[llvm-bugs] [Bug 35594] New: Missed optimization in math expression: simplify calls std::exp functions

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Sun Dec 10 04:24:31 PST 2017


https://bugs.llvm.org/show_bug.cgi?id=35594

            Bug ID: 35594
           Summary: Missed optimization in math expression: simplify calls
                    std::exp functions
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: zamazan4ik at tut.by
                CC: llvm-bugs at lists.llvm.org

clang(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags for this
code:

#include <cmath>

double f(double a, double b)
{
    return std::exp(a) * std::exp(b);
}

generates this assembly:


f(double, double): # @f(double, double)
  sub rsp, 24
  vmovsd qword ptr [rsp + 16], xmm1 # 8-byte Spill
  call exp
  vmovsd qword ptr [rsp + 8], xmm0 # 8-byte Spill
  vmovsd xmm0, qword ptr [rsp + 16] # 8-byte Reload
  call exp
  vmulsd xmm0, xmm0, qword ptr [rsp + 8] # 8-byte Folded Reload
  add rsp, 24
  ret


GCC (trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags generates
more optimal code:


f(double, double):
        vaddsd  xmm0, xmm0, xmm1
        jmp     __exp_finite

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