[Bug optimization/14796] New: [tree-ssa] combine two shifts into one

[kazu at cs dot umass dot edu]

void bar (void);

unsigned int
foo (unsigned int a)
{
  a <<= 16;
  a >>= 16;
  return a;
}

unsigned int
baz (unsigned int a)
{
  a >>= 1;
  a >>= 1;
  return a;
}

I get:

;; Function foo (foo)

foo (a)
{
<bb 0>:
  a_2 = a_1 << 16;
  a_3 = a_2 >> 16;
  return a_3;

}



;; Function baz (baz)

baz (a)
{
  unsigned int pretmp.0;

<bb 0>:
  a_2 = a_1 >> 1;
  a_3 = a_2 >> 1;
  return a_3;

}

foo() could use an AND.  baz() could simply say "a_3 = a_1 >> 2;".

-- 
           Summary: [tree-ssa] combine two shifts into one
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: enhancement
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kazu at cs dot umass dot edu
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14796