Re: [PATCH] Split vector load from parm_del to elemental loads to avoid STLF stalls.

[Hongtao Liu <crazylht@gmail.com>]

On Fri, Apr 1, 2022 at 2:54 PM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Fri, Apr 1, 2022 at 8:47 AM liuhongt via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > Update in V2:
> > 1. Use get_insns instead of FOR_EACH_BB_CFUN and FOR_BB_INSNS.
> > 2. Return for any_uncondjump_p and ANY_RETURN_P.
> > 3. Add dump info for spliting instruction.
> > 4. Restrict ix86_split_stlf_stall_load under TARGET_SSE2.
> >
> > Since cfg is freed before machine_reorg, just do a rough calculation
> > of the window according to the layout.
> > Also according to an experiment on CLX, set window size to 64.
> >
> > Currently only handle V2DFmode load since it doesn't need any scratch
> > registers, and it's sufficient to recover cray performance for -O2
> > compared to GCC11.
> >
> > gcc/ChangeLog:
> >
> >         PR target/101908
> >         * config/i386/i386.cc (ix86_split_stlf_stall_load): New
> >         function
> >         (ix86_reorg): Call ix86_split_stlf_stall_load.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * gcc.target/i386/pr101908-1.c: New test.
> >         * gcc.target/i386/pr101908-2.c: New test.
> > ---
> >  gcc/config/i386/i386.cc                    | 60 ++++++++++++++++++++++
> >  gcc/testsuite/gcc.target/i386/pr101908-1.c | 12 +++++
> >  gcc/testsuite/gcc.target/i386/pr101908-2.c | 12 +++++
> >  3 files changed, 84 insertions(+)
> >  create mode 100644 gcc/testsuite/gcc.target/i386/pr101908-1.c
> >  create mode 100644 gcc/testsuite/gcc.target/i386/pr101908-2.c
> >
> > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> > index 5a561966eb4..c88a689f32b 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -21933,6 +21933,64 @@ ix86_seh_fixup_eh_fallthru (void)
> >        emit_insn_after (gen_nops (const1_rtx), insn);
> >      }
> >  }
> > +/* Split vector load from parm_decl to elemental loads to avoid STLF
> > +   stalls.  */
> > +static void
> > +ix86_split_stlf_stall_load ()
> > +{
> > +  rtx_insn* insn, *start = get_insns ();
> > +  unsigned window = 0;
> > +
> > +  for (insn = start; insn; insn = NEXT_INSN (insn))
> > +    {
> > +      if (!NONDEBUG_INSN_P (insn))
> > +       continue;
> > +      window++;
> > +      /* Insert 64 vaddps %xmm18, %xmm19, %xmm20(no dependence between each
> > +        other, just emulate for pipeline) before stalled load, stlf stall
> > +        case is as fast as no stall cases on CLX.
> > +        Since CFG is freed before machine_reorg, just do a rough
> > +        calculation of the window according to the layout.  */
> > +      if (window > 64)
>
> I think we want to turn the '64' into a --param at least.  You can add
>
> -param=x86-stlf-window-ninsns=
>
> into i386.opt (see -param= examples in aarch64/ for example).
Sure.
>
> > +       return;
> > +
> > +      if (any_uncondjump_p (insn)
> > +         || ANY_RETURN_P (PATTERN (insn)))
>
> You made a point about calls - does any_uncondjump_p cover them?
>
No, I prefer excluding calls which could take sufficient time to
compensate for the STLF stall.
> otherwise I think this is fine, Honza, do you agree?
>
> Thanks,
> Richard.
>
> > +       return;
> > +
> > +      rtx set = single_set (insn);
> > +      if (!set)
> > +       continue;
> > +      rtx src = SET_SRC (set);
> > +      if (!MEM_P (src)
> > +         /* Only handle V2DFmode load since it doesn't need any scratch
> > +            register.  */
> > +         || GET_MODE (src) != E_V2DFmode
> > +         || !MEM_EXPR (src)
> > +         || TREE_CODE (get_base_address (MEM_EXPR (src))) != PARM_DECL
> > +       continue;
> > +
> > +      rtx zero = CONST0_RTX (V2DFmode);
> > +      rtx dest = SET_DEST (set);
> > +      rtx m = adjust_address (src, DFmode, 0);
> > +      rtx loadlpd = gen_sse2_loadlpd (dest, zero, m);
> > +      emit_insn_before (loadlpd, insn);
> > +      m = adjust_address (src, DFmode, 8);
> > +      rtx loadhpd = gen_sse2_loadhpd (dest, dest, m);
> > +      if (dump_file && (dump_flags & TDF_DETAILS))
> > +       {
> > +         fputs ("Due to potential STLF stall, split instruction:\n",
> > +                dump_file);
> > +         print_rtl_single (dump_file, insn);
> > +         fputs ("To:\n", dump_file);
> > +         print_rtl_single (dump_file, loadlpd);
> > +         print_rtl_single (dump_file, loadhpd);
> > +       }
> > +      PATTERN (insn) = loadhpd;
> > +      INSN_CODE (insn) = -1;
> > +      gcc_assert (recog_memoized (insn) != -1);
> > +    }
> > +}
> >
> >  /* Implement machine specific optimizations.  We implement padding of returns
> >     for K8 CPUs and pass to avoid 4 jumps in the single 16 byte window.  */
> > @@ -21948,6 +22006,8 @@ ix86_reorg (void)
> >
> >    if (optimize && optimize_function_for_speed_p (cfun))
> >      {
> > +      if (TARGET_SSE2)
> > +       ix86_split_stlf_stall_load ();
> >        if (TARGET_PAD_SHORT_FUNCTION)
> >         ix86_pad_short_function ();
> >        else if (TARGET_PAD_RETURNS)
> > diff --git a/gcc/testsuite/gcc.target/i386/pr101908-1.c b/gcc/testsuite/gcc.target/i386/pr101908-1.c
> > new file mode 100644
> > index 00000000000..33d9684f0ad
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/i386/pr101908-1.c
> > @@ -0,0 +1,12 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O2 -msse2 -mno-avx" } */
> > +/* { dg-final { scan-assembler-not {(?n)movhpd[ \t]} } } */
> > +
> > +struct X { double x[2]; };
> > +typedef double v2df __attribute__((vector_size(16)));
> > +
> > +v2df __attribute__((noipa))
> > +foo (struct X* x, struct X* y)
> > +{
> > +  return (v2df) {x->x[1], x->x[0] } + (v2df) { y->x[1], y->x[0] };
> > +}
> > diff --git a/gcc/testsuite/gcc.target/i386/pr101908-2.c b/gcc/testsuite/gcc.target/i386/pr101908-2.c
> > new file mode 100644
> > index 00000000000..45060b73c06
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/i386/pr101908-2.c
> > @@ -0,0 +1,12 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O2 -msse2 -mno-avx" } */
> > +/* { dg-final { scan-assembler-times {(?n)movhpd[ \t]+} "2" } }  */
> > +
> > +struct X { double x[4]; };
> > +typedef double v2df __attribute__((vector_size(16)));
> > +
> > +v2df __attribute__((noipa))
> > +foo (struct X x, struct X y)
> > +{
> > +  return (v2df) {x.x[1], x.x[0] } + (v2df) { y.x[1], y.x[0] };
> > +}
> > --
> > 2.18.1
> >



-- 
BR,
Hongtao