Re: [Qemu-devel] [PATCH 1/2] target-i386: Use 1UL for bit shift

[Laszlo Ersek <lersek@redhat.com>]

On 10/01/15 15:52, Paolo Bonzini wrote:
> 
> 
> On 01/10/2015 11:24, Peter Maydell wrote:
>> On 30 September 2015 at 21:24, Richard Henderson <rth@twiddle.net> wrote:
>>> On 09/30/2015 11:27 PM, Paolo Bonzini wrote:
>>>> ps: Ego ceterum censeo that these warnings are useless and uglify the
>>>> code unnecessarily.  But it looks like I'm in a minority so the patch is
>>>> okay.
>>
>>> I totally agree.  There are no ones-compliment machines anymore, and so the
>>> whole point of that "undefined" in the C standard is moot.  Let's all accept
>>> that shifts of signed quantities do exactly what we expect.
>>
>> I'd rather not do that without a documented statement from both
>> clang and gcc teams that they won't use this UB to do optimizations
>> that might break programs relying on it. History suggests they
>> will happily do so if it improves a benchmark at all.
> 
> Well, this is pretty much the only ubsan issue that we stumble upon.
> You can imagine how common that is in the wild and how good a move that
> would be to rely on that undefined behavior.
> 
> In addition, C89 didn't say at all what the result was for signed data
> types, so technically we could compile QEMU with -std=gnu89 (the default
> until GCC5) and call it a day.
> 
> Really the C standard should make this implementation-defined.

Obligatory link: http://blog.regehr.org/archives/1180

:)

>>> Without looking, I don't suppose either compiler has a switch to disable
>>> just the shift part of ubsan?
>>
>> Not without turning off other shift checks which we would want to
>> retain (like shifts greater than the bitwidth), I think.
> 
> I agree those are valuable.
> 
> Paolo
>