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From: "ktietz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/51781] New: Missed optimization for ==/!= comparison type-sinking
Date: Sat, 07 Jan 2012 11:00:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51781

             Bug #: 51781
           Summary: Missed optimization for ==/!= comparison type-sinking
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ktietz@gcc.gnu.org
            Target: *-*-*


The following code shows an issue about missed optimization for ==/!=
comparisons.

extern unsigned char ar[256];

int foo (int x, int y)
{
  int i = (int) ar[x];
  return (i == (y & 0xff));
}

Compiled with 4.7 using -O2 we get the following optimized tree:

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

foo (int x, int y)
{
  int i;
  _Bool D.2716;
  int D.2715;
  int D.2714;
  unsigned char D.2713;

<bb 2>:
  D.2713_2 = ar[x_1(D)];
  i_3 = (int) D.2713_2;
  D.2715_5 = y_4(D) & 255;
  D.2716_6 = D.2715_5 == i_3;
  D.2714_7 = (int) D.2716_6;
  return D.2714_7;

}

But to be expected would be:

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

foo (int x, int y)
{
  unsigned char D.1719;
  _Bool D.1716;
  int D.1714;
  unsigned char D.1713;

<bb 2>:
  D.1713_2 = ar[x_1(D)];
  D.1719_9 = (unsigned char) y_4(D);
  D.1716_6 = D.1713_2 == D.1719_9;
  D.1714_7 = (int) D.1716_6;
  return D.1714_7;

}