This patch enhances x86's STV pass to handle VEC_SELECT during general
scalar chain conversion, performing SImode scalar extraction from V4SI
and DImode scalar extraction from V2DI vector registers.

The motivating test case from bugzilla is:

typedef unsigned int v4si __attribute__((vector_size(16)));

unsigned int f (v4si a, v4si b)
{
  a[0] += b[0];
  return a[0] + a[1];
}

currently with -O2 -march=znver2 this generates:

        vpextrd $1, %xmm0, %edx
        vmovd   %xmm0, %eax
        addl    %edx, %eax
        vmovd   %xmm1, %edx
        addl    %edx, %eax
        ret

which performs three transfers from the vector unit to the scalar unit,
and performs the two additions there.  With this patch, we now generate:

        vmovdqa %xmm0, %xmm2
        vpshufd $85, %xmm0, %xmm0
        vpaddd  %xmm0, %xmm2, %xmm0
        vpaddd  %xmm1, %xmm0, %xmm0
        vmovd   %xmm0, %eax
        ret

which performs the two additions in the vector unit, and then transfers
the result to the scalar unit.  Technically the (cheap) movdqa isn't
needed with better register allocation (or this could be cleaned up
during peephole2), but even so this transform is still a win.

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?


2022-12-22  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        PR target/107548
        * config/i386/i386-features.cc (scalar_chain::add_insn): The
        operands of a VEC_SELECT don't need to added to the scalar chain.
        (general_scalar_chain::compute_convert_gain) <case VEC_SELECT>:
        Provide gains for performing STV on a VEC_SELECT.
        (general_scalar_chain::convert_insn): Convert VEC_SELECT to pshufd,
        psrldq or no-op.
        (general_scalar_to_vector_candidate_p): Handle VEC_SELECT of a
        single element from a vector register to a scalar register.

gcc/testsuite/ChangeLog
        PR target/107548
        * gcc.target/i386/pr107548-1.c: New test V4SI case.
        * gcc.target/i386/pr107548-1.c: New test V2DI case.


Thanks in advance,
Roger
--