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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/112835] inverting the result of memcmp() produces
 inefficient code
Date: Sun, 03 Dec 2023 20:35:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112835

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2023-12-03
             Target|                            |x86_64-linux-gnu
                   |                            |aarch64-linux-gnu
          Component|target                      |middle-end
           Keywords|                            |missed-optimization
     Ever confirmed|0                           |1

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

aarch64 code generation is slightly better:
...
.L2:
        mov     w0, 1
        eor     w0, w0, 1
        ret
...
        mov     w0, 0
        eor     w0, w0, 1
        ret

But there is no reason for the mov followed by eor there, it should just be=
 a
mov.

Note the gimple level is fine:
  _1 =3D __builtin_memcmp_eq (a_3(D), b_4(D), 12);
  _5 =3D _1 =3D=3D 0;


Expansion does:
```
(jump_insn 14 13 33 4 (set (pc)
        (if_then_else (ne (reg:CC 66 cc)
                (const_int 0 [0]))
            (label_ref 19)
            (pc))) "/app/example.cpp":5:24 -1
     (int_list:REG_BR_PROB 708669604 (nil))
 -> 19)
(note 33 14 15 5 [bb 5] NOTE_INSN_BASIC_BLOCK)
(insn 15 33 16 5 (set (reg:SI 92 [ _1 ])
        (const_int 0 [0])) "/app/example.cpp":5:24 -1
     (nil))
(jump_insn 16 15 17 5 (set (pc)
        (label_ref 21)) "/app/example.cpp":5:24 -1
     (nil)
 -> 21)
(barrier 17 16 18)
(barrier 18 17 19)
(code_label 19 18 34 6 2 (nil) [2 uses])
(note 34 19 20 6 [bb 6] NOTE_INSN_BASIC_BLOCK)
(insn 20 34 21 6 (set (reg:SI 92 [ _1 ])
        (const_int 1 [0x1])) "/app/example.cpp":5:24 -1
     (nil))
(code_label 21 20 35 7 3 (nil) [1 uses])
(note 35 21 22 7 [bb 7] NOTE_INSN_BASIC_BLOCK)
(insn 22 35 23 7 (set (reg:CC 66 cc)
        (compare:CC (reg:SI 92 [ _1 ])
            (const_int 0 [0]))) "/app/example.cpp":5:33 discrim 1 -1
     (nil))
(insn 23 22 24 7 (set (reg:SI 102)
        (eq:SI (reg:CC 66 cc)
            (const_int 0 [0]))) "/app/example.cpp":5:33 discrim 1 -1
     (nil))

```

Seems like we could do a better job at expansion here ...=