Re: [Qemu-ppc] [PULL 00/40] ppc patch queue 2015-06-03

[Peter Maydell]

On 5 June 2015 at 16:20, Paolo Bonzini <address@hidden> wrote:
>
>
> On 05/06/2015 17:08, Peter Maydell wrote:
>>> > Can someone add a checkpatch rule that forbids shifting left U or UL
>>> > constants (i.e. only ULL)?  That would alleviate my concerns with these
>>> > ubsan warnings.
>>
>> ...but things like "(1U << 31)" are entirely valid.
>
> They're only valid until someone does a ~ on them.  I think it's
> reasonable to forbid them in our coding standards, if we want to fix
> ubsan's warning of (1 << 31).
>
> I don't think it's reasonable for compiler writers to exploit the
> undefinedness of (1 << 31) anyway, and if it were possible to shut up
> ubsan about this particular kind of undefined behavior, I would prefer it.

I don't think it's reasonable for compiler writers to exploit
undefined behaviour either, but historically they absolutely
have done. Absent a guarantee from gcc that it will never do
so, I think we should avoid any UB in our code.

There's also the usual rationale that it's worth fixing the
borderline things that provoke complaints from lint-like tools
in order that new warnings don't get lost in the pile of old
uninteresting warnings.

thanks
-- PMM