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

[Uros Bizjak <ubizjak@gmail.com>]

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.

   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.

   op1 = gen_rtx_ZERO_EXTEND (TImode, op1);
   op1 = gen_rtx_ASHIFT (TImode, op1, GEN_INT (64));
   op1 = gen_rtx_IOR (TImode, tmp, op1);

Uros.