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.


Thanks in advance,
Roger
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