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

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/11/2015 19:24, Peter Maydell wrote:
> On 17 November 2015 at 18:21, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 17/11/2015 19:19, Peter Maydell wrote:
>>> That doesn't seem like more than half-good news to me. In particular,
>>> if ubsan is still diagnosing these cases and they're still a
>>> problem in some constant expressions
>>
>> Constant expressions are standardese for e.g.
>>
>> static int x = 1 << 31;
>>
>> It doesn't mean _all_ constants, and the warning only triggers with
>> -pedantic.
> 
> But if "-fwrapv" means "this dialect of C makes shifts of
> negative numbers well defined and OK" then "-1 << 31"
> should be fine and should not provoke a warning (whether in
> a constant expression or not). If that's not what -fwrapv means,
> then we shouldn't be using it as if it did.

Since we don't use -pedantic, we don't care.  But I agree that it could
be the subject of a separate fix.  For now I'll focus on ubsan since
that's the more important one.

Paolo