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From: "antoshkka at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/110170] New: Sub-optimal conditional jumps in
 conditional-swap with floating point
Date: Thu, 08 Jun 2023 11:31:05 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: antoshkka at gmail dot com
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D110170

            Bug ID: 110170
           Summary: Sub-optimal conditional jumps in conditional-swap with
                    floating point
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Some of the C++ algorithms are written in attempt to avoid conditional jump=
s in
tight loops. For example, code close the following could be seen in libc++:

void __cond_swap(double* __x, double* __y) {
  bool __r =3D (*__x < *__y);
  auto __tmp =3D __r ? *__x : *__y;
  *__y =3D __r ? *__y : *__x;
  *__x =3D __tmp;
}


GCC-14 with -O2 and -march=3Dx86-64 options generates the following code:

__cond_swap(double*, double*):
        movsd   xmm1, QWORD PTR [rdi]
        movsd   xmm0, QWORD PTR [rsi]
        comisd  xmm0, xmm1
        jbe     .L2
        movq    rax, xmm1
        movapd  xmm1, xmm0
        movq    xmm0, rax
.L2:
        movsd   QWORD PTR [rsi], xmm1
        movsd   QWORD PTR [rdi], xmm0
        ret


A conditional jump could be probably avoided in the following way:

__cond_swap(double*, double*):
        movsd   xmm0, qword ptr [rdi]
        movsd   xmm1, qword ptr [rsi]
        movapd  xmm2, xmm0
        minsd   xmm2, xmm1
        maxsd   xmm1, xmm0
        movsd   qword ptr [rsi], xmm1
        movsd   qword ptr [rdi], xmm2
        ret


Playground: https://godbolt.org/z/v3jW67x91=