[Bug other/17101] New: Opportunity to collapse and eliminate loop

["gcc-bugzilla at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

Description:
A non-optimal code sequence is illustraded.    Duplicate using gcc 3.5 and
command line:

gcc -O3 -m64 -funroll-loops -c test.c


Testcase:
int st;

void foo ()
{

  int i;
  int j;

  for (i=0; i < 10; i++)
    for (j=0; j < 10; j++)
      st = st + j;

}

Assembly:
Code currently generated collapses the outer loop, but not the inner loop:

.foo:
      li 0,10
      ld 11,.LC0@toc(2)
      mtctr 0
.L2:
      lwz 9,0(11)
      addi 9,9,45
      stw 9,0(11)
      bdnz .L2
      blr

Forward store motion could remove the stw from the loop and allow it to
be collapsed to look like this:

 .foo:
       ld 11,.LC0@toc(2)
       lwz 3,0(11)
       addi 9,3,450
       stw 9,0(11)
       blr




-- 
           Summary: Opportunity to collapse and eliminate loop
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: other
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steinmtz at us dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org,steinmtz at us dot ibm
                    dot com
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17101