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

[Peter Maydell <peter.maydell@linaro.org>]

On 26 November 2015 at 11:23, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 26/11/2015 11:56, Peter Maydell wrote:
>> 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_.

Until and unless the compiler guarantees us the semantics that
we want, then silently hoping we get something we're not getting
is even more dangerous, and avoiding the UB is better than
just crossing our fingers and hoping.

> 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.

I don't see much point in documenting what we rely on
if we can't rely on it and need to stop relying on it.

thanks
-- PMM