What is the fastest way to check if XMM contains only zeros?

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peter_k

Joined: 27 Dec 2006
Posts: 6
Location: Poland

peter_k 27 Dec 2006, 13:59

I'm coding engine to logical game and i'm using SSE & SSE2 technology for doing bitmasks.
I need fast checking if 128 bits XMM register contains only zeros. The trivial way to do this is sth like this (below), but i need sth much more efficient. I'm new to this forum and SSE technology Smile

Code:

;in data section
tmp        dd        4 dup(?)

;in code section; checking if xmm0 contains only zeros
movdqa [tmp], xmm0
mov    eax, [tmp+0]
mov    ebx, [tmp+4]
or     eax, [tmp+8]
or     ebx, [tmp+12]
or     eax, ebx
jz     xmm0_contain_only_zeros

27 Dec 2006, 13:59

Goplat

Joined: 15 Sep 2006
Posts: 181

Goplat 27 Dec 2006, 15:56

Here are a couple of alternative ways to do it

Code:

packssdw xmm1,xmm0
packsswb xmm1,xmm1
movd eax,xmm1
test eax,eax
jz xmm0_contain_only_zeros

pxor xmm1,xmm1
pcmpeqb xmm1,xmm0
pmovmskb eax,xmm1
inc eax
jz xmm0_contain_only_zeros

27 Dec 2006, 15:56

asmfan

Joined: 11 Aug 2006
Posts: 392
Location: Russian

asmfan 27 Dec 2006, 16:46

Actually, Goplat is basically right, but the second one way is wrong cuz if XMM reg contains zeros then after

Code:

pmovmskb eax,xmm1

eax = 0FFFFh
and we should eather

Code:

cmp eax, 0FFFFh
jz xmm0_contain_only_zeros

Code:

inc ax
jz xmm0_contain_only_zeros

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Any offers?

27 Dec 2006, 16:46

MCD

Joined: 21 Aug 2004
Posts: 602
Location: Germany

MCD 28 Dec 2006, 18:07

my try

Code:

        pxor    xmm1,xmm1
        packssdw        xmm0,xmm0
        comisd  xmm0,xmm1;0000000000000000h = +0.0
        je    xmm0_contain_only_zeros

maybe the mixing of floating point and integer numbers is not such a good idea Question

EDIT: the COMISD definately generates a reformatting delay, so this example is the shortest, but maybe not the fastest one

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Last edited by MCD on 06 Jan 2007, 21:23; edited 1 time in total

28 Dec 2006, 18:07

MazeGen

Joined: 06 Oct 2003
Posts: 977
Location: Czechoslovakia

MazeGen 28 Dec 2006, 18:44

Yep, I was also thinking about COMISD and it should work since it doesn't check the destination for a NaN.

28 Dec 2006, 18:44

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8466
Location: Kraków, Poland

Tomasz Grysztar 29 Dec 2006, 12:40

The PTEST instruction from SSE4 set seems like a promise to simplify this problem in future. Wink

29 Dec 2006, 12:40

MCD

Joined: 21 Aug 2004
Posts: 602
Location: Germany

MCD 29 Dec 2006, 19:24

Tomasz Grysztar wrote:

The PTEST instruction from SSE4 set seems like a promise to simplify this problem in future.

You must be joking. my CPU only got MMX and SSE1 Sad

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29 Dec 2006, 19:24

peter_k

Joined: 27 Dec 2006
Posts: 6
Location: Poland

peter_k 29 Dec 2006, 21:03

Thanks everybody for reply!

MCD code seems to be the shortest. I'll profile and test it.

My processor is Intel Pentium M Processor 730 so i have SSE2.

29 Dec 2006, 21:03

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