[llvm-bugs] [Bug 35602] New: Missed optimization in math expression: tan(a) * cos(a) == sin(a)

via llvm-bugs llvm-bugs at lists.llvm.org
Sun Dec 10 07:25:38 PST 2017


            Bug ID: 35602
           Summary: Missed optimization in math expression: tan(a) *
                    cos(a) == sin(a)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            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

#include <cmath>

double test(double a)
    return tan(a) * cos(a);

generates this assembly:

test(double): # @test(double)
  sub rsp, 24
  vmovsd qword ptr [rsp + 8], xmm0 # 8-byte Spill
  call tan
  vmovsd qword ptr [rsp + 16], xmm0 # 8-byte Spill
  vmovsd xmm0, qword ptr [rsp + 8] # 8-byte Reload
  call cos
  vmulsd xmm0, xmm0, qword ptr [rsp + 16] # 8-byte Folded Reload
  add rsp, 24

gcc(trunk) with '--std=c++17 -O3 -march=native -ffast-math' flags generates

        jmp     sin

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