[PATCH v2] Add vec_const_duplicate optab and TARGET_GEN_MEMSET_SCRATCH_RTX

H.J. Lu hjl.tools@gmail.com
Tue Jun 1 13:05:13 GMT 2021


On Mon, May 31, 2021 at 11:54:53PM -0600, Jeff Law wrote:
> 
> 
> On 5/31/2021 11:50 PM, Richard Sandiford wrote:
> > "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
> > > On Mon, May 31, 2021 at 06:32:04AM -0700, H.J. Lu wrote:
> > > > On Mon, May 31, 2021 at 6:26 AM Richard Biener
> > > > <richard.guenther@gmail.com> wrote:
> > > > > On Mon, May 31, 2021 at 3:12 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > > > On Mon, May 31, 2021 at 5:46 AM Richard Biener
> > > > > > <richard.guenther@gmail.com> wrote:
> > > > > > > On Mon, May 31, 2021 at 2:09 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > > > > > > On Wed, May 26, 2021 at 10:28:16AM +0200, Richard Biener wrote:
> > > > > > > > > > > >   -- Target Hook: rtx TARGET_GEN_MEMSET_VALUE (rtx DATA, scalar_int_mode
> > > > > > > > > > > >            MODE)
> > > > > > > > > > > >       This function returns the RTL of a register containing
> > > > > > > > > > > >       'GET_MODE_SIZE (MODE)' consecutive copies of the unsigned char
> > > > > > > > > > > >       value given in the RTL register DATA.  For example, if MODE is 4
> > > > > > > > > > > >       bytes wide, return the RTL for 0x01010101*DATA.
> > > > > > > > > > > For this one I wonder if it should be an optab instead.  Couldn't you
> > > > > > > > > > > use the existing vec_duplicate for this by using (paradoxical) subregs
> > > > > > > > > > > like (subreg:TI (vec_duplicate:VnQI (subreg:VnQI (reg:QI ...)))?
> > > > > > > > > > I tried.   It doesn't even work on x86.  See:
> > > > > > > > > > 
> > > > > > > > > > https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570661.html
> > > > > > > > > Not sure what I should read from there...
> > > > > > > > > 
> > > > > > > > > > There are special cases to subreg HI, SI and DI modes of TI mode in
> > > > > > > > > > ix86_gen_memset_value_from_prev.   simplify_gen_subreg doesn't
> > > > > > > > > > work here.   Each backend may need its own special handling.
> > > > > > > > > OK, I guess I'm not (RTL) qualified enough to further review these parts,
> > > > > > > > > sorry.  Since we're doing code generation the canonical way to communicate
> > > > > > > > > with backends should be optabs, not some set of disconnected target hooks.
> > > > > > > > > But as said, I probably don't know enough of RTL to see why it's the only way.
> > > > > > > > > 
> > > > > > > > > Richard.
> > > > > > > > Here is the patch to add optabs instead.  Does it look OK?
> > > > > > > > 
> > > > > > > > Thanks.
> > > > > > > > 
> > > > > > > > H.J.
> > > > > > > > ---
> > > > > > > > Add 2 optabs:
> > > > > > > > 
> > > > > > > > 1. integer_extract: Extract lower bit value from the integer value in
> > > > > > > > TImode, OImode or XImode.
> > > > > > > That sounds very specific, esp. the restriction to {TI,OI,XI}mode.
> > > > > > > It also sounds like it matches (subreg:{TI,OI,XI} (...) 0).  There are
> > > > > > > existing target hooks verifying subreg validity - why's that not a good
> > > > > > > fit here?  ISTR you say gen_lowpart () doesn't work (or was it
> > > > > > > simplify_gen_subreg?), why's that so?
> > > > > > {TI,OI,XI}mode are storage only integer types.   subreg doesn't work
> > > > > > well on them.  I got
> > > > > > 
> > > > > > [hjl@gnu-cfl-2 pieces]$ cat s2.i
> > > > > > extern void *ops;
> > > > > > 
> > > > > > void
> > > > > > foo (int c)
> > > > > > {
> > > > > >    __builtin_memset (ops, c, 34);
> > > > > > }
> > > > > > [hjl@gnu-cfl-2 pieces]$ make s2.s
> > > > > > /export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/xgcc
> > > > > > -B/export/build/gnu/tools-build/gcc-gitlab-debug/build-x86_64-linux/gcc/
> > > > > > -O2 -march=haswell -S s2.i
> > > > > > during RTL pass: reload
> > > > > > s2.i: In function ‘foo’:
