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From: "core13 at gmx dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/99930] New: Failure to optimize floating point -abs(x)
 in nontrivial code at -O2/3
Date: Tue, 06 Apr 2021 10:38:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99930

            Bug ID: 99930
           Summary: Failure to optimize floating point -abs(x) in
                    nontrivial code at -O2/3
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: core13 at gmx dot net
  Target Milestone: ---

Expected compiler output for -abs(x) is an orps setting the sign bit.


It works as expected with trivial code at -O1/2/3 optimization levels:

float q(float p)
{
    return -std::abs(p);
}

orps    xmm0, XMMWORD PTR .LC1[rip]
ret


With more complex code the compiler uses orps at -O1 but andps + xorps at
-O2/3:

bool t(float n[2], float m)
{
    for (int i =3D 0; i < 2; i++)
        if (m > -std::abs(n[i]))
            return true;
    return false;
}

-O1
movss   xmm1, DWORD PTR [rdi]
orps    xmm1, XMMWORD PTR .LC1[rip]
comiss  xmm0, xmm1
ja      .L3
movss   xmm1, DWORD PTR [rdi+4]
orps    xmm1, XMMWORD PTR .LC1[rip]
comiss  xmm0, xmm1
seta    al
ret

-O2/3
movss   xmm1, DWORD PTR [rdi]
movss   xmm3, DWORD PTR .LC0[rip]
movss   xmm2, DWORD PTR .LC1[rip]
andps   xmm1, xmm3
xorps   xmm1, xmm2
comiss  xmm0, xmm1
ja      .L3
movss   xmm1, DWORD PTR [rdi+4]
andps   xmm1, xmm3
xorps   xmm1, xmm2
comiss  xmm0, xmm1
seta    al
ret

https://godbolt.org/z/5ch5ceEj7=