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Subject: [Bug middle-end/98865] Missed transform of (a >> 63) * b
Date: Fri, 27 May 2022 08:02:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98865

--- Comment #8 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:8fb94fc6097c0a934aac0d89c9c5e2038da67655

commit r13-793-g8fb94fc6097c0a934aac0d89c9c5e2038da67655
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Fri May 27 08:57:46 2022 +0100

    Canonicalize X&-Y as X*Y in match.pd when Y is [0,1].

    "For every pessimization, there's an equal and opposite optimization".

    In the review of my original patch for PR middle-end/98865, Richard
    Biener pointed out that match.pd shouldn't be transforming X*Y into
    X&-Y as the former is considered cheaper by tree-ssa's cost model
    (operator count).  A corollary of this is that we should instead be
    transforming X&-Y into the cheaper X*Y as a preferred canonical form
    (especially as RTL expansion now intelligently selects the appropriate
    implementation based on the target's costs).

    With this patch we now generate identical code for:
    int foo(int x, int y) { return -(x&1) & y; }
    int bar(int x, int y) { return (x&1) * y; }

    specifically on x86_64-pc-linux-gnu both use and/neg/and with -O2,
    but both use and/mul with -Os.

    One minor wrinkle/improvement is that this patch includes three
    additional optimizations (that account for the change in canonical
    form) to continue to optimize PR92834 and PR94786.

    2022-05-27  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            * match.pd (match_zero_one_valued_p): New predicate.
            (mult @0 @1): Use zero_one_valued_p for optimization to the
            expression "bit_and @0 @1".
            (bit_and (negate zero_one_valued_p@0) @1): Optimize to MULT_EXP=
R.
            (plus @0 (mult (minus @1 @0) zero_one_valued_p@2)): New transfo=
rm.
            (minus @0 (mult (minus @0 @1) zero_one_valued_p@2)): Likewise.
            (bit_xor @0 (mult (bit_xor @0 @1) zero_one_valued_p@2)): Likewi=
se.
            Remove three redundant transforms obsoleted by the three above.

    gcc/testsuite/ChangeLog
            * gcc.dg/pr98865.c: New test case.=