I decided to perform a test between the following versions:
- std::min & std::max
- inline min & max using pure C
- inline min & max using SSE intrinsics
inline const float& min( const float &a, const float &b )The SSE version:
{
return a < b ? a : b;
}
inline const float min( const float &left, const float &right )
{
float retVal;
_mm_store_ss( &retVal, _mm_min_ss( _mm_set_ss( left ), _mm_set_ss( right ) ) );
return retVal;
}
Before I continue, a couple notes:
- Everything compiled with /arch:SSE2. Not setting it produced the same code. Setting /arch:IA32 produced ugly & long x87 code.
- /O2 was enabled.
- /LCGT was disabled.
- The same tests were applied to "max" versions, as well as their double counterparts. I got the same results.
- VS2012 Express Edition
- This was a win32 build, not x64
The Test
int main( int argc, char **argv )I cout the result to ensure the compiler doesn't wipe the whole code. I cast argv to ensure the compiler doesn't do constant propagation. I could use atof, but that only clutters the assembly output. I'm not interested in whether the floats are nans or the pointers are valid.
{
float a = *(float*)( &argv[0][0] );
float b = *(float*)( &argv[0][1] );
float c = *(float*)( &argv[0][2] );
float result = min( (-1.0f + 20.5f), a * b + c );
result = max( result, c );
std::cout << result;
return 0;
}
Additionally, I even do a constant operation to ensure constant propagation still works as expected (which does).
Using std::min
VS2012 is using comiss + cmovbe to perform conditional movement. Definitely an improvement over previous compiler which generated jumps.
_main PROC ; COMDATpush ebpmov ebp, espsub esp, 8mov eax, DWORD PTR _argv$[ebp]movss xmm0, DWORD PTR __real@419c0000mov eax, DWORD PTR [eax]lea ecx, DWORD PTR $T1[ebp]movss xmm1, DWORD PTR [eax+1]mulss xmm1, DWORD PTR [eax]movss xmm2, DWORD PTR [eax+2]lea eax, DWORD PTR $T2[ebp]addss xmm1, xmm2mov DWORD PTR $T1[ebp], 1100742656 ; 419c0000Hmovss DWORD PTR _c$[ebp], xmm2comiss xmm0, xmm1movss DWORD PTR $T2[ebp], xmm1cmovbe eax, ecxlea ecx, DWORD PTR _result$[ebp]movss xmm0, DWORD PTR [eax]comiss xmm2, xmm0lea eax, DWORD PTR _c$[ebp]cmovbe eax, ecxpush ecxmov ecx, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@Amovss DWORD PTR _result$[ebp], xmm0movss xmm0, DWORD PTR [eax]movss DWORD PTR [esp], xmm0call DWORD PTR __imp_??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@M@Zxor eax, eaxmov esp, ebppop ebpret 0_main ENDP
The good:
- Code is branchless
The bad:
- There's an interleave of xmm& gpr registers in the process. The data is also copied to two locations in RAM. Interleaving like this plus moving to memory back and forth doesn't look good at all.
The inline C version
The generated assembly is almost identical to std::min case, with the exception that it saves one instruction by executing comiss directly on memory instead of caching the value first in an xmm register. This value by the way, is -21.5, which is the constant arithmetic we did in the C++ code._main PROC ; COMDATpush ebpmov ebp, espsub esp, 8mov eax, DWORD PTR _argv$[ebp]lea ecx, DWORD PTR $T1[ebp]mov eax, DWORD PTR [eax]mov DWORD PTR $T2[ebp], 1100742656 ; 419c0000Hmovss xmm0, DWORD PTR [eax+1]mulss xmm0, DWORD PTR [eax]movss xmm1, DWORD PTR [eax+2]lea eax, DWORD PTR $T2[ebp]addss xmm0, xmm1movss DWORD PTR _c$[ebp], xmm1comiss xmm0, DWORD PTR __real@419c0000movss DWORD PTR $T1[ebp], xmm0cmovbe eax, ecxlea ecx, DWORD PTR _c$[ebp]movss xmm0, DWORD PTR [eax]comiss xmm0, xmm1lea eax, DWORD PTR _result$[ebp]cmovbe eax, ecxpush ecxmov ecx, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@Amovss DWORD PTR _result$[ebp], xmm0movss xmm0, DWORD PTR [eax]movss DWORD PTR [esp], xmm0call DWORD PTR __imp_??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@M@Zxor eax, eaxmov esp, ebppop ebpret 0_main ENDP
I haven't checked whether the actual binary size of the instructions is equal. Also note some instructions are following a different order, which may pipeline better/worse.
