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Performance testing with bit operations, SSE2 and asm

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Paul C Skertich


Today I figure I sit down and see how SSE2 is benefitial to a game. I don't come fromt a strong background knowledge of ASM code. However, I found it fascinating to say the least! I can understand the headaches that go into ASM but if you can get around knowing some ASM then you're good to go on SSE2 instructions.

As I already aknowledged from previous issues with DIrectX Math is the vectors have to be 16 bit aligned or there will be access violations exceptions thrown about. I recently had issues like this when I changed the camera class.

On my performance test I tested out a simple function that would return the minimal of two floating points.inline float _b_min(float a, float b) { return (a < b ? a : b);}inline float _a_nin(float a, float b) { float result = 0.0f; __asm { mov eax,a mov ebx,b cmp eax,ebx mov [result], eax }return result;}
the function _a_min was a bit faster than _b_min. _a_min gave around 900 microseconds.
_b_min function gave over 1 millisecond time elapsed from the high resolution timer.

The sse2 min function gave me around 0.003 milliseconds.inline float *_sse2_min(float a, float b) { __m128 _a = _mm_set_ps1(a); //-- set _a to the floating point a; __m128 _b = _mm_set_ps1(b); //-- set _b to floating point b; __m128 c = _mm_min_ps(_a,_b); //-- return in C the minimal of _a,_b; float *result = (float*)_aligned_malloc(sizeof(float), sizeof(float)); _mm_store_ps(result, c); //-- store the result float* result; return result;}
my question is for whomever the reader is - why is SSE2 and bit operator maniupation a bit slower than ASM or does it not matter? Possible SSE2 is better with bigger data than just comparing minimal of floating points?
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I'm guessing that was a typo. 0.003 microseconds *IS* faster than 1 microsecond.

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so the SSE2 timing gave me 3 milliseconds and the asm timing gave me 900 microseconds. I got confused with the whole second conversion to microseconds and milliseconds. The output of the time elapsed is in milliseconds.

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Yeah, SSE is not made for operating on regular floats. You eigther need to process 4 values at the same time, and/or perform a series of SSE-operations on the same values to minimize time spent on shoving values between registers for there to be a performance improvement.

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I see what you guys are getting at. Yeah I didn't expect to run small floating operations using SSE and had the idea that SSE had to have 16-bit aligned or else exceptions would be thrown. I started working with the DIrectXMath not XNA like I was using before. An exception kept on being thrown each time the camera class wasn't aligned properly. Again you're absolutely right Juliean about the vectors. I wanted to dive into knowing SSE2 and gain some knowledge in ASM a bit. Again Aressera you're code fits the bill because I was wasting memory and it was a potentially more hazardous on memory leak. The code wasn't meant to be used in game engine - just seeing if I could learn more about SSE2. I didn't know the aligned_malloc was a wrapper function of malloc.

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You realize that aligned malloc is usually just a wrapper around regular malloc that enforces the alignment? and that malloc is a kernel call that requires context switch, acquiring a lock, and various data structure manipulation? You're doing like 1000x the work in that one call than the rest of the function. You should never return memory like that from a function, it'd bad form and requires the caller to free it (and know to use aligned free).

That's not quite accurate, malloc is not a kernel call (thankfully) except perhaps when it needs to grow the heap. I agree returning memory to the caller like that is pretty nasty though.

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