[Bug tree-optimization/20580] New: Using ASSERT_EXPR to improve constant propagation of conditional constants

[steven at gcc dot gnu dot org]

This is an enhancement request for constant propagation on the 
tree-cleanup-branch.  Consider the following small test case: 
 
int foo (int a, int b) { 
        if (a || b) { 
                a = 3; 
                return a + b; 
        } 
        return a + b; 
} 
 
We currently compile this to the following .optimized dump: 
foo (a, b) 
{ 
  int D.1465; 
  int D.1464; 
<bb 0>: 
  if ((a | b) != 0) goto <L0>; else goto <L1>; 
<L0>:; 
  D.1465 = b + 3; 
  goto <bb 3> (<L2>); 
<L1>:; 
  D.1465 = a + b;  /* Ah-ha! But we know both a and b must be 0!  */ 
<L2>:; 
  return D.1465; 
} 
 
Notice the "D.1465 = a + b;".  We can know that both a and b are 0 at 
this point, or the condition "(a | b)" could not have been false.  So 
this test case could be optimized to something like this: 
 
foo (a, b) 
{ 
  int D.1465; 
  int D.1464; 
<bb 0>: 
  if ((a | b) != 0) goto <L0>; else goto <L1>; 
 <L0>:; 
  D.1465 = b + 3; 
  goto <bb 3> (<L2>); 
<L1>:; 
  D.1465 = 0; 
<L2>:; 
  return D.1465; 
} 
 
In the traditional SSA-CCP algorithm this is actually difficult to do, 
but on the tree-cleanup-branch, we can insert ASSERT_EXPRs for a and b 
in the false arm of the conditional: 
 
foo (a, b) 
{ 
  int D.1465; 
  int D.1464; 
<bb 0>: 
  D.1464_4 = a_2 | b_3; 
  if (D.1464_4 != 0) goto <L0>; else goto <L1>; 
<L0>:; 
  D.1465_8 = b_3 + 3; 
  goto <bb 3> (<L2>); 
<L1>:; 
  a_7 = ASSERT_EXPR <a_2, 0>; 
  b_8 = ASSERT_EXPR <a_3, 0> 
  D.1465_6 = a_7 + b_8; 
  # D.1465_1 = PHI <D.1465_8(1), D.1465_6(2)>; 
<L2>:; 
  return D.1465_1; 
} 
 
This should allow us to propagate more constants.

-- 
           Summary: Using ASSERT_EXPR to improve constant propagation of
                    conditional constants
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, TREE
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: dnovillo at gcc dot gnu dot org
        ReportedBy: steven at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20580