Fast normalization - slightly off topic

[DFrey]

Here is a 3DNow! vector normalization:

 
#include <AMD3D/amd3dx.h> // 3DNow! opcode macros
 
void Normalize3f_3DNow(float *vec)
{
	_asm
	{
		femms
		mov	eax, dword ptr [vec]
		movq	mm0, [eax]
		movq	mm3, mm0
		pfmul	(m0,m0)
		movd	mm1, [eax+8]
		movq	mm4, mm1
		pfmul	(m1,m1)
		pfacc	(m0,m0)
		pfadd	(m0,m1)
		pfrsqrt (m1,m0)
		movq	mm2,mm1
		pfmul	(m2,m2)
		pfrsqit1 (m2,m0)
		pfrcpit2 (m2,m1)
		punpckldq mm2,mm2
		pfmul	(m3,m2)
		movq	[eax],mm3
		pfmul	(m4,m2)
		movd	[eax+8],mm4
		femms
	}
}

I believe I simply copied that routine from the 3DNow! SDK. To get maximum performance when working on a bunch of vectors it is much better to use most of the above code inlined and use prefetch (or prefetchw) to fetch the next vector into cache while working with the current vector. Oh and you can lose the pfrsqit1, pfrcpit2, and one pfmul instruction if 15 bit precision is good enough.


[This message has been edited by DFrey (edited 02-13-2001).]

[Relic]

There is a fastmath.cpp source on NVIDIA's developer relations page containing an approximation method for sqrt(): http://www.nvidia.com/Marketing/deve...Frame?OpenPage

[Humus]

DFrey:
While your code probably works I don't understand why you start with FEMMS? It should only be at the end. You signal that you're exiting the multimedia state before you entering it?

[Relic]

No, that's ok.
Read the 3Dnow! specs coming with the SDK:

"Like the EMMS instruction, the FEMMS instruction can be used to clear the MMX
state following the execution of a block of MMX instructions. Because the MMX
registers and tag words are shared with the floating-point unit, it is necessary to clear
the state before executing floating-point instructions. Unlike the EMMS instruction,
the contents of the MMX/floating-point registers are undefined after a FEMMS
instruction is executed. Therefore, the FEMMS instruction offers a faster context
switch at the end of an MMX routine where the values in the MMX registers are no
longer required. FEMMS can also be used prior to executing MMX instructions where
the preceding floating-point register values are no longer required, which facilitates
faster context switching."

[DFrey]

That's how it was in the 3DNow! SDK. From my understanding, they tacked it onto the beginning just to put the mmx registers into a known (undefined ) state. I understand perfectly why it is on the end, and thought it odd at first when I saw it at the start too. But the white paper on it says the FEMMS instruction is to facilitate "Faster Enter/Exit of MMX or floating-point state".

[Humus]

Hmm ... that's cool .

One learns something new each day