[Bug tree-optimization/23855] loop header should also be pulled out of the inner loop too

[rguenth at gcc dot gnu dot org]

------- Comment #11 from rguenth at gcc dot gnu dot org  2006-04-07 11:34 
-------
I updated the patch for current mainline, but it still has issues for some
common uses of loops:

void foo(int *ie, int *je, double *x)
{
  int i, j;
  for (j=0; j<*je; ++j)
    for (i=0; i<*ie; ++i)
      x[i+j] = 0.0;
}

After loop header copying we have

  if (*je > 0)
    for (j=0; j<*je; ++j)
      if (*ie > 0)
        for (i=0; i<*ie; ++i)
          x[i+j ] = 0.0;

note how in this form we see the condition *ie > 0 not invariant wrt the
outer loop (because it does a memory load), but if we would run load-PRE
between loop header copying and guard hoisting we would get

  pretmp.1 = *je;
  if (pretmp.1 > 0)
    pretmp.2 = *ie;
    for (j=0; j<pretmp.1; ++j)
      if (pretmp.2 > 0)
        for (i=0; i<pretmp.2; ++i)
          x[i+j] = 0.0;

which would enable us to hoist the guard out of the outer loop.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23855