[Bug tree-optimization/114230] New: Missed optimization of loop deletion: a=0||a

[Bug tree-optimization/114230] New: Missed optimization of loop deletion: a=0||a

[652023330028 at smail dot nju.edu.cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114230

            Bug ID: 114230
           Summary: Missed optimization of loop deletion: a=0||a
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: 652023330028 at smail dot nju.edu.cn
  Target Milestone: ---

Hello, we noticed that in the code below, looping is not necessary (the value
of 0||a doesn't change), but gcc seems to have missed this optimization.

https://godbolt.org/z/bx9jEfb63

int a;
void func(){
    for(int i=0;i<100000;i++){
        a=0||a;
    }
}

GCC -O3:
func():
        mov     edx, DWORD PTR a[rip]
        mov     eax, 100000
.L2:
        test    edx, edx
        setne   dl
        movzx   edx, dl
        sub     eax, 1
        jne     .L2
        mov     DWORD PTR a[rip], edx
        ret

Expected code (Clang):
func():                               # @func()
        xor     eax, eax
        cmp     dword ptr [rip + a], 0
        setne   al
        mov     dword ptr [rip + a], eax
        ret

Thank you very much for your time and effort! We look forward to hearing from
you.

[Bug tree-optimization/114230] Missed optimization of loop deletion: `a!=0`

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114230

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
            Summary|Missed optimization of loop |Missed optimization of loop
                   |deletion: a=0||a            |deletion: `a!=0`
                 CC|                            |pinskia at gcc dot gnu.org
           Keywords|                            |missed-optimization
           Severity|normal                      |enhancement
   Last reconfirmed|                            |2024-03-05

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.
we have:
```
  <bb 3> [local count: 1063004408]:
  # i_11 = PHI <i_8(5), 0(2)>
  # a_lsm.4_13 = PHI <_3(5), a_lsm.4_5(2)>
  _2 = a_lsm.4_13 != 0;
  _3 = (int) _2;
  i_8 = i_11 + 1;
  if (i_8 != 100000)
    goto <bb 5>; [98.99%]
  else
    goto <bb 4>; [1.01%]

  <bb 5> [local count: 1052266995]:
  goto <bb 3>; [100.00%]
```

Which sccp does not handle `(int)a != 0` currently. It does handle `a|=b;`,
`a^=b;`, and `a&=b;` though.