[Bug rtl-optimization/19702] New: suboptimal multiple branch comparison code produced

[yuri at tsoft dot com]

When two comparisons immediately follow each other that compare same numbers but
branch twice based on 3 possible comparison outcomes actual comparison is
performed twice in asm code. One comparision suffices.

Also if second branches are strongly disadvantaged even pre-jxx mov for each
branch should be moved outside of the main body of the function.

Yuri (yuri at ONLYtsoftLOWER CASEcomMATTERS)


-- begin code --
int f(int k) {
  if (k < 300)
    return (10);
  if (k > 300)
    return (-10);
  return (0);
}

int main() { f(299); }
-- end code --

obtained assembly log:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   8b 55 08                mov    0x8(%ebp),%edx
   6:   81 fa 2b 01 00 00       cmp    $0x12b,%edx
   c:   b8 0a 00 00 00          mov    $0xa,%eax
  11:   7e 0f                   jle    22 <_Z1fi+0x22>
  13:   81 fa 2c 01 00 00       cmp    $0x12c,%edx
  19:   b8 f6 ff ff ff          mov    $0xfffffff6,%eax
  1e:   7f 02                   jg     22 <_Z1fi+0x22>
  20:   31 c0                   xor    %eax,%eax
  22:   c9                      leave
  23:   c3                      ret

expected assembly log:
  :     55                      push   %ebp
  :     89 e5                   mov    %esp,%ebp
  :     8b 55 08                mov    0x8(%ebp),%edx
  :     81 fa 2b 01 00 00       cmp    $0x12b,%edx
  :     b8 0a 00 00 00          mov    $0xa,%eax
  :     7e 0f                   jle    22 <_Z1fi+ret>
  :     b8 f6 ff ff ff          mov    $0xfffffff6,%eax
  :     7f 02                   je     22 <_Z1fi+ret>
  :     31 c0                   xor    %eax,%eax
ret:    c9                      leave
  :     c3                      ret

-- 
           Summary: suboptimal multiple branch comparison code produced
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: yuri at tsoft dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19702