> -----Original Message----- > From: Richard Biener <richard.guent...@gmail.com> > Sent: Wednesday, May 15, 2024 12:20 PM > To: Tamar Christina <tamar.christ...@arm.com> > Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; Richard Earnshaw > <richard.earns...@arm.com>; Marcus Shawcroft > <marcus.shawcr...@arm.com>; ktkac...@gcc.gnu.org; Richard Sandiford > <richard.sandif...@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.christ...@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 > > > > --- > > > > --
