -fwrapv option
> docs. The clang folks haven't replied yet so we don't know.)