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Subject: [Bug target/102117] s390: Inefficient code for 64x64=128 signed
 multiply for <= z13
Date: Sun, 21 Nov 2021 11:41:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102117

--- 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:dc915b361bbc99da83fc53db7f7e0e28d0ce12c8

commit r12-5436-gdc915b361bbc99da83fc53db7f7e0e28d0ce12c8
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Sun Nov 21 11:40:08 2021 +0000

    Tweak tree-ssa-math-opts.c to solve PR target/102117.

    This patch resolves PR target/102117 on s390.  The problem is that
    some of the functionality of GCC's RTL expanders is no longer triggered
    following the transition to tree SSA form.  On s390, unsigned widening
    multiplications are converted into WIDEN_MULT_EXPR (aka w* in tree dump=
s),
    but signed widening multiplies are left in their original form, which
    alas doesn't benefit from the clever logic in expand_widening_mult.

    The fix is to teach convert_mult_to_widen, that RTL expansion can
    synthesize a signed widening multiplication if the target provides
    a suitable umul_widen_optab.

    On s390-linux-gnu with -O2 -m64, the code in the bugzilla PR currently
    generates:

    imul128:
            stmg    %r12,%r13,96(%r15)
            srag    %r0,%r4,63
            srag    %r1,%r3,63
            lgr     %r13,%r3
            mlgr    %r12,%r4
            msgr    %r1,%r4
            msgr    %r0,%r3
            lgr     %r4,%r12
            agr     %r1,%r0
            lgr     %r5,%r13
            agr     %r4,%r1
            stmg    %r4,%r5,0(%r2)
            lmg     %r12,%r13,96(%r15)
            br      %r14

    but with this patch should now generate the more efficient:

    imul128:
            lgr     %r1,%r3
            mlgr    %r0,%r4
            srag    %r5,%r3,63
            ngr     %r5,%r4
            srag    %r4,%r4,63
            sgr     %r0,%r5
            ngr     %r4,%r3
            sgr     %r0,%r4
            stmg    %r0,%r1,0(%r2)
            br      %r14

    2021-11-21  Roger Sayle  <roger@nextmovesoftware.com>
                Robin Dapp  <rdapp@linux.ibm.com>

    gcc/ChangeLog
            PR target/102117
            * tree-ssa-math-opts.c (convert_mult_to_widen): Recognize
            signed WIDEN_MULT_EXPR if the target supports umul_widen_optab.

    gcc/testsuite/ChangeLog
            PR target/102117
            * gcc.target/s390/mul-wide.c: New test case.
            * gcc.target/s390/umul-wide.c: New test case.=