> > > > > > s2.i:7:1: internal compiler error: maximum number of generated reload
> > > > > > insns per insn achieved (90)
> > > > > >      7 | }
> > > > > >        | ^
> > > > > > 0x1050734 lra_constraints(bool)
> > > > > > /export/gnu/import/git/gitlab/x86-gcc/gcc/lra-constraints.c:5091
> > > > > > 0x1039536 lra(_IO_FILE*)
> > > > > > /export/gnu/import/git/gitlab/x86-gcc/gcc/lra.c:2336
> > > > > > 0xfe1140 do_reload
> > > > > > /export/gnu/import/git/gitlab/x86-gcc/gcc/ira.c:5822
> > > > > > 0xfe162e execute
> > > > > > /export/gnu/import/git/gitlab/x86-gcc/gcc/ira.c:6008
> > > > > > Please submit a full bug report,
> > > > > > with preprocessed source if appropriate.
> > > > > > Please include the complete backtrace with any bug report.
> > > > > > See <https://gcc.gnu.org/bugs/> for instructions.
> > > > > > make: *** [Makefile:32: s2.s] Error 1
> > > > > > [hjl@gnu-cfl-2 pieces]$
> > > > > > 
> > > > > > due to
> > > > > > 
> > > > > > (insn 12 11 0 (set (mem:HI (plus:DI (reg/f:DI 84)
> > > > > >                  (const_int 32 [0x20])) [0 MEM <char[1:34]> [(void
> > > > > > *)ops.0_1]+32 S2 A8])
> > > > > >          (subreg:HI (reg:OI 51 xmm15) 0)) "s2.i":6:3 -1
> > > > > >       (nil))
> > > > > > 
> > > > > > The new optab gives us
> > > > > > 
> > > > > > (insn 12 11 13 2 (set (reg:TI 88)
> > > > > >          (reg:TI 51 xmm15)) "s2.i":6:3 -1
> > > > > >       (nil))
> > > > > > (insn 13 12 14 2 (set (reg:SI 89)
> > > > > >          (subreg:SI (reg:TI 88) 0)) "s2.i":6:3 -1
> > > > > >       (nil))
> > > > > > (insn 14 13 15 2 (set (reg:HI 87)
> > > > > >          (subreg:HI (reg:SI 89) 0)) "s2.i":6:3 -1
> > > > > >       (nil))
> > > > > that looks odd to me - what's the final result after LRA?  I think
> > > > I got:
> > > > 
> > > > vmovd %edi, %xmm15
> > > > movq ops(%rip), %rdx
> > > > vpbroadcastb %xmm15, %ymm15
> > > > vmovq %xmm15, %rax    <<<< move to GPR
> > > > vmovdqu %ymm15, (%rdx)
> > > > movw %ax, 32(%rdx)   <<<< subreg of GPR
> > > > vzeroupper
> > > > ret
> > > > 
> > > > > we should see to make lowpart_subreg work on {XI,OI,TI}mode.
> > > > > Only two steps should be necessary at most:
> > > > > xmm -> gpr, grp -> subreg, or gpr -> subreg.  So the expander
> > > > > code in memset should try to generate the subreg directly
> > > > subreg didn't fail on x86 when I tried.
> > > > 
> > > > > and if that fails, try a word_mode subreg followed by the subreg.
> > > > I will try word_mode subreg.
> > > > 
> > > Here is the v2 patch to use word_mode subreg.  For
> > > 
> > > ---
> > > extern void *ops;
> > > 
> > > void
> > > foo (int c)
> > > {
> > >    __builtin_memset (ops, 4, 32);
> > > }
> > > ---
> > > 
> > > without vec_const_duplicate, I got
> > > 
> > > 	movl	$4, %eax
> > > 	movq	ops(%rip), %rdx
> > > 	movd	%eax, %xmm0
> > > 	punpcklbw	%xmm0, %xmm0
> > > 	punpcklwd	%xmm0, %xmm0
> > > 	pshufd	$0, %xmm0, %xmm0
> > > 	movups	%xmm0, (%rdx)
> > > 	movups	%xmm0, 16(%rdx)
> > > 	ret
> > > 
> > > With vec_const_duplicate, I got
> > > 
> > > 	movq	ops(%rip), %rax
> > > 	movdqa	.LC0(%rip), %xmm0
> > > 	movups	%xmm0, (%rax)
> > > 	movups	%xmm0, 16(%rax)
> > > 	ret
> > > 
> > > If vec_duplicate is allowed to fail, I don't need vec_const_duplicate.
> > I don't understand why we need an optab for this though.  If the operand
> > is constant then we should just be doing an ordinary move in which the
> > source is a CONST_VECTOR.  It's then up to the move patterns to handle
> > duplicated constants as efficiently as possible.  (Sorry if this was
> > discussed upthread and I missed it.)
> That's exactly the point I'm trying to get across as well.
> 

This is what we do today.  But I'd like to generate

	movl	$4, %eax
	vpbroadcastb	%eax, %ymm15
	movq	ops(%rip), %rax
	vmovdqu	%ymm15, (%rax)
	vzeroupper
	ret

instead of

	vmovdqa	.LC0(%rip), %ymm15
	movq	ops(%rip), %rax
	vmovdqu	%ymm15, (%rax)
	vzeroupper
	ret

Do I need a vec_dup pattern for it?

H.J.


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