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Subject: [Bug tree-optimization/106245] Failure to optimize (u8)(a << 7) >> 7
 pattern to a & 1
Date: Fri, 20 Oct 2023 23:08:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106245

--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Roger Sayle <sayle@gcc.gnu.org>:

https://gcc.gnu.org/g:e28869670c9879fe7c67caf6cc11af202509ef78

commit r14-4810-ge28869670c9879fe7c67caf6cc11af202509ef78
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sat Oct 21 00:06:02 2023 +0100

    PR 106245: Split (x<<31)>>31 as -(x&1) in i386.md

    This patch is the backend piece of a solution to PRs 101955 and 106245,
    that adds a define_insn_and_split to the i386 backend, to perform sign
    extension of a single (least significant) bit using and $1 then neg.

    Previously, (x<<31)>>31 would be generated as

            sall    $31, %eax       // 3 bytes
            sarl    $31, %eax       // 3 bytes

    with this patch the backend now generates:

            andl    $1, %eax        // 3 bytes
            negl    %eax            // 2 bytes

    Not only is this smaller in size, but microbenchmarking confirms
    that it's a performance win on both Intel and AMD; Intel sees only a
    2% improvement (perhaps just a size effect), but AMD sees a 7% win.

    2023-10-21  Roger Sayle  <roger@nextmovesoftware.com>
                Uros Bizjak  <ubizjak@gmail.com>

    gcc/ChangeLog
            PR middle-end/101955
            PR tree-optimization/106245
            * config/i386/i386.md (*extv<mode>_1_0): New define_insn_and_sp=
lit.

    gcc/testsuite/ChangeLog
            PR middle-end/101955
            PR tree-optimization/106245
            * gcc.target/i386/pr106245-2.c: New test case.
            * gcc.target/i386/pr106245-3.c: New 32-bit test case.
            * gcc.target/i386/pr106245-4.c: New 64-bit test case.
            * gcc.target/i386/pr106245-5.c: Likewise.=