[Bug optimization/14341] Missed comparision optimization (jump threading related)

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

------- Additional Comments From steven at gcc dot gnu dot org  2004-02-29 14:19 -------
Tree dump after DCE1: 
 
t () 
{ 
  int x; 
  int i; 
  int T.0; 
 
  # BLOCK 0 
  # PRED: ENTRY (fallthru) 
  i_2 = 0; 
  goto <bb 2> (<L1>); 
  # SUCC: 2 (fallthru) 
 
  # BLOCK 1 
  # PRED: 2 (true) 
<L0>:; 
  T.0_3 = i_1 <= 99999999; 
  f (T.0_3); 
  i_4 = i_1 + 1; 
  # SUCC: 2 (fallthru) 
 
  # BLOCK 2 
  # PRED: 1 (fallthru) 0 (fallthru) 
  # i_1 = PHI <i_2(0), i_4(1)>; 
<L1>:; 
  if (i_1 <= 99999999) goto <L0>; else goto <L2>; 
  # SUCC: 3 (false) 1 (true) 
 
  # BLOCK 3 
  # PRED: 2 (false) 
<L2>:; 
  return; 
  # SUCC: EXIT 
 
} 
 
The dominator tree looks like this: 
 
bb0 
| 
bb2 (contains loop exit condition "i < 100000000") 
|\ 
| \ 
|  \ 
|   bb1 (contains "f(i < 100000000);") 
| 
bb3 
 
So the loop exit condition dominates the function call and we should be able 
to optimize that test. 
 
 
 
Tree dump after DOM1: 
 
t () 
{ 
  int x; 
  int i; 
  int T.0; 
 
  # BLOCK 0 
  # PRED: ENTRY (fallthru) 
  i_5 = 0; 
  i_6 = 0; 
  # SUCC: 1 (fallthru) 
 
  # BLOCK 1 
  # PRED: 0 (fallthru) 1 (true) 
  # i_1 = PHI <0(0), i_7(1)>; 
<L0>:; 
  T.0_3 = i_1 <= 99999999; 
  f (T.0_3); 
  i_7 = i_1 + 1; 
  if (i_7 <= 99999999) goto <L0>; else goto <L2>; 
  # SUCC: 2 (false) 1 (true) 
 
  # BLOCK 2 
  # PRED: 1 (false) 
<L2>:; 
  return; 
  # SUCC: EXIT 
 
} 
 
 
The dominator tree looks like this: 
 
bb0 
| 
bb1 (contains loop exit condition "i < 100000000" 
|    and "f(i < 100000000);") 
| 
bb3 
 
So the loop exit condition no longer dominates the function call, and we lose. 
 
So perhaps we are threading jumps too early. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14341