# Xna Math Performance

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I've done a little research to check performance of Xna math library. For test I changed a little code for counting normals:

DX Math:

 for(int i=0;i<primitives*3;i++) { D3DXVECTOR3 nor; D3DXVECTOR3 v1 = pos[rand()%primitives]-pos[rand()%primitives]; D3DXVECTOR3 v2 = pos[rand()%primitives]-pos[rand()%primitives]; D3DXVec3Cross(&nor,&v1,&v2); D3DXVec3Normalize(&nor,&nor); pos[rand()%primitives] += nor; pos[rand()%primitives] +=nor; pos[rand()%primitives] += nor; } for(int i=0;i<primitives;i++) { D3DXVec3Normalize(&pos,&pos); } 

Xna Math:

 for(int i=0;i<primitives*3;i++) { XMVECTOR nor; nor = XMVector3Cross(xmpos[rand()%primitives]-xmpos[rand()%primitives],xmpos[rand()%primitives]-xmpos[rand()%primitives]); nor = XMVector3Normalize(nor); xmpos[rand()%primitives] += nor; xmpos[rand()%primitives] += nor; xmpos[rand()%primitives] += nor; } for(int i=0;i<primitives;i++) { xmpos =XMVector3Normalize(xmpos); } 

Well, I run it for 10^6 primitives and my time results for this code parts:
D3DX Math : 1.31335
XNA Math : 2.04672

After reading a bit I found out that XMVECTOR should be 16 byte aligned on heap, so I changed new to (XMVECTOR*)_aligned_malloc(sizeof(XMVECTOR)*primitives,16);

New results:
D3DX Math : 1.32109
XNA Math : 2.05517

Visual studio instructions set: Streaming SIMD Extensions 2 (/arch:SSE2) (/arch:SSE2)..

Now my questions is: what have I done wrong? I did also test with storing data as XMFLOAT3 with loading it for computations, than storing, and it was 3 times slower than simple and convenient DX math.

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XNA math is optimized for the XBox while DirectX math is optimized for the PC.

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XNA math is optimized for the XBox while DirectX math is optimized for the PC.

Thats not true by the way, MS renamed the math library to XNA math when they released XNA the first time. Ms actually urge you to use XNA Math as DX11 doesn't contain the D3DX math library (http://msdn.microsoft.com/en-us/library/ff729728(v=vs.85).aspx). And subsequently XNA math has been renamed to DirectXMath now, and should support SSE2 instructions.
It might be that your array acces is causing conversions between SIMD registers and normal float registers, these cause severe LHS overhead.

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Having just switched to XNA math, I'm very interested to see what the problem is. I vaguely remember reading that it is recommended not to use the overloaded operators for XNA Math where performance is critical, and to use the function name equivalents instead. Does it help if you replace those for your subtractions and additions?

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CPU SIMD performance is very tricky. If you look through the general programming forums you'll find plenty of threads with in-depth discussions of the potential pitfalls.

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Does it help if you replace those for your subtractions and additions?

It helps. But still:

DX Math : 1.30931
Xna Math : 1.64447

I changed Normalize to NormalizeEst. Now it is:

DX Math : 1.30711
Xna Math : 1.36559

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