[Bug tree-optimization/23346] New: [4.1 Regression] FRE before DCE makes a mess of loads or need to sink loads

[pinskia at gcc dot gnu dot org]

Take the following code:
int a, b;

int f(int bool1)
{
  int c;
  int c1 = a/b;
  if(bool1)
    c = 0;
  else
    c = a/b;
  return c;
}

On the mainline, we produce (at -O2 -fomit-frame-pointer):
f:
        subl    $8, %esp
        xorl    %ecx, %ecx
        movl    12(%esp), %eax
        movl    %ebx, (%esp)
        movl    b, %edx
        movl    %esi, 4(%esp)
        movl    a, %ebx
        testl   %eax, %eax
        jne     .L4
        movl    %ebx, %eax
        movl    %edx, %esi
        cltd
        idivl   %esi
        movl    %eax, %ecx
.L4:
        movl    (%esp), %ebx
        movl    %ecx, %eax
        movl    4(%esp), %esi
        addl    $8, %esp
        ret

Which is just a mess.
in 4.0.0, we produced:
f:
        movl    4(%esp), %edx
        xorl    %eax, %eax
        testl   %edx, %edx
        jne     .L4
        movl    a, %eax
        cltd
        idivl   b
.L4:
        ret

There are two ways of fixing this, one way is to sink loads; the other simple way (most likely for 4.1) is 
to call dce before fre.  I think the reason why the testcase which Steven committed did not catch this is 
because sink will sink the divide which is what is only in Steven's testcase.

-- 
           Summary: [4.1 Regression] FRE before DCE makes a mess of loads or
                    need to sink loads
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23346