On Thu, Aug 3, 2023 at 9:10 AM Roger Sayle <ro...@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 <ro...@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
> --
>
