This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: [PATCH, i386]: Add SSE4.2 support - pcmpstr part
> > No, it is also needed for new __cconly instructions to instantiate
> > instruction that has free register to clobber. IMO there is no other way
> > for allocator to choose between two alternative instructions.
>
> Ah, you are right here, I was looking for instructions having 'z' in
> multiple alternatives, but I've missed this one.
> If we really need use xmm0 constraint in alternatives, we are screwed
> and we need class.
Hi,
looking into the pattern sse.md more, the pcmpistri pcmpistrm really
only differs in the ignored output operand. I wonder if the performance
characteristics was already published for SSE4.2 implementation. At
least from AMD chip experience the SSE instructions communicating output
to integer registers are a lot more expensive than pure SSE
instructions.
That would nullify the need for multiple alternatives (we would want to
always use the second alternative). Which also suggest that reordering
the pattern to make pcmpistrm come out with priority now might be a good
step.
Honza