Re: [x86 PATCH] Split SUBREGs of SSE vector registers into vec_select insns.

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Aug 3, 2023 at 9:10 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch is the final piece in the series to improve the ABI issues
> affecting PR 88873.  The previous patches tackled inserting DFmode
> values into V2DFmode registers, by introducing insvti_{low,high}part
> patterns.  This patch improves the extraction of DFmode values from
> v2DFmode registers via TImode intermediates.
>
> I'd initially thought this would require new extvti_{low,high}part
> patterns to be defined, but all that's required is to recognize that
> the SUBREG idioms produced by combine are equivalent to (forms of)
> vec_select patterns.  The target-independent middle-end can't be sure
> that the appropriate vec_select instruction exists on the target,
> hence doesn't canonicalize a SUBREG of a vector mode as a vec_select,
> but the backend can provide a define_split stating where and when
> this is useful, for example, considering whether the operand is in
> memory, or whether !TARGET_SSE_MATH and the destination is i387.
>
> For pr88873.c, gcc -O2 -march=cascadelake currently generates:
>
> 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
>
> with this patch, we now generate:
>
> foo:    vpunpcklqdq     %xmm1, %xmm0, %xmm6
>         vpunpcklqdq     %xmm3, %xmm2, %xmm7
>         vpunpcklqdq     %xmm5, %xmm4, %xmm2
>         vmovdqa %xmm6, %xmm1
>         vfmadd132pd     %xmm7, %xmm2, %xmm1
>         vmovsd  %xmm1, %xmm1, %xmm0
>         vunpckhpd       %xmm1, %xmm1, %xmm1
>         ret
>
> The improvement is even more dramatic when compared to the original
> 29 instructions shown in comment #8.  GCC 13, for example, required
> 12 transfers to/from memory.
>
> 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?
>
>
> 2023-08-03  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * config/i386/sse.md (define_split): Convert highpart:DF extract
>         from V2DFmode register into a sse2_storehpd instruction.
>         (define_split): Likewise, convert lowpart:DF extract from V2DF
>         register into a sse2_storelpd instruction.
>
> gcc/testsuite/ChangeLog
>         * gcc.target/i386/pr88873.c: Tweak to check for improved code.

OK.

Thanks,
Uros.

>
>
> Thanks in advance,
> Roger
> --
>