[PATCH] move x86 to use gather/scatter internal functions

Hongtao Liu crazylht@gmail.com
Wed Aug 18 09:17:23 GMT 2021


On Wed, Aug 18, 2021 at 4:32 PM Richard Biener <rguenther@suse.de> wrote:
>
> On Wed, 18 Aug 2021, Hongtao Liu wrote:
>
> > On Wed, Aug 18, 2021 at 11:24 AM Hongtao Liu <crazylht@gmail.com> wrote:
> > >
> > > On Tue, Aug 17, 2021 at 10:43 PM Richard Biener via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > > >
> > > > On Tue, Aug 17, 2021 at 3:29 PM Richard Biener via Gcc-patches
> > > > <gcc-patches@gcc.gnu.org> wrote:
> > > > >
> > > > > This is an attempt to start moving the x86 backend to use
> > > > > standard pattern names for [mask_]gather_load and [mask_]scatter_store
> > > > > rather than using the builtin_{gather,scatter} target hooks.
> > > > >
> > > > > I've started with AVX2 gathers and given x86 only supports masked
> > > > > gather I only implemented mask_gather_load.  Note while for
> > > > > the builtin_gather case the vectorizer will provide an all-true
> > > > > mask operand for non-masked gathers this capability does not
> > > > > exist for the IFN path yet, so only testcases with actual masked
> > > > > gathers will work.
> > > > >
> > > > > If this looks reasonable on the backend side I'll see to first
> > > > > complete the vectorizer part, ripping out the target hook and
> > > > > arranging for the missing pieces.  Another one is the support
> > > > > for SImode indices with DFmode data which requires unpacking
> > > > > the index vector and actually recognizing the IFN.
> > > > >
> > > > > 2021-08-17  Richard Biener  <rguenther@suse.de>
> > > > >
> > > > >         * tree-vect-data-refs.c (vect_check_gather_scatter):
> > > > >         Always use internal functions.
> > > > >         * config/i386/sse.md
> > > > >         (mask_gather_load<mode><vec_gather_idxsi>): New expander.
> > > > >         (mask_gather_load<mode><vec_gather_idxdi>): Likewise.
> > > > > ---
> > > > >  gcc/config/i386/sse.md    | 56 +++++++++++++++++++++++++++++++++++++++
> > > > >  gcc/tree-vect-data-refs.c |  4 +--
> > > > >  2 files changed, 57 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/gcc/config/i386/sse.md b/gcc/config/i386/sse.md
> > > > > index 3957c86c3df..40bec98d9f7 100644
> > > > > --- a/gcc/config/i386/sse.md
> > > > > +++ b/gcc/config/i386/sse.md
> > > > > @@ -23232,12 +23232,22 @@
> > > > >                        (V2DF "V4SI") (V4DF "V4SI") (V8DF "V8SI")
> > > > >                        (V4SI "V4SI") (V8SI "V8SI") (V16SI "V16SI")
> > > > >                        (V4SF "V4SI") (V8SF "V8SI") (V16SF "V16SI")])
> > > > > +(define_mode_attr vec_gather_idxsi
> > > > > +                     [(V2DI "v4si") (V4DI "v4si") (V8DI "v8si")
> > > > > +                      (V2DF "v4si") (V4DF "v4si") (V8DF "v8si")
> > > > > +                      (V4SI "v4si") (V8SI "v8si") (V16SI "v16si")
> > > > > +                      (V4SF "v4si") (V8SF "v8si") (V16SF "v16si")])
> > > > >
> > > > >  (define_mode_attr VEC_GATHER_IDXDI
> > > > >                       [(V2DI "V2DI") (V4DI "V4DI") (V8DI "V8DI")
> > > > >                        (V2DF "V2DI") (V4DF "V4DI") (V8DF "V8DI")
> > > > >                        (V4SI "V2DI") (V8SI "V4DI") (V16SI "V8DI")
> > > > >                        (V4SF "V2DI") (V8SF "V4DI") (V16SF "V8DI")])
> > > > > +(define_mode_attr vec_gather_idxdi
> > > > > +                     [(V2DI "v2di") (V4DI "v4di") (V8DI "v8di")
> > > > > +                      (V2DF "v2di") (V4DF "v4di") (V8DF "v8di")
> > > > > +                      (V4SI "v2di") (V8SI "v4di") (V16SI "v8di")
> > > > > +                      (V4SF "v2di") (V8SF "v4di") (V16SF "v8di")])
> > > > >
> > > > >  (define_mode_attr VEC_GATHER_SRCDI
> > > > >                       [(V2DI "V2DI") (V4DI "V4DI") (V8DI "V8DI")
> > > > > @@ -23245,6 +23255,29 @@
> > > > >                        (V4SI "V4SI") (V8SI "V4SI") (V16SI "V8SI")
> > > > >                        (V4SF "V4SF") (V8SF "V4SF") (V16SF "V8SF")])
> > > > >
> > > > > +(define_expand "mask_gather_load<mode><vec_gather_idxsi>"
> > > > > +  [(parallel [(set (match_operand:VEC_GATHER_MODE 0 "register_operand")
> > > > > +                  (unspec:VEC_GATHER_MODE
> > > > > +                    [(pc)
> > > > > +                     (mem:<ssescalarmode>
> > > > > +                       (match_par_dup 6
> > > > > +                         [(match_operand 1 "vsib_address_operand")
> > > > > +                          (match_operand:<VEC_GATHER_IDXSI>
> > > > > +                             2 "register_operand")
> > > > > +                          (match_operand:SI 4 "const1248_operand")
