[Bug tree-optimization/24696] New: missing optimization in comparison of results of bit operations

["drepper at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Take this little program:

int
f (unsigned long a, unsigned long b, unsigned long c)
{
  return (a & (c - 1)) != 0 || (b & (c - 1)) != 0;
}

Compiled on x86-64 with gcc 4.0.2 (but I think also with the current mainline)
yields with -O2 the following code:

0000000000000000 <f>:
   0:   48 ff ca                dec    %rdx
   3:   48 85 d7                test   %rdx,%rdi
   6:   75 07                   jne    f <f+0xf>
   8:   31 c0                   xor    %eax,%eax
   a:   48 85 d6                test   %rdx,%rsi
   d:   74 05                   je     14 <f+0x14>
   f:   b8 01 00 00 00          mov    $0x1,%eax
  14:   f3 c3                   repz retq

As can be seen, both comparisons are executed individually.  This is
unnecessarily slow.  Since the right operand for & is the same and this is a
pure bit-test it is perfectly fine to compile the code to the equivalent of

int
f (unsigned long a, unsigned long b, unsigned long c)
{
  return ((a | b) & (c - 1)) != 0;
}

This would be significantly faster.  On archs like x86-64 no conditional jump
(just a setne) would be needed.


-- 
           Summary: missing optimization in comparison of results of bit
                    operations
           Product: gcc
           Version: 4.0.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: drepper at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24696