[Bug tree-optimization/48732] New: Nested loops with small iteration count gobble time in "tree reassociation"

[arthur.j.odwyer at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48732

           Summary: Nested loops with small iteration count gobble time in
                    "tree reassociation"
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: arthur.j.odwyer@gmail.com


Created attachment 24076
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24076
Output of "gcc-4.5 -w -O1 -S test.c -Q -v"

The following test case takes inordinately long to compile on my machine
(Ubuntu 10.10, 64-bit), starting with gcc-4.5.

cat >test.c <<EOF
void func_47()
{
  int a, b, c, d, e;
  for (a=0; a < 8; ++a) {
    for (b=0; b < 8; ++b) {
      for (c=0; c < 8; ++c) {
        for (d=0; d < 8; ++d) {
          int l_752[8];
          int j;
          for (j = 0; j < 8; j++)
            l_752[j] = 0;
        }
      }
    }
  }
}
EOF
time gcc -w -O1 -S test.c -Q

On my machine, the timings are as follows:
With gcc-4.4: real 0.027s
With gcc-4.5: real 5.801s, spent 94% of that time in "tree reassociation"
With trunk:   real 3.567s, spent 77% of that time in "tree reassociation"

The only difference in the assembly output is that gcc-4.4 uses "ret" and
gcc-4.5 uses "rep ret".
Adding a fifth nested loop with 8 iterations makes compilation take more than
300 seconds; I killed it at that point.