[Bug c/105276] New: [12 Regression] executed once loop not optimized anymore

[denis.campredon at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105276

            Bug ID: 105276
           Summary: [12 Regression]  executed once loop not optimized
                    anymore
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: denis.campredon at gmail dot com
  Target Milestone: ---

bool foo(unsigned i) {
    bool result = true;
    while (i) {
        i = i % 3;
        i = i - (i==2 ? 2 : i ? 1 : 0);
        result = !result;
    }
    return result;
}
--------------
compiled with g++ 11.2 and -O2 it produces:

-----------------
foo(unsigned int):
        test    edi, edi
        sete    al
        ret
----------------

With current trunk and -02 lots of instructions are generated, the loop is
still present, about 30 instructions are produced.


Also, when compiled with -Os trunk produces loopless assembly:
------------------
foo(unsigned int):
        mov     dl, 1
        test    edi, edi
        je      .L1
        xor     edx, edx
.L1:
        mov     eax, edx
        ret
-------------------
Whereas using -Os and g++ 11.2 it uses one less register:
------------------
foo(unsigned int):
        mov     al, 1
        test    edi, edi
        je      .L4
        xor     eax, eax
.L4:
        ret