[Bug tree-optimization/51780] New: Missed optimization for ==/!= comparison

[ktietz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51780

             Bug #: 51780
           Summary: Missed optimization for ==/!= comparison
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: kti...@gcc.gnu.org
            Target: *-*-*


Hi,

optimization misses to eliminate addition for 'b' in following code:

extern unsigned int ar[256];

int foo (int a, unsigned int b)
{
  int c = ar[a] + b;
  int d = (int) b;
  return c != d;
}

compiled this code with -O2 we get tree-optimized as follwing:

;; Function foo (foo, funcdef_no=0, decl_uid=1600, cgraph_uid=0)

foo (int a, unsigned int b)
{
  int d;
  int c;
  _Bool D.2717;
  int D.2716;
  unsigned int D.2715;
  unsigned int D.2714;

<bb 2>:
  D.2714_2 = ar[a_1(D)];
  D.2715_4 = D.2714_2 + b_3(D);
  c_5 = (int) D.2715_4;
  d_6 = (int) b_3(D);
  D.2717_7 = c_5 != d_6;
  D.2716_8 = (int) D.2717_7;

<L0>:
  return D.2716_8;

}

but optimized variant should be:

;; Function foo (foo, funcdef_no=0, decl_uid=1709, cgraph_uid=0)

foo (int a, unsigned int b)
{
  _Bool D.1717;
  int D.1716;
  unsigned int D.1714;

<bb 2>:
  D.1714_2 = ar[a_1(D)];
  D.1717_7 = D.1714_2 != 0;
  D.1716_8 = (int) D.1717_7;
  return D.1716_8;

}