RE: [PATCH 0/4]AArch64: support conditional early clobbers on certain operations.

[Tamar Christina <Tamar.Christina@arm.com>]

> -----Original Message-----
> From: Richard Biener <richard.guenther@gmail.com>
> Sent: Wednesday, May 15, 2024 12:20 PM
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; Richard Earnshaw
> <Richard.Earnshaw@arm.com>; Marcus Shawcroft
> <Marcus.Shawcroft@arm.com>; ktkachov@gcc.gnu.org; Richard Sandiford
> <Richard.Sandiford@arm.com>
> Subject: Re: [PATCH 0/4]AArch64: support conditional early clobbers on certain
> operations.
> 
> On Wed, May 15, 2024 at 12:29 PM Tamar Christina
> <tamar.christina@arm.com> wrote:
> >
> > Hi All,
> >
> > Some Neoverse Software Optimization Guides (SWoG) have a clause that state
> > that for predicated operations that also produce a predicate it is preferred
> > that the codegen should use a different register for the destination than that
> > of the input predicate in order to avoid a performance overhead.
> >
> > This of course has the problem that it increases register pressure and so should
> > be done with care.  Additionally not all micro-architectures have this
> > consideration and so it shouldn't be done as a default thing.
> >
> > The patch series adds support for doing conditional early clobbers through a
> > combination of new alternatives and attributes to control their availability.
> 
> You could have two alternatives, one with early clobber and one with
> a matching constraint where you'd disparage the matching constraint one?
> 

Yeah, that's what I do, though there's no need to disparage the non-early clobber
alternative as the early clobber alternative will naturally get a penalty if it needs a
reload.

Cheers,
Tamar

> > On high register pressure we also use LRA's costing to prefer not to use the
> > alternative and instead just use the tie as this is preferable to a reload.
> >
> > Concretely this patch series does:
> >
> > > aarch64-none-elf-gcc -O3 -g0 -S -o - pred-clobber.c -mcpu=neoverse-n2
> >
> > foo:
> >         mov     z31.h, w0
> >         ptrue   p3.b, all
> >         cmplo   p0.h, p3/z, z0.h, z31.h
> >         b       use
> >
> > > aarch64-none-elf-gcc -O3 -g0 -S -o - pred-clobber.c -mcpu=neoverse-n1+sve
> >
> > foo:
> >         mov     z31.h, w0
> >         ptrue   p0.b, all
> >         cmplo   p0.h, p0/z, z0.h, z31.h
> >         b       use
> >
> > > aarch64-none-elf-gcc -O3 -g0 -S -o - pred-clobber.c -mcpu=neoverse-n2 -
> ffixed-p[1-15]
> >
> > foo:
> >         mov     z31.h, w0
> >         ptrue   p0.b, all
> >         cmplo   p0.h, p0/z, z0.h, z31.h
> >         b       use
> >
> > Testcases for the changes are in the last patch of the series.
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
> >
> > Thanks,
> > Tamar
> >
> > ---
> >
> > --