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From: "zhiwuyazhe154 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/115350] New: Missing optimzation: fold `n = std::min(f ? 0
 : 3, -a)` to `n = -a`
Date: Wed, 05 Jun 2024 00:53:28 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 15.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: zhiwuyazhe154 at gmail dot com
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D115350

            Bug ID: 115350
           Summary: Missing optimzation: fold `n =3D std::min(f ? 0 : 3,
                    -a)` to `n =3D -a`
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          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/j9rj5q6W5

Code Example:
unsigned short n;
#include <algorithm>
void fn1( unsigned short f, unsigned char a) {
    n =3D std::min(f ? 0 : 3, -a); // equals to "n =3D -a"
}

During the operation, -a will be converted to int type, and its value range=
 is
[-255, 0], which must be less than 0 or 3, so =E2=80=9Cn =3D std::min(f ? 0=
 : 3, -a)=E2=80=9D is
equivalent to "n =3D -a".

GCC -O3:
fn1(unsigned short, unsigned char):
        test    di, di
        je      .L5
        xor     eax, eax
        test    sil, sil
        jne     .L5
        mov     WORD PTR n[rip], ax
        ret
.L5:
        movzx   eax, sil
        neg     eax
        mov     WORD PTR n[rip], ax
        ret
n:
        .zero   2

Expected code (CLANG -O3):
fn1(unsigned short, unsigned char):=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20
        neg     esi
        mov     word ptr [rip + n], si
        ret
n:
        .short  0=