Re: [Qemu-devel] [PATCH v2 for 2.5] QEMU does not care about left shifts of signed negative values

[Markus Armbruster <armbru@redhat.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 17/11/2015 18:39, Markus Armbruster wrote:
>> The kernel switched from -fwrapv to -fno-strict-overflow in '09, because
>> -fwrapv was buggy in gcc 4.1 (already old then, completely irrelevant
>> now), and because it "seems to be much less disturbing to gcc too: the
>> difference in the generated code by -fno-strict-overflow are smaller
>> (compared to using neither flag) than when using -fwrapv", which may or
>> may not be still the case with compilers that matter today.
>> 
>> Could you briefly explain why you picked -fwrapv and not
>> -fno-strict-overflow?
>
> Because -fno-strict-overflow doesn't silence ubsan, only -fwrapv does
> (it doesn't silence it for negative left shifts, but I've asked on
> gcc-patches whether they'd like to have that fixed as well).

Add something like that to the commit message, and you have my
Reviewed-by: Markus Armbruster <armbru@redhat.com>

> In the meanwhile I got some good news from the GCC folks:
>
>>> I think we should remove the ", but this is subject to change" in 
>>> implement-c.texi (while replacing it with noting that ubsan will still 
>>> diagnose such cases, and they will also be diagnosed where constant 
>>> expressions are required).

Makes only sense.