Re: [PATCH 02/17] [APX NDD] Restrict TImode register usage when NDD enabled

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Dec 6, 2023 at 2:31 AM Hongyu Wang <wwwhhhyyy333@gmail.com> wrote:
>
> Uros Bizjak <ubizjak@gmail.com> 于2023年12月5日周二 18:46写道：
>
> >
> > On Tue, Dec 5, 2023 at 3:29 AM Hongyu Wang <hongyu.wang@intel.com> wrote:
> > >
> > > Under APX NDD, previous TImode allocation will have issue that it was
> > > originally allocated using continuous pair, like rax:rdi, rdi:rdx.
> > >
> > > This will cause issue for all TImode NDD patterns. For NDD we will not
> > > assume the arithmetic operations like add have dependency between dest
> > > and src1, then write to 1st highpart rdi will be overrided by the 2nd
> > > lowpart rdi if 2nd lowpart rdi have different src as input, then the write
> > > to 1st highpart rdi will missed and cause miscompliation.
> > >
> > > To resolve this, under TARGET_APX_NDD we'd only allow register with even
> > > regno to be allocated with TImode, then TImode registers will be allocated
> > > with non-overlapping pairs.
> >
> > Perhaps you could use earlyclobber with __doubleword instructions:
> >
> > (define_insn_and_split "*add<dwi>3_doubleword"
> >   [(set (match_operand:<DWI> 0 "nonimmediate_operand" "=ro,r")
> >     (plus:<DWI>
> >       (match_operand:<DWI> 1 "nonimmediate_operand" "%0,0")
> >       (match_operand:<DWI> 2 "x86_64_hilo_general_operand" "r<di>,o")))
> >    (clobber (reg:CC FLAGS_REG))]
> >
> > For the above pattern, you can add earlyclobbered &r output
> > alternative that guarantees that output won't be allocated to any of
> > the input registers.
> >
>
> Yes, it does resolve the dest/src overlapping issue we met, thanks!
> I tried it and no fails in gcc-testsuite and spec. Suppose for
> different src1/src2 RA can handle them correctly.

Yes, and when memory input operand is used in doubleword patterns, you
need earlyclobber anyway, otherwise nothing prevents the compiler from
clobbering address registers. When addr registers are dead, the
compiler can (and will) allocate output register to the same regno as
address register.

Uros,

> Will update in V3 patches with the changes of get_attr_isa (insn) == ISA_APX_NDD