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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/59239] [SH] Improve decrement-and-test insn
Date: Sat, 10 May 2014 13:03:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59239

--- Comment #3 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Oleg Endo from comment #0)
> 
> The decrement-and-test insn seems to rely on the define_peephole.  At least
> taking it out shows some missed opportunities in the CSiBE set.
> I've tried replacing it with a define_peephole2, but it would still show
> missed cases such as:
>    mov.l  @(...), Rn
>    add    #-1,Rn
>    mov.l  Rn,@(...)
>    tst    Rn,Rn

This happens especially when the mems are volatile.  An isolated example:

int test (volatile int* x, int a, int b, int c, int d)
{
  int xx = x[2] - 1;
  x[2] = xx;
  if (xx == 0)
    return a;

  return b + c;
}

Currently compiles to (-O2):
        mov.l   @(8,r4),r1
        add     #-1,r1
        mov.l   r1,@(8,r4)
        tst     r1,r1
        bt      .L5
        mov     r6,r5
        add     r7,r5
.L5:
        rts
        mov     r5,r0

which could be:
        mov.l   @(8,r4),r1
        dt      r1
        mov.l   r1,@(8,r4)
        bt      .L5
        mov     r6,r5
        add     r7,r5
.L5:
        rts
        mov     r5,r0

The problem seems that combine will not look across volatile mems when picking
insns for combination.

In this particular case it could be fixed by a manual combine step, i.e. make
the cmpeqsi_t an insn_and_split and in the split pass after combine check the
previous insns if they are a (plus (reg) (const_int -1)), then replace the
insns if matched.