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

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/11/2015 12:22, Peter Maydell wrote:
> On 17 November 2015 at 10:57, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 17/11/2015 11:55, Peter Maydell wrote:
>>> If you pass clang -fwrapv then this causes -fsanitize=undefined to
>>> no longer complain about signed integer overflows from addition.
>>> However the sanitizer will still complain about left shifts of
>>> negative values. The conclusion I draw is that clang (as per
>>> the documentation) applies fwrapv only to addition &c, not to
>>> shifts.
>>
>> Ok, I'll open a bug for this.  It's probably unintended, they've already
>> fixed -fwrapv once for pointers.
> 
> Thanks. Other things that would need to be fixed for -fwrapv to
> apply to shifts:
>  * gcc need to document this (this is a stronger statement than
>    what they currently have since it is a guarantee not to change
>    the semantics in the future if -fwrapv is set)

Actually they document it under -fstrict-overflow ("Using '-fwrapv'
means that integer signed overflow is fully defined: it wraps") but it
would be nice to add it under -fwrapv as well.  I'll send a patch.

>  * clang need to document this
>  * -fwrapv in clang should suppress -Wshift-negative-value
>  * ideally, test cases in both the clang and gcc test suites to
>    defend the 2s-complement signed shift semantics

This + the ubsan issue is now https://llvm.org/bugs/show_bug.cgi?id=25552

Paolo