On Thu, Mar 31, 2022 at 6:45 PM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Thu, Mar 31, 2022 at 7:51 AM liuhongt <hongtao....@intel.com> wrote:
> >
> > 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.
> >
> > Bootstrap and regtest on x86_64-pc-linux-gnu{-m32,}.
> > No impact for SPEC2017(same binary for both O2 and Ofast).
> > Ok for trunk?
> >
> > 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 | 47 ++++++++++++++++++++++
> > gcc/testsuite/gcc.target/i386/pr101908-1.c | 12 ++++++
> > gcc/testsuite/gcc.target/i386/pr101908-2.c | 12 ++++++
> > 3 files changed, 71 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..f9169b04d43 100644
> > --- a/gcc/config/i386/i386.cc
> > +++ b/gcc/config/i386/i386.cc
> > @@ -21933,7 +21933,53 @@ 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 ()
> > +{
> > + basic_block bb;
> > + unsigned window = 0;
> > + FOR_EACH_BB_FN (bb, cfun)
> > + {
> > + rtx_insn *insn;
> > + FOR_BB_INSNS (bb, 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)
> > + return;
>
> I wonder if we should also return for any_uncondjump_p (insn)
> (not sure if that captures returnjump_p), or maybe just explicitely
> allow any_condjump_p and reject other PC setters.
>
I guess it doesn't include call.
> Likewise we might want to stop at a LABEL that can be backwards reached.
>
I think checking load from parm_decl can somehow avoid split load in a
loop(assume optimizer will hoist that out).
> I suppose people more familiar with cfgrtl can suggest something better.
>
> > + 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)
>
> I wonder if we have (easy) ways to detect whether XEXP (src, 0) is
> frame/stack based
> rather than requiring a MEM_EXPR. There is may_be_sp_based_p ()
may_be_sp_based_p just checks stack pointer which is not suitable after RA.
> exported from alias.c
> for example, but I'm not sure whether that works after RA & frame elimination.
>
> > + continue;
> > +
> > + rtx zero = CONST0_RTX (V2DFmode);
> > + rtx dest = SET_DEST (set);
> > + rtx m = adjust_address (src, DFmode, 0);
> > + emit_insn_before (gen_sse2_loadlpd (dest, zero, m), insn);
>
> Can SSE1 also do this?
X86 does have movlps and movhps, but the problem is movlps load 64bit
memory to xmm w/o change upper bits which may cause partial register
dependence.
>
> > + m = adjust_address (src, DFmode, 8);
> > + PATTERN (insn) = gen_sse2_loadhpd (dest, dest, m);
> > + INSN_CODE (insn) = -1;
> > + gcc_assert (recog_memoized (insn) != -1);
>
> I think we want to dump something into dump_file when we split an insn here.
Good idea.
>
> > + }
> > + }
> > +
> > +}
> > /* Implement machine specific optimizations. We implement padding of
> > returns
> > for K8 CPUs and pass to avoid 4 jumps in the single 16 byte window. */
> > static void
> > @@ -21948,6 +21994,7 @@ ix86_reorg (void)
> >
> > if (optimize && optimize_function_for_speed_p (cfun))
> > {
> > + ix86_split_stlf_stall_load ();
> > if (TARGET_PAD_SHORT_FUNCTION)
> > ix86_pad_short_function ();
>
> btw. this function suggests we do have edges, so doing something "better"
> than FOR_EACH_BB_FN, aka walking blocks in layout order, might be
> possible after all. For example ix86_avoid_jump_mispredicts just walks
> the function by looking at get_insns(), that might be more closely what
> "as laid out" is.
I prefer to use get_insns ().
>
> > 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
