Re: [Qemu-devel] [PATCH 5/6] Do constant folding for shift operations.

[Blue Swirl]

On Thu, May 26, 2011 at 11:10 PM, Richard Henderson <address@hidden> wrote:
> On 05/26/2011 12:14 PM, Blue Swirl wrote:
>> On Thu, May 26, 2011 at 4:56 PM, Richard Henderson <address@hidden> wrote:
>>> On 05/26/2011 05:36 AM, Kirill Batuzov wrote:
>>>>>   x = (int32_t)x >> (int32_t)y;
>>>>>
>>>> This expression has an implementation-defined behavior accroding to
>>>> C99 6.5.7 so we decided to emulate signed shifts by hand.
>>>
>>> Technically, yes.  In practice, no.  GCC, ICC, LLVM, MSVC all know
>>> what the user wants here and will implement it "properly".
>>
>> Can't this be probed by configure? Then a wrapper could be introduced
>> for signed shifts.
>
> I don't see the point.  The C99 implementation defined escape hatch
> exists for weird cpus.  Which we won't be supporting as a QEMU host.

Maybe not, but a compiler with this property could arrive. For
example, GCC developers could decide that since this weirdness is
allowed by the standard, it may be implemented as well.