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From: "denis.campredon at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/105276] New: [12 Regression]  executed once loop not
 optimized anymore
Date: Thu, 14 Apr 2022 13:47:31 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105276

            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 =3D true;
    while (i) {
        i =3D i % 3;
        i =3D i - (i=3D=3D2 ? 2 : i ? 1 : 0);
        result =3D !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=