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

[Paolo Bonzini]

On 05/26/2011 09:14 PM, Blue Swirl 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.

The reason for implementation-defined behavior is basically to allow for 
non-two's-complement machine, which isn't really practical to support.

Paolo