[Bug rtl-optimization/58033] New: counterproductive bb-reorder

[olegendo at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58033

            Bug ID: 58033
           Summary: counterproductive bb-reorder
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: olegendo at gcc dot gnu.org
                CC: steven at gcc dot gnu.org, tejohnson at google dot com
            Target: sh*-*-*

On SH, compiling the following code with -O2

#include <bitset>

std::bitset<32> make_bits (void)
{
  std::bitset<32> r;
  for (auto&& i : { 4, 5, 6, 10 })
    if (i < r.size ())
      r.set (i);

  return r;
}

results in the following code:

        mov.l   .L8,r1
        mov     #0,r0
        mov     #31,r7
        mov     #1,r6
        mov     #4,r2
.L2:
        mov.l   @r1,r3
        cmp/hi  r7,r3
        bf/s    .L7
        mov     r6,r5
.L3:
        dt      r2
        bf/s    .L2     // branch if value not > 31, i.e. in each iteration
        add     #4,r1
        rts
        nop
        .align 1
.L7:
        shld    r3,r5
        bra     .L3
        or      r5,r0
.L9:
        .align 2
.L8:
        .long   _._45+0

_._45:
        .long   4
        .long   5
        .long   6
        .long   10

Disabling the bb-reorder pass or using -Os results in more compact and faster
code:

        mov.l   .L7,r1
        mov     #0,r0
        mov     #31,r7
        mov     #1,r6
        mov     #4,r2
.L2:
        mov.l   @r1,r3
        cmp/hi  r7,r3
        bt/s    .L3    // branch if value > 31, i.e. never.
        mov     r6,r5
        shld    r3,r5
        or      r5,r0
.L3:
        dt      r2
        bf/s    .L2
        add     #4,r1
        rts    
        nop

Of course the bb-reorder pass doesn't know that the values in this case are
always in range.  Still, maybe it could be improved by not splitting out a BB
if it consists only of a few insns?  I've tried increasing the branch cost but
it won't do anything.

Teresa, Steven,