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

[Jamie Lokier <jamie@shareable.org>]

Richard Henderson wrote:
> On 05/26/2011 01:25 PM, Blue Swirl wrote:
> >> 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.
> 
> If you like, you can write a configure test for it.  But, honestly,
> essentially every place in qemu that uses shifts on signed types
> would have to be audited.  Really.

I agree, the chance of qemu ever working, or needing to work, on a non
two's complement machine is pretty remote!

> The C99 hook exists to efficiently support targets that don't have
> arithmetic shift operations.  Honestly.

If you care, this should be portable without a configure test, as
constant folding should have the same behaviour:

    (((int32_t)-3 >> 1 == (int32_t)-2)
     ? (int32_t)x >> (int32_t)y
     : long_winded_portable_shift_right(x, y))

-- Jamie