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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/67635] New: [SH] ifcvt missed optimization
Date: Sat, 19 Sep 2015 04:09:00 -0000	[thread overview]
Message-ID: <bug-67635-4@http.gcc.gnu.org/bugzilla/> (raw)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67635

            Bug ID: 67635
           Summary: [SH] ifcvt missed optimization
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
  Target Milestone: ---
            Target: sh*-*-*

At least on SH, the following:

bool test (int a, int b, int* r)
{
  return __builtin_mul_overflow (a, b, r);
}

compiled with -m4 -ml -O2 results in:

        dmuls.l r5,r4
        mov     #0,r0
        sts     macl,r1
        sts     mach,r2
        cmp/gt  r1,r0
        subc    r3,r3
        cmp/eq  r2,r3    <<
        bf      .L6      <<
.L2:
        rts
        mov.l   r1,@r6
        .align 1
.L6:
        bra     .L2
        mov     #1,r0


The expected code would be:
        dmuls.l r5,r4
        mov     #0,r0
        sts     macl,r1
        sts     mach,r2
        cmp/gt  r1,r0
        subc    r3,r3
        cmp/eq  r2,r3    // T = r2 == r3
        mov     #-1,r0
        negc    r0,r0    // T = r2 != r3
        rts
        mov.l   r1,@r6


Alternatively, in cases zero-displacement branches are fast (e.g. SH4):
        dmuls.l r5,r4
        mov     #0,r0
        sts     macl,r1
        sts     mach,r2
        cmp/gt  r1,r0
        subc    r3,r3
        cmp/eq  r2,r3
        bt      0f
        mov     #1,r0
0:
        rts
        mov.l   r1,@r6


I think I have seen similar cases before, where something tries to preserve the
zero constant in a reg.  Instead of overwriting it with a cstore value {0,1},
conditional branches are created for the non-zero paths.

If conditional move patterns are enabled on SH with -mpretend-cmove the
conditional branches go away:

        dmuls.l r5,r4
        mov     #-1,r0
        sts     macl,r2
        sts     mach,r3
        sts     macl,r1
        shll    r2
        subc    r2,r2
        cmp/eq  r3,r2
        mov.l   r1,@r6
        rts
        negc    r0,r0

But enabling -mpretend-cmove on SH has some other side effects and currently is
not safe (PR 58517).


             reply	other threads:[~2015-09-19  4:09 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-19  4:09 olegendo at gcc dot gnu.org [this message]
2015-09-30 12:49 ` [Bug rtl-optimization/67635] " olegendo at gcc dot gnu.org
2015-10-19 16:06 ` ktkachov at gcc dot gnu.org
2015-10-21 13:51 ` olegendo at gcc dot gnu.org
2023-08-08  7:02 ` pinskia at gcc dot gnu.org

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