Newbie in SSE intrinsics:: How to fix _mm_cvtpi16_ps convert error ?

Newbie in SSE intrinsics:: How to fix _mm_cvtpi16_ps convert error ?

Ritratto di Gaiger Chen
Gaiger Chen
Mer, 01/12/2010 - 02:44

Hi:

I have write a stupid code to practiceSSE:

#include
#include
#include
#include /*SSE*/

#define MALLOC_ALIGN_16BYTE(_size) _aligned_malloc( _size, 16)
#define FREE_ALIGN_16BYTE(ptr) _aligned_free(ptr)

int main(void)
{

int inN;

short *input;
float *yIn;

inN = 10;

input = (short*)MALLOC_ALIGN_16BYTE(inN*sizeof(short));
yIn = (float*)MALLOC_ALIGN_16BYTE(inN*sizeof(float));

for(int i = 0; i< inN; i++){
input[i] = (short)i;
}/*for i*/

__m64 *pShort = (__m64*)input;
__m128 *pFloat = (__m128*)yIn;

int m = inN/4;

#if(1)
for(int i = 0; i< m; i++){
pFloat[i] = _mm_cvtpi16_ps(pShort[i]);
}/*for i*/

for(int i = m*4; i< inN;i++){
yIn[i] = (float)input[i];
}
for(int i = 0; i< inN; i++){
printf("i = %d, yout = %4.3f\\n", i,yIn[i] );
}/*for i*/
#else

for(int i = 0; i< inN;i++){
yIn[i] = (float)input[i];
}

for(int i = 0; i< inN; i++){
printf("i = %d, yout = %4.3f\\n", i,yIn[i] );
}/*for i*/
#endif

FREE_ALIGN_16BYTE(input);
FREE_ALIGN_16BYTE(yIn);

}/*main*/

the printf result is :

i = 0, yout = 0.000
i = 1, yout = 1.000
i = 2, yout = 2.000
i = 3, yout = 3.000
i = 4, yout = 4.000
i = 5, yout = 5.000
i = 6, yout = 6.000
i = 7, yout = 7.000
i = 8, yout = -1.#IO
i = 9, yout = 9.000

if I set inN = 12; the result is

i = 0, yout = -1.#IO
i = 1, yout = 1.000
i = 2, yout = 2.000
i = 3, yout = 3.000
i = 4, yout = 4.000
i = 5, yout = 5.000
i = 6, yout = 6.000
i = 7, yout = 7.000
i = 8, yout = 8.000
i = 9, yout = 9.000
i = 10, yout = 10.000
i = 11, yout = 11.000

They is both the same for VC8 or ICC10.1.

I do not know what mistake I make.... i am newbie in use SSE intrinsics.

could someone help me ?

thank you.

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Ritratto di Nicolae Popovici (Intel)
Nicolae Popovici (Intel)
Mer, 01/12/2010 - 03:39

on my end your code runs OK
i'll take the inN=12 case (the 10 one does not make much sense to me)
essentially you transform each 4 short values into 4 sp float values (in 128bit format) which is OK
nownext step would be to apply some sort of arithmetic/logica operation on your sp floats (eg mm_add_ps)

for inN=12 case this loop does not make sense(maybe you need to rethink your code)

for(int i = m*4; i< inN;i++){
yIn[i] = (float)input[i];
}

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Ritratto di Gaiger Chen
Gaiger Chen
Mer, 01/12/2010 - 08:30

I think I found the error :

I use intel 10.1 compiler, there is warning #965 and #964:

this explain I should insert _mm_empty() after use SSE point:

for(int i = 0; i< m; i++){
pFloat[i] = _mm_cvtpi16_ps(pShort[i]);
}/*for i*/

_mm_empty();

for(int i = m*4 ; i< inN;i++){
yIn[i] = (float)input[i];
}/*for i*/

and the output would be regular.

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Ritratto di cod3r
cod3r
Gio, 02/12/2010 - 02:26

A missing emms causes often really strange results. I can't imagine how much time I've spent debugging such code :(

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