Double.IsNaN test 100 times faster?

Question

I found this in the .NET Source Code: It claims to be 100 times faster than System.Double.IsNaN. Is there a reason to not use this function instead of Sys

Answer 1

I call shenanigans. The "fast" version has a considerably larger number of ops and even performs more reads from memory, (stack, so in L1 but still slower than registers).

00007FFAC53D3D01  movups      xmmword ptr [rsp+8],xmm0  
00007FFAC53D3D06  sub         rsp,48h  
00007FFAC53D3D0A  mov         qword ptr [rsp+20h],0  
00007FFAC53D3D13  mov         qword ptr [rsp+28h],0  
00007FFAC53D3D1C  mov         qword ptr [rsp+30h],0  
00007FFAC53D3D25  mov         rax,7FFAC5423D40h  
00007FFAC53D3D2F  mov         eax,dword ptr [rax]  
00007FFAC53D3D31  test        eax,eax  
00007FFAC53D3D33  je          00007FFAC53D3D3A  
00007FFAC53D3D35  call        00007FFB24EE39F0  
00007FFAC53D3D3A  mov         r8d,8  
00007FFAC53D3D40  xor         edx,edx  
00007FFAC53D3D42  lea         rcx,[rsp+20h]  
00007FFAC53D3D47  call        00007FFB24A21680  
            t.DoubleValue = value;
00007FFAC53D3D4C  movsd       xmm5,mmword ptr [rsp+50h]  
00007FFAC53D3D52  movsd       mmword ptr [rsp+20h],xmm5  

            UInt64 exp = t.UintValue & 0xfff0000000000000;
00007FFAC53D3D58  mov         rax,qword ptr [rsp+20h]  
00007FFAC53D3D5D  mov         rcx,0FFF0000000000000h  
00007FFAC53D3D67  and         rax,rcx  
00007FFAC53D3D6A  mov         qword ptr [rsp+28h],rax  
            UInt64 man = t.UintValue & 0x000fffffffffffff;
00007FFAC53D3D6F  mov         rax,qword ptr [rsp+20h]  
00007FFAC53D3D74  mov         rcx,0FFFFFFFFFFFFFh  
00007FFAC53D3D7E  and         rax,rcx  
00007FFAC53D3D81  mov         qword ptr [rsp+30h],rax  

            return (exp == 0x7ff0000000000000 || exp == 0xfff0000000000000) && (man != 0);
00007FFAC53D3D86  mov         rax,7FF0000000000000h  
00007FFAC53D3D90  cmp         qword ptr [rsp+28h],rax  
00007FFAC53D3D95  je          00007FFAC53D3DA8  
00007FFAC53D3D97  mov         rax,0FFF0000000000000h  
00007FFAC53D3DA1  cmp         qword ptr [rsp+28h],rax  
00007FFAC53D3DA6  jne         00007FFAC53D3DBD  
00007FFAC53D3DA8  xor         eax,eax  
00007FFAC53D3DAA  cmp         qword ptr [rsp+30h],0  
00007FFAC53D3DB0  setne       al  
00007FFAC53D3DB3  mov         dword ptr [rsp+38h],eax  
00007FFAC53D3DB7  mov         al,byte ptr [rsp+38h]  
00007FFAC53D3DBB  jmp         00007FFAC53D3DC1  
00007FFAC53D3DBD  xor         eax,eax  
00007FFAC53D3DBF  jmp         00007FFAC53D3DC1  
00007FFAC53D3DC1  nop  
00007FFAC53D3DC2  add         rsp,48h  
00007FFAC53D3DC6  ret  

Versus the .NET version:

            return (*(UInt64*)(&d) & 0x7FFFFFFFFFFFFFFFL) > 0x7FF0000000000000L;
00007FFAC53D3DE0  movsd       mmword ptr [rsp+8],xmm0  
00007FFAC53D3DE6  sub         rsp,38h  
00007FFAC53D3DEA  mov         rax,7FFAC5423D40h  
00007FFAC53D3DF4  mov         eax,dword ptr [rax]  
00007FFAC53D3DF6  test        eax,eax  
00007FFAC53D3DF8  je          00007FFAC53D3DFF  
00007FFAC53D3DFA  call        00007FFB24EE39F0  
00007FFAC53D3DFF  mov         rdx,qword ptr [rsp+40h]  
00007FFAC53D3E04  mov         rax,7FFFFFFFFFFFFFFFh  
00007FFAC53D3E0E  and         rdx,rax  
00007FFAC53D3E11  xor         ecx,ecx  
00007FFAC53D3E13  mov         rax,7FF0000000000000h  
00007FFAC53D3E1D  cmp         rdx,rax  
00007FFAC53D3E20  seta        cl  
00007FFAC53D3E23  mov         dword ptr [rsp+20h],ecx  
00007FFAC53D3E27  movzx       eax,byte ptr [rsp+20h]  
00007FFAC53D3E2C  jmp         00007FFAC53D3E2E  
00007FFAC53D3E2E  nop  
00007FFAC53D3E2F  add         rsp,38h  
00007FFAC53D3E33  ret