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Re: [i386] Replace builtins with vector extensions


On Tue, Apr 9, 2013 at 9:15 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Tue, 9 Apr 2013, Marc Glisse wrote:
>
>> On Tue, 9 Apr 2013, Richard Biener wrote:
>>
>>> I seem to remember discussion in the PR(s) that the intrinsics should
>>> (and do for other compilers) expand to the desired instructions even when
>>> the corresponding instruction set is disabled.
>>
>>
>> emmintrin.h starts with:
>> #ifndef __SSE2__
>> # error "SSE2 instruction set not enabled"
>
>
> Oh, re-reading your post, it looks like you mean we should change the
> current behavior, not just avoid regressions...
>
> My opinion on the intrinsics is that they are the portable way to use
> vectors on x86, but they are not equivalent to asm (which people should use
> if they don't want the compiler looking at their code). Knowingly generating
> SSE code with -mno-sse is not very appealing.
>
> However, the arguments in:
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56298
> make sense. I guess I'll forget about this patch.

Note that to fully support emitting intrinsics "correctly" even without -msse
x86 specific builtins need to be used and they need to conditionally expand
to either UNSPECs (if the required instriuction set / modes are not available)
or regular RTL (where they can be folded to generic GIMPLE earlier
then as well).
A complication is register allocation which would need to understand how to
allocate registers for the UNSPECs - even if some of the modes would not
be "available".  So it's indeed a mess ...

That said, folding of the x86 builtins to GIMPLE looks like a more
viable approach
that would not interfere too much with any possible route we would go here.
As suggested previously please add a new target hook with the same interface
as fold_stmt in case you want to work on this.

Thanks,
Richard.

> --
> Marc Glisse


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