[Bug ada/12966] New: x86 array comparison optimization

[Bug ada/12966] New: x86 array comparison optimization

[dave at synergy dot org]

gnatmake -O3 q

See OPTIMIZE below.

Ada source:

with Ada.Text_IO;
use Ada.Text_IO;

procedure Q is

type Unsigned_32 is
       mod 2 ** 32;

type A_Type is
       array (1..4) of Unsigned_32;

X: A_Type := (0, 0, 0, 0);
Y: A_Type := (0, 0, 0, 0);

begin
  if X = Y then
    Put_Line(Standard_Output, "=");
  else
    Put_Line(Standard_Output, "/=");
  end if;
end Q;

x86 object:

q.o:     file format elf32-i386

Disassembly of section .text:

--
-- procedure Q is
-- begin
--

00000000 <_ada_q>:
   0:   55                      push   %ebp
   6:   89 e5                   mov    %esp,%ebp
   b:   83 ec 38                sub    $0x38,%esp
   e:   89 7d fc                mov    %edi,0xfffffffc(%ebp)
  16:   89 75 f8                mov    %esi,0xfffffff8(%ebp)

--
-- X := (0, 0, 0, 0);
--

   8:   fc                      cld    
   9:   31 c0                   xor    %eax,%eax
   1:   ba 04 00 00 00          mov    $0x4,%edx
  11:   89 d1                   mov    %edx,%ecx
  13:   8d 7d e8                lea    0xffffffe8(%ebp),%edi
  1c:   f3 ab                   repz stos %eax,%es:(%edi)

--
-- Y := (0, 0, 0, 0);
--

  21:   89 d1                   mov    %edx,%ecx
  1e:   8d 7d d8                lea    0xffffffd8(%ebp),%edi
  23:   f3 ab                   repz stos %eax,%es:(%edi)

--
-- OPTIMIZE
--
-- if X = Y then
--
-- In the previous two sequences we knew enough to use stosl loops
-- to initialize the X and Y arrays.  In this sequence we're comparing the
-- same two arrays for equality.  However, this sequence reverts to a
-- cmpsb (not a cmpsl) loop.  There is a 4:1 execution penalty for making
-- this decision with no impact on code size.
--

  28:   b9 10 00 00 00          mov    $0x10,%ecx
  19:   8d 75 d8                lea    0xffffffd8(%ebp),%esi
  25:   8d 7d e8                lea    0xffffffe8(%ebp),%edi
  2d:   f3 a6                   repz cmpsb %es:(%edi),%ds:(%esi)

--
-- Put_Line(Standard_Output, "=");
--

  2f:   75 2f                   jne    60 <_ada_q+0x60>
  31:   e8 fc ff ff ff          call   32 <_ada_q+0x32>
                        32: R_386_PC32  ada__text_io__standard_output
  36:   ba 10 00 00 00          mov    $0x10,%edx
                        37: R_386_32    .rodata
  3b:   b9 00 00 00 00          mov    $0x0,%ecx
                        3c: R_386_32    .rodata
  40:   89 54 24 04             mov    %edx,0x4(%esp,1)
  44:   89 4c 24 08             mov    %ecx,0x8(%esp,1)
  48:   89 04 24                mov    %eax,(%esp,1)
  4b:   e8 fc ff ff ff          call   4c <_ada_q+0x4c>
                        4c: R_386_PC32  ada__text_io__put_line

--
-- end if;
-- end Q;
--

  50:   8b 75 f8                mov    0xfffffff8(%ebp),%esi
  53:   8b 7d fc                mov    0xfffffffc(%ebp),%edi
  56:   89 ec                   mov    %ebp,%esp
  58:   5d                      pop    %ebp
  59:   c3                      ret    

--
-- Put_Line(Standard_Output, "/=");
--

  5a:   8d b6 00 00 00 00       lea    0x0(%esi),%esi
  60:   e8 fc ff ff ff          call   61 <_ada_q+0x61>
                        61: R_386_PC32  ada__text_io__standard_output
  65:   ba 11 00 00 00          mov    $0x11,%edx
                        66: R_386_32    .rodata
  6a:   b9 08 00 00 00          mov    $0x8,%ecx
                        6b: R_386_32    .rodata
  6f:   eb cf                   jmp    40 <_ada_q+0x40>

-- 
           Summary: x86 array comparison optimization
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ada
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dave at synergy dot org
                CC: gcc-bugs at gcc dot gnu dot org
 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=12966

[Bug ada/12966] x86 array comparison optimization

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-08 18:24 -------
The problem is that ada is makes a "call" to memcmp which has to be done with byte wise, so 
maybe the builtin expand needs to be help to "fix" (help out) for the call to memcmp.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
  GCC build triplet|i686-pc-linux-gnu           |
   GCC host triplet|i686-pc-linux-gnu           |
 GCC target triplet|i686-pc-linux-gnu           |
           Keywords|                            |pessimizes-code
   Last reconfirmed|0000-00-00 00:00:00         |2003-11-08 18:24:13
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12966

[Bug ada/12966] x86 array comparison optimization

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |16996
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12966

[Bug middle-end/12966] x86 array comparison optimization

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-10-29 14:05 -------
Well Ada does not build a memcmp but the middle-end does and we don't optimize memcmp well in 
the middle-end/back-end that well.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|ada                         |middle-end
   Last reconfirmed|2004-10-11 13:51:30         |2004-10-29 14:05:26
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12966

[Bug middle-end/12966] x86 array comparison optimization

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
   Last reconfirmed|2004-10-29 14:05:26         |2005-04-07 07:59:45
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12966