SSE2 Question

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Kuemmel

Joined: 30 Jan 2006
Posts: 200
Location: Stuttgart, Germany

Kuemmel 28 Feb 2006, 17:55

Hi,

I try to get into SSE2 and got some problems. Whereas

Code:

...
  plot_double       rq 2           ;reserve 2 quad words = 128 bit sse register
...
        mov ecx,plot_double    ; get address of 
        movapd xmm0,[ecx]    ; get whole xmm0 register
        mulpd  xmm0,xmm0    ; xmm0*xmm0
        movapd [ecx],xmm0    ; store result at same address
...

doesn't work (crash), the following works correctly:

Code:

...
  plot_double       rq 2           ;reserve 2 quad words = 128 bit sse register
...
         mov ecx,plot_double
         movhpd xmm0,[ecx]
         movlpd xmm0,[ecx+8]
         mulpd  xmm0,xmm0
         movhpd [ecx],xmm0
         movlpd [ecx+8],xmm0
...

Why the first version doesn't work !?

28 Feb 2006, 17:55

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 28 Feb 2006, 18:28

This one crashes

Code:

format PE GUI 4.0
entry start

start:
        mov ecx,plot_double    ; get address of  
        movapd xmm0,[ecx]    ; get whole xmm0 register 
        mulpd  xmm0,xmm0    ; xmm0*xmm0 
        movapd [ecx],xmm0    ; store result at same address
        ret

  plot_double       rq 2           ;reserve 2 quad words = 128 bit sse register

This not:

Code:

format PE GUI 4.0
entry start

start:
        mov ecx,plot_double    ; get address of  
        movapd xmm0,[ecx]    ; get whole xmm0 register 
        mulpd  xmm0,xmm0    ; xmm0*xmm0 
        movapd [ecx],xmm0    ; store result at same address
        ret

  align 16
  plot_double       rq 2           ;reserve 2 quad words = 128 bit sse register

I'd never programmed using SSE but there are instructions to load unaligned data, I think.

28 Feb 2006, 18:28

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend 28 Feb 2006, 21:08

Yes, align the SSE data to 16 and use 'movapd' or otherwise 'movupd' if data not aligned.

28 Feb 2006, 21:08

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