Important remarks:
If the code is changed so that the arguments are not references, but rather copy by value, a jump is generated:inline const float min( const float a, const float b )Note the lack of '&'. If compiled with that definition...
{
return a < b ? a : b;
}
_main PROC ; COMDATpush ebpmov ebp, espmov eax, DWORD PTR _argv$[ebp]movss xmm0, DWORD PTR __real@419c0000mov eax, DWORD PTR [eax]movss xmm1, DWORD PTR [eax+1]mulss xmm1, DWORD PTR [eax]movss xmm2, DWORD PTR [eax+2]addss xmm1, xmm2comiss xmm1, xmm0ja SHORT $LN6@mainmovaps xmm0, xmm1$LN6@main:comiss xmm0, xmm2ja SHORT $LN10@mainmovaps xmm0, xmm2$LN10@main:push ecxmov ecx, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@Amovss DWORD PTR [esp], xmm0call DWORD PTR __imp_??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@M@Zxor eax, eaxpop ebpret 0_main ENDP
The good
- No mixing between general purpose & SSE registers. Everything stays whithin xmm regs. That's much better.
The bad
- Code contains jumps (branches)
Using SSE2 intrinsics
This is the assembly output from using _mm_min_ss & _mm_max_ss:Sweet! That's exactly what we were aiming for. The code translates to minss & maxss instructions._main PROC ; COMDATpush ebpmov ebp, espmov eax, DWORD PTR _argv$[ebp]push ecxmov eax, DWORD PTR [eax]mov ecx, DWORD PTR __imp_?cout@std@@3V?$basic_ostream@DU?$char_traits@D@std@@@1@Amovss xmm0, DWORD PTR [eax+1]mulss xmm0, DWORD PTR [eax]movss xmm2, DWORD PTR [eax+2]addss xmm0, xmm2movaps xmm1, xmm0movss xmm0, DWORD PTR __real@419c0000minss xmm0, xmm1movaps xmm1, xmm0movaps xmm0, xmm2maxss xmm1, xmm0
movss DWORD PTR [esp], xmm1call DWORD PTR __imp_??6?$basic_ostream@DU?$char_traits@D@std@@@std@@QAEAAV01@M@Zxor eax, eaxpop ebpret 0_main ENDP
I was also afraid that the compiler wouldn't do this efficiently (i.e. store xmm0's result from the first minss unnecessarily into stack and then load it back) but note that it writes to memory after the second one, maxss, is executed!
Not only that, the code is also smaller. It couldn't have gone better. All advantages, no disadvantage.
Completely branchless, no exchange with gpr registers, no conditional moves either!
Doubles
I won't post the results from using doubles, because it's the same: instead of comiss, comisd is used. Instead of minss, minsd is generated.Specializing std::min & std::max
I tried specializing std functions for float & double to use the SSE2 intrinsics. This actually worked and produced same optimal assembly with minss & maxss. However, std::min returns by reference. Not by value.The only way to implement it is using a local variable which is where _mm_store_ss will send the result.
But then the compiler warns that we're returning the address of a local variable. Which can be very dangerous, since "const float &r = std::min( a, b )" is valid, and would cause undefined behavior.
Conclusion
Given what I found, I will be using Ogre::min & Ogre::max to replace std::min & std::max (where floats & doubles are involved).On x86 architectures, it will use the SSE2 intrinsics as it translates to optimum assembly. On non-x86 architectures, it will default to the inline C version (or an architecture variant, ie. Neon intrinsics)
We still have to measure what happens with GCC though.
