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Hi all,

What is the fastest way to multiply four floats by 0.5 ? The floats are in a xmm register.

Posted on 2004-07-08 01:25:49 by Dr. Manhattan

Hello,

I have never worked with floats before, but I imagine the fastest way to multiply them by 0.5 is to divide them by 2.

But this is only a thought of mine.

Regards,
Claus

Posted on 2004-07-08 03:24:42 by ndn4u

Fast multiplication by 0.5

This probably won't work for everything and is more than likely misguided but essentially you have to shift right by 1. Since floats are stored as powers of 2, you have only to subract 1 from the power in order to divide by 2.

For example with REAL8:

DATA SECTION

somefp		DQ	210.244



CODE SECTION

sub D[somefp+4],00100000h ; divide REAL8 by 2

For REAL4 (MASM format this time):

.DATA

somefp REAL4 210.244



.CODE

sub DWORD PTR somefp,00800000h ; Divide REAL4 by 2

Posted on 2004-07-08 03:40:43 by donkey

Fast multiplication by 0.5

He's referring to the Exponent.
FloatingPoint values have a special binary encoding where the top handful of bits are used to keep the Exponent (and Sign bit) while the rest of the bits contain the Mantissa (also known as the Significand).
The fastest way to divide any floatingpoint value by a multiple of 2 would be to modify the Exponent, he's absolutely right, but I get the feeling you were looking for an MMX solution...

For anyone who isn't awake to this stuff,

S EEEEEEEE MMMMMMMMMMMMMMMMMMMMMMM

That's how a 32bit float is encoded under the IEEE standard.

Have a nice day :)

Posted on 2004-07-08 05:48:05 by Homer

Fast multiplication by 0.5

Evilhomer2k it is easy to do with SSE. You can copy the data to memory and subtract as Donkey has show below. Or if you have SSE2 you can do it in parallel. You have to have at least SSE2 to do that because you need to treat the data like integer data and SSE doesn't support that.



.data

align 16

sub_exponent dd 00800000h, 00800000h, 00800000h, 00800000h

.code

psubd xmm0,[sub_exponent]

DATA SECTION

somefp		DQ	210.244



CODE SECTION

sub D[somefp+4],00100000h ; divide REAL8 by 2

For REAL4 (MASM format this time):

.DATA

somefp REAL4 210.244



.CODE

sub DWORD PTR somefp,00800000h ; Divide REAL4 by 2

Posted on 2004-07-08 08:23:20 by mark_larson

Fast multiplication by 0.5

Thanks. Too bad it doesn't work for everything, 0.0 for example.

Posted on 2004-07-08 13:37:14 by Dr. Manhattan

Fast multiplication by 0.5

Well, you have only to make a special case for 0.0 :

DATA SECTION

somefp		DQ	0.0



CODE SECTION

cmp D[somefp+4],0

je >

	sub D[somefp+4],00100000h

:

MASM:

.DATA

somefp REAL8 0.0



.CODE

cmp DWORD PTR [somefp+4],0

je @F

	sub DWORD PTR [somefp+4],00100000h

@@:

Then it seems to work in all cases.

Posted on 2004-07-08 14:00:02 by donkey

Fast multiplication by 0.5

Well, you have only to make a special case for 0.0 :

DATA SECTION

somefp		DQ	0.0



CODE SECTION

cmp D[somefp+4],0

je >

	sub D[somefp+4],00100000h

:

MASM:

.DATA

somefp REAL8 0.0



.CODE

cmp DWORD PTR [somefp+4],0

je @F

	sub DWORD PTR [somefp+4],00100000h

@@:

Then it seems to work in all cases.

Using SSE2 that would be:



.data

align 16

   four_0s   real4   0.0,0.0,0.0,0.0

   sub_value dd     00800000h,00800000h,00800000h,00800000h

.code

;xmm0 holds the 4 real4 values you want to multiply by 0.5

    movaps  xmm5,[four_0s]		;value to look for.

    movaps  xmm6,[sub_value]		;value to subtract from the exponentf

    movaps  xmm1,xmm0               ;get backup copy of data

    cmpps	xmm1,xmm5,0			;xmm1 now has an FFFFFFFF in each dword location in XMM1 with a 0.0 all other locations have 0. 

    pandn   xmm1,xmm6               ;NOTs XMM1, and then ANDs it with XMM6.  So it will only subtract from the exponent if the value is non-zero

    psubd   xmm0,xmm1               ;only subtract the sub_value from those fields that aren't 0.0s'.

Posted on 2004-07-08 14:28:33 by mark_larson

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