This patch improves TImode STV by adding support for logical shifts by
integer constants that are multiples of 8.  For the test case:

__int128 a, b;
void foo() { a = b << 16; }

on x86_64, gcc -O2 currently generates:

        movq    b(%rip), %rax
        movq    b+8(%rip), %rdx
        shldq   $16, %rax, %rdx
        salq    $16, %rax
        movq    %rax, a(%rip)
        movq    %rdx, a+8(%rip)
        ret

with this patch we now generate:

        movdqa  b(%rip), %xmm0
        pslldq  $2, %xmm0
        movaps  %xmm0, a(%rip)
        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?


2022-07-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * config/i386/i386-features.cc (compute_convert_gain): Add gain
        for converting suitable TImode shift to a V1TImode shift.
        (timode_scalar_chain::convert_insn): Add support for converting
        suitable ASHIFT and LSHIFTRT.
        (timode_scalar_to_vector_candidate_p): Consider logical shifts
        by integer constants that are multiples of 8 to be candidates.

gcc/testsuite/ChangeLog
        * gcc.target/i386/sse4_1-stv-7.c: New test case.


Thanks again,
Roger
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