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From: "olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/65214] New: [SH] Optimize sign/zero extensions across basic blocks
Date: Wed, 25 Feb 2015 22:27:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65214

            Bug ID: 65214
           Summary: [SH] Optimize sign/zero extensions across basic blocks
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
            Target: sh*-*-*

This is an example from gcc.c-torture/execute/pr20187-1.c:

int a = 0x101;
int b = 0x100;

int
test (void)
{
  return (((unsigned char) (unsigned long long) ((a ? a : 1) & (a * b)))
      ? 0 : 1);
}

compiled with -m4 -ml -O2 -fdisable-rtl-dbr:

        mov.l   .L6,r1
        mov.l   @r1,r2
        mov     #1,r1     // r1 is zero extended after constant load
        tst     r2,r2
        bt      .L2
        exts.b  r2,r1
.L2:
        mov.l   .L7,r3
        extu.b  r1,r1     // r1 = extu.b (exts.b (r2))
        mov.l   @r3,r3
        mulu.w  r2,r3
        sts     macl,r3
        tst     r3,r1
        movt    r0
        rts
        nop

The extu.b basically overrides result of exts.b.  Thus we can replace the
exts.b with extu.b and eliminate one insn:

        mov.l   .L6,r1
        mov.l   @r1,r2
        mov     #1,r1
        tst     r2,r2
        bt      .L2
        extu.b  r2,r1
.L2:
        mov.l   .L7,r3
        mov.l   @r3,r3
        mulu.w  r2,r3
        sts     macl,r3
        tst     r3,r1
        movt    r0
        rts
        nop