[Bug optimization/13629] New: [tree-ssa][Regression] dominator optimizations can pessimize code

[steven at gcc dot gnu dot org]

If an expression dominates another one on a path, the redundant 
expression is always eliminated.  This can pessimize the code 
if the original expression was dead code: 
 
int *data; 
void partial_dead (int i, int j) 
{ 
  int k = i * j; 
  if (i & j) 
    data[0] = i * j; 
} 
 
--> 
 
;; Function partial_dead (partial_dead) 
 
partial_dead (i, j) 
{ 
  int k; 
 
<bb 0>: 
  k = i * j; 
  if ((i & j) != 0) goto <L0>; else goto <L1>; 
 
<L0>:; 
  *data = k; 
 
<L1>:; 
  return; 
} 
 
This is, in fact, a regression: 
 
Resulting assembly with tree-ssa: 
partial_dead: 
        movl    4(%esp), %edx   # i 
        movl    8(%esp), %eax   # j 
        movl    %edx, %ecx      # k = i 
        imull   %eax, %ecx      # k = i * j 
        testl   %eax, %edx      # if (j & i) 
        je      .L1             #   goto <L1> 
        movl    data, %eax 
        movl    %ecx, (%eax)    # *data = k 
.L1: 
        ret 
 
Resulting assembly with 3.3.1 (SuSE Linux): 
partial_dead: 
        movl    4(%esp), %edx   #  i 
        movl    8(%esp), %eax   #  j 
        testl   %eax, %edx      #  if (j &  i) 
        je      .L1		#    goto <L1> 
        imull   %eax, %edx 
        movl    data, %eax 
        movl    %edx, (%eax)    #  *data = i * j 
.L1: 
        ret

-- 
           Summary: [tree-ssa][Regression] dominator optimizations can
                    pessimize code
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steven at gcc dot gnu dot org
                CC: dnovillo at redhat dot com,gcc-bugs at gcc dot gnu dot
                    org,law at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629