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 13:04, Peter Maydell wrote:
> On 17 November 2015 at 11:59, Markus Armbruster <armbru@redhat.com> wrote:
>> Laszlo Ersek <lersek@redhat.com> writes:
>>
>>> 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.)
>>
>> One of them, not both.
>>
>> Quote gcc manual:
>>
>>     Using -fwrapv means that integer signed overflow is fully defined:
>>     it wraps.  When -fwrapv is used, there is no difference between
>>     -fstrict-overflow and -fno-strict-overflow for integers.
> 
> I thought this too, but note that it says "for integers". As Laszlo
> says, the reason to provide both is to get the -fno-strict-overflow
> behaviour for pointer arithmetic, which is not affected by -fwrapv.

Correct, that's what I meant.

> 
> thanks
> -- PMM
>