> > > > > +                          (match_operand:SI 3 "const0_operand")]))
> > > > > +                     (mem:BLK (scratch))
> > > > > +                     (match_operand:<sseintvecmode> 5 "register_operand")]
> > > >
> > > > One problem of these is that when AVX512[VL] is enabled we get a AVX512 mask
> > > > mode here and while the internal function expansion check succeeds (it
> > > > never checks
> > > > the mask operand!), RTL expansion fails unexpectedly because of this mismatch.
> > > >
> > > > I suppose a more complicated define_mode_attr might do the trick or do I
> > > I don't think define_mode_attr supports conditional selection based on
> > > target feature.
> > > > need to add && !TARGET_AVX512F to these expanders?
> > > There'll be a duplicated definition of  mask_loadv8sfv8si for avx512
> > > and no-avx512 versions.
> > Note gather_loadmn can be shared by avx512 and non-avx512 version, we
> > can handle mask mode in the preparation statements based on target
> > feature.
> > > >
> > > The best way is like maskloadmn to accept different mask modes in its
> > > name, but that may create too much complexity in the middle-end to
> > > choose between avx2 mask_gather_load and avx512 mask_gather_load.
>
> So when we have -mavx512f only then we can gather V8DF using a QImode
> maks with AVX512 gather but we have to use AVX2 for V4DF gather
> using a V4DImode mask.  With -mavx512vl we use a QImode mask for that
> case as well.
>
> Note the
>
>   operands[5] = gen_lowpart_SUBREG (<VEC_GATHER_SRCDI>mode, operands[5]);
>
> is because the define_insns for the gathers somehow expect odd mask
> modes (the vectorizer generates an integer vector mode matching the
> layout of the result vector).  I suppose we could somehow fix that
> but I wanted to change as little as possible for now.
>
> The following seems to "work" in generating AVX512 variants for
> -mavx512f -mavx512vl -mprefer-vector-width=512 and AVX2 variants
> when I drop the -mavx512vl arg:
>
Interesting.
> (define_expand "mask_gather_load<mode><vec_gather_idxdi>"
>   [(match_operand:VEC_GATHER_MODE 0 "register_operand")
>    (match_operand 1 "vsib_address_operand")
>    (match_operand:<VEC_GATHER_IDXDI> 2 "register_operand")
>    (match_operand:SI 3 "const0_operand")
>    (match_operand:SI 4 "const1248_operand")
>    (match_operand 5 "register_operand")]
>   "TARGET_AVX2 && TARGET_USE_GATHER"
> {
>   if (TARGET_AVX512VL)
>     {
>       rtx scratch = gen_reg_rtx (<MODE>mode);
>       emit_move_insn (scratch, CONST0_RTX(<MODE>mode));
>       emit_insn (gen_avx512vl_gatherdi<mode>
>         (operands[0], scratch,
>          operands[1], operands[2], operands[5], operands[4]));
>     }
>   else
>     {
>       operands[5] = gen_lowpart_SUBREG (<VEC_GATHER_SRCDI>mode,
> operands[5]);
>       rtx scratch = gen_reg_rtx (<MODE>mode);
>       emit_move_insn (scratch, CONST0_RTX(<MODE>mode));
>       emit_insn (gen_avx2_gatherdi<mode>
>         (operands[0], scratch,
>          operands[1], operands[2], operands[5], operands[4]));
>     }
>   DONE;
> })
>
> But I see quite horrible code for an always-true mask.  From GIMPLE
>
>   # ivtmp.17_34 = PHI <ivtmp.17_19(3), 0(2)>
>   vect__4.5_24 = MEM <vector(8) int> [(int *)idx_12(D) + ivtmp.17_34 * 1];
>   vect_patt_28.6_23 = [vec_unpack_lo_expr] vect__4.5_24;
>   vect_patt_28.6_22 = [vec_unpack_hi_expr] vect__4.5_24;
>   vect_patt_27.7_17 = .MASK_GATHER_LOAD (_18, vect_patt_28.6_23, 8, { 0.0,
> 0.0, 0.0, 0.0 }, { -1, -1, -1, -1 });
>   vect_patt_27.8_16 = .MASK_GATHER_LOAD (_18, vect_patt_28.6_22, 8, { 0.0,
> 0.0, 0.0, 0.0 }, { -1, -1, -1, -1 });
>   MEM <vector(4) double> [(double *)&a + ivtmp.17_34 * 2] =
> vect_patt_27.7_17;
>   MEM <vector(4) double> [(double *)&a + 32B + ivtmp.17_34 * 2] =
> vect_patt_27.8_16;
>   ivtmp.17_19 = ivtmp.17_34 + 32;
>   if (ivtmp.17_19 != 4096)
>
> we end up with
>
>         movl    $-1, %edx
>         xorl    %eax, %eax
>         vxorpd  %xmm1, %xmm1, %xmm1
>         kmovw   %edx, %k1
>         .p2align 4,,10
>         .p2align 3
> .L2:
>         vpmovsxdq       (%rsi,%rax), %ymm0
>         vmovdqu (%rsi,%rax), %ymm4
>         vmovapd %ymm1, %ymm3
>         kmovw   %k1, %k2
>         vmovapd %ymm1, %ymm2
>         kmovw   %k1, %k3
>         vgatherqpd      (%rdi,%ymm0,8), %ymm3{%k2}
>         vextracti128    $0x1, %ymm4, %xmm0
>         vpmovsxdq       %xmm0, %ymm0
>         vgatherqpd      (%rdi,%ymm0,8), %ymm2{%k3}
>         vmovapd %ymm3, a(%rax,%rax)
>         vmovapd %ymm2, a+32(%rax,%rax)
>         addq    $32, %rax
>         cmpq    $4096, %rax
>         jne     .L2
>
> I wonder why we have those extra kmovw here.  I see there's

