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Subject: [Bug rtl-optimization/101617] a ? -1 : 1 -> (-(type)a) | 1
Date: Mon, 30 May 2022 20:28:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101617

--- Comment #9 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:f1652e3343b1ec47035370801d9b9aca1f8b613f

commit r13-857-gf1652e3343b1ec47035370801d9b9aca1f8b613f
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon May 30 21:26:37 2022 +0100

    PR rtl-optimization/101617: Use neg/sbb in ix86_expand_int_movcc.

    This patch resolves PR rtl-optimization/101617 where we should generate
    the exact same code for (X ? -1 : 1) as we do for ((X ? -1 : 0) | 1).
    The cause of the current difference on x86_64 is actually in
    ix86_expand_int_movcc that doesn't know that negl;sbbl can be used
    to create a -1/0 result depending on whether the input is zero/nonzero.

    So for Andrew Pinski's test case:

    int f1(int i)
    {
      return i ? -1 : 1;
    }

    GCC currently generates:

    f1:     cmpl    $1, %edi
            sbbl    %eax, %eax      // x ? 0 : -1
            andl    $2, %eax        // x ? 0 : 2
            subl    $1, %eax        // x ? -1 : 1
            ret

    but with the attached patch, now generates:

    f1:     negl    %edi
            sbbl    %eax, %eax      // x ? -1 : 0
            orl     $1, %eax        // x ? -1 : 1
            ret

    To implement this I needed to add two expanders to i386.md to generate
    the required instructions (in both SImode and DImode) matching the
    pre-existing define_insns of the same name.

    2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR rtl-optimization/101617
            * config/i386/i386-expand.cc (ix86_expand_int_movcc): Add a
            special case (indicated by negate_cc_compare_p) to generate a
            -1/0 mask using neg;sbb.
            * config/i386/i386.md (x86_neg<mode>_ccc): New define_expand
            to generate an *x86_neg<mode>_ccc instruction.
            (x86_mov<mode>cc_0_m1_neg): Likewise, a new define_expand to
            generate a *x86_mov<mode>cc_0_m1_neg instruction.

    gcc/testsuite/ChangeLog
            PR rtl-optimization/101617
            * gcc.target/i386/pr101617.c: New test case.=