[Bug target/37734] New: Missing optimization: gcc fails to reuse flags from already calculated expression for condition check with zero

[siarhei dot siamashka at gmail dot com]

For the following source:
/****************************************/
extern void a();

int unrolled_loop_fn(int count)
{
    while ((count -= 2) >= 0) {
        a();
        a();
    }
    if (count & 1) {
        a();
    }
}
/****************************************/

'gcc -O2 -c test.c' produces the following quite suboptimal code:

00000000 <unrolled_loop_fn>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   56                      push   %esi
   4:   8b 75 08                mov    0x8(%ebp),%esi
   7:   53                      push   %ebx
   8:   83 ee 02                sub    $0x2,%esi
   b:   85 f6                   test   %esi,%esi
   d:   89 f0                   mov    %esi,%eax
   f:   78 1c                   js     2d <unrolled_loop_fn+0x2d>
  11:   89 f3                   mov    %esi,%ebx
  13:   90                      nop
  14:   8d 74 26 00             lea    0x0(%esi),%esi
  18:   e8 fc ff ff ff          call   19 <unrolled_loop_fn+0x19>
  1d:   e8 fc ff ff ff          call   1e <unrolled_loop_fn+0x1e>
  22:   83 eb 02                sub    $0x2,%ebx
  25:   79 f1                   jns    18 <unrolled_loop_fn+0x18>
  27:   83 e6 01                and    $0x1,%esi
  2a:   8d 46 fe                lea    -0x2(%esi),%eax
  2d:   a8 01                   test   $0x1,%al
  2f:   74 05                   je     36 <unrolled_loop_fn+0x36>
  31:   e8 fc ff ff ff          call   32 <unrolled_loop_fn+0x32>
  36:   5b                      pop    %ebx
  37:   5e                      pop    %esi
  38:   5d                      pop    %ebp
  39:   c3                      ret


-- 
           Summary: Missing optimization: gcc fails to reuse flags from
                    already calculated expression for condition check with
                    zero
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: siarhei dot siamashka at gmail dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37734