[Bug rtl-optimization/19078] [4.0 Regression] Poor quality code after loop unrolling.

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

------- Additional Comments From rakdver at gcc dot gnu dot org  2004-12-19 19:41 -------
Unroller splits the induction variables, so that the final code looks basically 
like

if (a[0] == 2)
  return a;
if (a[1] == 2)
  return a + 4;
if (a[2] == 2)
  return a + 8;
...
if (a[7] == 2)
  return a + 28;
a+=32;

Which is good in some cases, but obviously not here.

However even with -fno-split-ivs-in-unroller we do not get the autoincrements; 
we also need -fno-ivopts.  The reason is that with ivopts the code looks like

a = a.1;
a.1 = a + 1;
if (*a == 2)
  return a;

Whereas the old loop optimizer makes things look like

a = a + 1
if (*a == 2)
  return 0;

by changing the initial value of a, which enables the autoinc creation pass to 
work.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19078