paddw vs paddb

paddw vs paddb

[Ankit Jain]

      1 #include<stdio.h>
      2 #include<inttypes.h>
      3 int main()
      4 {
      5         uint8_t
a[8]={1,2,3,4,5,6,7,8},b[8]={1,2,3,4,5,6,7,8},i;
      6         asm("movq (%1), %%mm0 \n"
      7             "paddb (%0), %%mm0 \n"
      8             "movq  %%mm0, (%0) \n"
      9                 :
     10                 :"r"(b),"r"(a)
     11                 :"%mm0"
     12         );
     13         for(i=0;i<8;i++)
     14         printf("%d ",b[i]);
     15         return 0;
     16 }
     17
Both the programs give the same output

WHY IS IT SO? i am using paddb and paddw. it dosent
make any diff?

  1 #include<stdio.h>
      2 #include<inttypes.h>
      3 int main()
      4 {
      5         uint8_t
a[8]={1,2,3,4,5,6,7,8},b[8]={1,2,3,4,5,6,7,8},i;
      6         asm("movq (%1), %%mm0 \n"
      7             "paddd (%0), %%mm0 \n"
      8             "movq  %%mm0, (%0) \n"
      9                 :
     10                 :"r"(b),"r"(a)
     11                 :"%mm0"
     12         );
     13         for(i=0;i<8;i++)
     14         printf("%d ",b[i]);
     15         return 0;
     16 }
     17
~

ankit jain



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Re: paddw vs paddb

[Maciej Hrebien]

Ankit Jain wrote:
> 
>       1 #include<stdio.h>
>       2 #include<inttypes.h>
>       3 int main()
>       4 {
>       5         uint8_t
> a[8]={1,2,3,4,5,6,7,8},b[8]={1,2,3,4,5,6,7,8},i;
>       6         asm("movq (%1), %%mm0 \n"
>       7             "paddb (%0), %%mm0 \n"
>       8             "movq  %%mm0, (%0) \n"
>       9                 :
>      10                 :"r"(b),"r"(a)
>      11                 :"%mm0"
>      12         );
>      13         for(i=0;i<8;i++)
>      14         printf("%d ",b[i]);
>      15         return 0;
>      16 }
>      17
> Both the programs give the same output
> 
> WHY IS IT SO? i am using paddb and paddw. it dosent
> make any diff?

Insert to a or b value for which a[i]+b[i]>255 where i is odd
and you will see the difference, ie. a[7]=255.

-- 
Maciej Hrebien

Re: paddw vs paddb

[Ankit Jain]

thanks

but if u can solve one more problem

'a' is uint8_t type and i want to extract only 4 int
and store it as a word in mmx register

i am not able to understand that because movq is
extracting 8 bytes at a time. 

actually the problem is overflow. saturation is not
tht helpfull

thanks

ankit jain
 --- Maciej Hrebien <m_hrebien@wp.pl> wrote: 
> Ankit Jain wrote:
> > 
> >       1 #include<stdio.h>
> >       2 #include<inttypes.h>
> >       3 int main()
> >       4 {
> >       5         uint8_t
> > a[8]={1,2,3,4,5,6,7,8},b[8]={1,2,3,4,5,6,7,8},i;
> >       6         asm("movq (%1), %%mm0 \n"
> >       7             "paddb (%0), %%mm0 \n"
> >       8             "movq  %%mm0, (%0) \n"
> >       9                 :
> >      10                 :"r"(b),"r"(a)
> >      11                 :"%mm0"
> >      12         );
> >      13         for(i=0;i<8;i++)
> >      14         printf("%d ",b[i]);
> >      15         return 0;
> >      16 }
> >      17
> > Both the programs give the same output
> > 
> > WHY IS IT SO? i am using paddb and paddw. it
> dosent
> > make any diff?
> 
> Insert to a or b value for which a[i]+b[i]>255 where
> i is odd
> and you will see the difference, ie. a[7]=255.
> 
> -- 
> Maciej Hrebien
> 
> -
> To unsubscribe from this list: send the line
> "unsubscribe linux-assembly" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at 
> http://vger.kernel.org/majordomo-info.html
>  

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Re: paddw vs paddb

[Maciej Hrebien]

Ankit Jain wrote:
> 
> thanks
> 
> but if u can solve one more problem
> 
> 'a' is uint8_t type and i want to extract only 4 int
> and store it as a word in mmx register
> 
> i am not able to understand that because movq is
> extracting 8 bytes at a time.
> 
> actually the problem is overflow. saturation is not
> tht helpfull

I don't know if i understand your question right but for
32-bit transfers (4 bytes) you have MOVD instruction.

-- 
Maciej Hrebien

Re: paddw vs paddb

[Ankit Jain]

hi

i know about movd but i want to move 16bit atr a time

thanks
--- Maciej Hrebien <m_hrebien@wp.pl> wrote: 
> Ankit Jain wrote:
> > 
> > thanks
> > 
> > but if u can solve one more problem
> > 
> > 'a' is uint8_t type and i want to extract only 4
> int
> > and store it as a word in mmx register
> > 
> > i am not able to understand that because movq is
> > extracting 8 bytes at a time.
> > 
> > actually the problem is overflow. saturation is
> not
> > tht helpfull
> 
> I don't know if i understand your question right but
> for
> 32-bit transfers (4 bytes) you have MOVD
> instruction.
> 
> -- 
> Maciej Hrebien
> 
> -
> To unsubscribe from this list: send the line
> "unsubscribe linux-assembly" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at 
> http://vger.kernel.org/majordomo-info.html
>  

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Re: paddw vs paddb

[Maciej Hrebien]

Ankit Jain wrote:
> 
> hi
> 
> i know about movd but i want to move 16bit atr a time

You can't move a word to a mmx register directly. You have to
simulate it ie. with a temp ored with the destination register:

pxor %mm7,%mm7
xor  %eax,%eax

movw your_word,%ax

movd %eax,%mm7
por  %mm7,%mm0

which moves your_word to the lowest part of mm0 and you have
to be sure that low word of mm0 is zeroed before the operation.
Or you can:

movd %mm0,%eax
movw your_word,%ax
movd %eax,%mm0

etc. but i don't recomend this kind of coding. Keep your words
together and use movq.

-- 
Maciej Hrebien