RE: [x86_64 PATCH] More TImode parameter passing improvements.

["Roger Sayle" <roger@nextmovesoftware.com>]

Hi Uros,

> From: Uros Bizjak <ubizjak@gmail.com>
> Sent: 20 July 2023 07:50
> 
> On Wed, Jul 19, 2023 at 10:07 PM Roger Sayle <roger@nextmovesoftware.com>
> wrote:
> >
> > This patch is the next piece of a solution to the x86_64 ABI issues in
> > PR 88873.  This splits the *concat<mode><dwi>3_3 define_insn_and_split
> > into two patterns, a TARGET_64BIT *concatditi3_3 and a !TARGET_64BIT
> > *concatsidi3_3.  This allows us to add an additional alternative to
> > the the 64-bit version, enabling the register allocator to perform
> > this operation using SSE registers, which is implemented/split after
> > reload using vec_concatv2di.
> >
> > To demonstrate the improvement, the test case from PR88873:
> >
> > typedef struct { double x, y; } s_t;
> >
> > s_t foo (s_t a, s_t b, s_t c)
> > {
> >   return (s_t){ __builtin_fma(a.x, b.x, c.x), __builtin_fma (a.y, b.y,
> > c.y) }; }
> >
> > when compiled with -O2 -march=cascadelake, currently generates:
> >
> > foo:    vmovq   %xmm2, -56(%rsp)
> >         movq    -56(%rsp), %rax
> >         vmovq   %xmm3, -48(%rsp)
> >         vmovq   %xmm4, -40(%rsp)
> >         movq    -48(%rsp), %rcx
> >         vmovq   %xmm5, -32(%rsp)
> >         vmovq   %rax, %xmm6
> >         movq    -40(%rsp), %rax
> >         movq    -32(%rsp), %rsi
> >         vpinsrq $1, %rcx, %xmm6, %xmm6
> >         vmovq   %xmm0, -24(%rsp)
> >         vmovq   %rax, %xmm7
> >         vmovq   %xmm1, -16(%rsp)
> >         vmovapd %xmm6, %xmm2
> >         vpinsrq $1, %rsi, %xmm7, %xmm7
> >         vfmadd132pd     -24(%rsp), %xmm7, %xmm2
> >         vmovapd %xmm2, -56(%rsp)
> >         vmovsd  -48(%rsp), %xmm1
> >         vmovsd  -56(%rsp), %xmm0
> >         ret
> >
> > with this change, we avoid many of the reloads via memory,
> >
> > foo:    vpunpcklqdq     %xmm3, %xmm2, %xmm7
> >         vpunpcklqdq     %xmm1, %xmm0, %xmm6
> >         vpunpcklqdq     %xmm5, %xmm4, %xmm2
> >         vmovdqa %xmm7, -24(%rsp)
> >         vmovdqa %xmm6, %xmm1
> >         movq    -16(%rsp), %rax
> >         vpinsrq $1, %rax, %xmm7, %xmm4
> >         vmovapd %xmm4, %xmm6
> >         vfmadd132pd     %xmm1, %xmm2, %xmm6
> >         vmovapd %xmm6, -24(%rsp)
> >         vmovsd  -16(%rsp), %xmm1
> >         vmovsd  -24(%rsp), %xmm0
> >         ret
> >
> >
> > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> > and make -k check, both with and without --target_board=unix{-m32}
> > with no new failures.  Ok for mainline?
> >
> >
> > 2023-07-19  Roger Sayle  <roger@nextmovesoftware.com>
> >
> > gcc/ChangeLog
> >         * config/i386/i386-expand.cc (ix86_expand_move): Don't call
> >         force_reg, to use SUBREG rather than create a new pseudo when
> >         inserting DFmode fields into TImode with insvti_{high,low}part.
> >         (*concat<mode><dwi>3_3): Split into two define_insn_and_split...
> >         (*concatditi3_3): 64-bit implementation.  Provide alternative
> >         that allows register allocation to use SSE registers that is
> >         split into vec_concatv2di after reload.
> >         (*concatsidi3_3): 32-bit implementation.
> >
> > gcc/testsuite/ChangeLog
> >         * gcc.target/i386/pr88873.c: New test case.
> 
> diff --git a/gcc/config/i386/i386-expand.cc b/gcc/config/i386/i386-expand.cc
> index f9b0dc6..9c3febe 100644
> --- a/gcc/config/i386/i386-expand.cc
> +++ b/gcc/config/i386/i386-expand.cc
> @@ -558,7 +558,7 @@ ix86_expand_move (machine_mode mode, rtx
> operands[])
>    op0 = SUBREG_REG (op0);
>    tmp = gen_rtx_AND (TImode, copy_rtx (op0), tmp);
>    if (mode == DFmode)
> -    op1 = force_reg (DImode, gen_lowpart (DImode, op1));
> +    op1 = gen_lowpart (DImode, op1);
> 
> Please note that gen_lowpart will ICE when op1 is a SUBREG. This is the reason
> that we need to first force a SUBREG to a register and then perform gen_lowpart,
> and it is necessary to avoid ICE.

The good news is that we know op1 is a register, as this is tested by
"&& REG_P (op1)" on line 551.  You'll also notice that I'm not removing
the force_reg from before the call to gen_lowpart, but removing the call
to force_reg after the call to gen_lowpart.  When I originally wrote this,
the hope was that placing this SUBREG in its own pseudo would help
with register allocation/CSE.  Unfortunately, increasing the number of
pseudos (in this case) increases compile-time (due to quadratic behaviour
in LRA), as shown by PR rtl-optimization/110587, and keeping the DF->DI
conversion in a SUBREG inside the insvti_{high,low}part allows the
register allocator to see the DF->DI->TI sequence in a single pattern,
and hence choose to keep the TI mode in SSE registers, rather than use
a pair of reloads, to write the DF value to memory, then read it back as
a scalar in DImode, and perhaps the same again to go the other way.

>    op1 = gen_rtx_ZERO_EXTEND (TImode, op1);
>    op1 = gen_rtx_IOR (TImode, tmp, op1);
>   }
> @@ -570,7 +570,7 @@ ix86_expand_move (machine_mode mode, rtx
> operands[])
>    op0 = SUBREG_REG (op0);
>    tmp = gen_rtx_AND (TImode, copy_rtx (op0), tmp);
>    if (mode == DFmode)
> -    op1 = force_reg (DImode, gen_lowpart (DImode, op1));
> +    op1 = gen_lowpart (DImode, op1);
> 
> Also here.