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Subject: [Bug middle-end/98865] Missed transform of (a >> 63) * b
Date: Thu, 19 May 2022 16:55:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98865

--- Comment #7 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:d863ba23fb16122bb0547b0c678173be0d98f43c

commit r13-673-gd863ba23fb16122bb0547b0c678173be0d98f43c
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Thu May 19 17:54:38 2022 +0100

    PR middle-end/98865: Expand X*Y as X&-Y when Y is [0,1].

    The patch is a revised solution for PR middle-end/98865 incorporating
    the feedback/suggestions from Richard Biener's review here:
    https://gcc.gnu.org/pipermail/gcc-patches/2022-May/593928.html
    Most significantly, this patch now performs the transformation/optimiza=
tion
    during RTL expansion, where the target's rtx_costs can be used to deter=
mine
    whether the original multiplication (that may potentially be implemente=
d by
    a shift or lea) is cheaper than a negation and a bit-wise and.

    Previously the expression (x>>63)*y would be compiled with -O2 as
            shrq    $63, %rdi
            movq    %rdi, %rax
            imulq   %rsi, %rax

    but with this patch now produces:
            sarq    $63, %rdi
            movq    %rdi, %rax
            andq    %rsi, %rax

    Likewise the expression (x>>63)*135 [that appears in a hot-spot of the
    Botan AES-128 benchmark] was previously:

            shrq    $63, %rdi
            leaq    (%rdi,%rdi,8), %rdx
            movq    %rdx, %rax
            salq    $4, %rax
            subq    %rdx, %rax

    now becomes:
            movq    %rdi, %rax
            sarq    $63, %rax
            andl    $135, %eax

    2022-05-19  Roger Sayle  <roger@nextmovesoftware.com>

    gcc/ChangeLog
            PR middle-end/98865
            * expr.cc (expand_expr_real_2) [MULT_EXPR]:  Expand X*Y as X&Y
            when both X and Y are [0, 1], X*Y as X&-Y when Y is [0,1] and
            likewise X*Y as -X&Y when X is [0,1] using tree_nonzero_bits.

    gcc/testsuite/ChangeLog
            PR middle-end/98865
            * gcc.target/i386/pr98865.c: New test case.=