(define_insn "*avx512f_gathersi<VI48F:mode>"
  [(set (match_operand:VI48F 0 "register_operand" "=&v")
        (unspec:VI48F
          [(match_operand:VI48F 1 "register_operand" "0")
           (match_operand:<avx512fmaskmode> 7 "register_operand" "2")
           (match_operator:<ssescalarmode> 6 "vsib_mem_operator"
             [(unspec:P
                [(match_operand:P 4 "vsib_address_operand" "Tv")
                 (match_operand:<VEC_GATHER_IDXSI> 3 "register_operand" "v")
                 (match_operand:SI 5 "const1248_operand" "n")]
                UNSPEC_VSIBADDR)])]
          UNSPEC_GATHER))
   (clobber (match_scratch:<avx512fmaskmode> 2 "=&Yk"))]
I think it's because this clobber, and the clobber here is because
mask register will be set to zero by this instruction.
cut from intel sdm
----cut first-------
VGATHERDPS/VGATHERDPD—Gather Packed Single, Packed Double with Signed Dword
......The entire mask register will be set to zero by this instruction
unless it trig-
gers an exception.
----cut end------
Similar for the avx2 version.

> extra (pc) patterns for always-true masks(?) like in the AVX2
> case.  But in the end we have to use {%kN} instructions, no?
> We can always provide non-masked gather_load expanders if that's
> better here.
>
> Richard.



-- 
BR,
Hongtao


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