[Bug c++/115392] New: Missed optimization: fold `3 / (b ^ a)` to `b < 4`

[Bug c++/115392] New: Missed optimization: fold `3 / (b ^ a)` to `b < 4`

[zhiwuyazhe154 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115392

            Bug ID: 115392
           Summary: Missed optimization: fold `3 / (b ^ a)` to `b < 4`
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhiwuyazhe154 at gmail dot com
  Target Milestone: ---

Godbolt example: https://godbolt.org/z/7YE4Eq9a7

Code example:
bool m;
bool a;
void fn1(unsigned int b) {
    if((b^a)!=0) 
        m = 3 / (b ^ a); // equals to `m = b < 4`
}

Since the range of a is {0,1}, we can discuss it in different cases:
When a is 0, b^a = b^0 = b;
When a is 1, b^a = b^1 = {b, b is an even number; b - 1, b is an odd number}
In summary, b^a is less than or equal to 3 only when b < 4

GCC -O3:
fn1(unsigned int):
        movzx   eax, BYTE PTR a[rip]
        cmp     eax, edi
        je      .L1
        xor     eax, edi
        cmp     eax, 3
        setbe   BYTE PTR m[rip]
.L1:
        ret
a:
        .zero   1
m:
        .zero   1

Expected code(CLANG -O3):
fn1(unsigned int):                                
        movzx   eax, byte ptr [rip + a]
        cmp     eax, edi
        je      .LBB0_2
        cmp     edi, 4
        setb    byte ptr [rip + m]
.LBB0_2:                                
        ret
m:
        .byte   0                               

a:
        .byte   0

[Bug tree-optimization/115392] Missed optimization: fold `(a^b) <= 3` to `b < 4` for bool a and know that (a^b) is non zero

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115392

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
           Severity|normal                      |enhancement
            Summary|Missed optimization: fold   |Missed optimization: fold
                   |`3 / (b ^ a)` to `b < 4`    |`(a^b) <= 3` to `b < 4` for
                   |                            |bool a and know that (a^b)
                   |                            |is non zero
   Last reconfirmed|                            |2024-06-08
          Component|c++                         |tree-optimization
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |missed-optimization

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed. Note GCC already transform `3 / (b ^ a)` into `(a^b) <= 3`. But does
not transform that into `b <= 3` when we know that `a` has a [0,1] range.


A simple pattern like:
```
(for cmp (gt le)
 (simplify
  (cmp:c (bit_xor @0 zero_one_valued_p@1) INTEGER_CST@2)
  (if ((wi::to_wide (2) & 1) == 1)
   (cmp @0 @2)))
```
Should work.

Note this can be expanded into more if needed