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From: "soonts at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96733] New: std::clamp for floats and doubles produces
 worse code than a combo of std::min / std::max
Date: Fri, 21 Aug 2020 10:04:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96733

            Bug ID: 96733
           Summary: std::clamp for floats and doubles produces worse code
                    than a combo of std::min / std::max
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: soonts at gmail dot com
  Target Milestone: ---

C++/17 introduced std::clamp in the standard library.

Very useful, but the code gcc generates for it is not optimal for doubles a=
nd
floats. Source code:

// This version is good in gcc, not so on vc++
double clamp_minmax( double x, double i, double ax )
{
    return std::min( std::max( x, i ), ax );
}

// This version should be identical but is not.
// gcc compiles to scalar comparisons
// vc++ code is outright horrible, compiles into RAM access.
double clamp_builtin( double x, double i, double ax )
{
    return std::clamp( x, i, ax );
}=20

Demo there: https://godbolt.org/z/Gnvc1s=