[Bug rtl-optimization/85950] New: Unsafe-math-optimizations regresses optimization using SSE4.1 roundss

[linux at carewolf dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85950

            Bug ID: 85950
           Summary: Unsafe-math-optimizations regresses optimization using
                    SSE4.1 roundss
           Product: gcc
           Version: 8.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linux at carewolf dot com
  Target Milestone: ---

When SSE4.1 is available, std::floor, std::ceil and their C counterparts are
inlined to being a single roundss instruction.

However if compiled with -Ofast, -ffast-math or -funsafe-math-optimization
specifically, then you instead get a slightly improved version of the much
slower SSE2 implementation of the same functions.

For instance compiling this with -msse4.1:

#include <cmath>
double stdfloor(double a)
{
    return std::floor(a);
}

double stdceil(double a)
{
    return std::ceil(a);
}