Brian Dessent wrote:
You're violates the C aliasing rules. You can't store through a casted
pointer like that. You also don't have to do the load/store, the
compiler know what you want when you use a union instead:
union { __m128i v; long l[2]; } a, b, c;
a.l[0] = a.l[1] = 1;
b.l[0] = b.l[1] = 1;
c.v = _mm_add_epi8 (a.v, b.v);
printf("c0=%ld c1=%ld\n", c.l[0], c.l[1]);
Many Thanks Brian. My little program now behaves better:
bash-3.1$ gcc -O2 -msse2 -o sse2 sse2-1.c
bash-3.1$ ./sse2
c0=2 c1=2
#include <stdio.h>
#include <emmintrin.h>
void test_int() {
union { __m128i v; long l[2]; } a, b, c;
a.l[0] = a.l[1] = 1;
b.l[0] = b.l[1] = 1;
c.l[0] = c.l[1] = 0;
c.v = _mm_add_epi8( a.v, b.v );
printf("c0=%ld c1=%ld\n", c.l[0], c.l[1] );
}
int main( int count, char ** args ) {
test_int();
return 0;
}
There's an even more natural way to do this though using gcc's built-in
vector extensions without any of the Intel mmintrin.h stuff. This way
will result in code that will vectorize to altivec, sse2, spu, whatever
the machine supports, it's not hardware specific:
typedef int v4si __attribute__ ((vector_size (16)));
v4si a = { 1, 2, 3, 4 }, b = { 5, 6, 7, 8 }, c;
c = a + b;
You can use all the normal C operators like + and * as if they were
scalars but they will be compiled using the corresponding SIMD
instructions. See
<http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html> for more. If
you want access to the individual parts you can again use the union,
My thinking is that I'd like try to be compiler independent, so by using
the intel intrinsics I figure I should be able to get gcc and the intel
compiler to work as a start.
What I am _really_ trying to do is to implement is the addition of
elements of two arrays.
Is there a more efficient way of doing this than this way?:
#include <stdio.h>
#include <emmintrin.h>
void test_add_long(long * result, long * a, long * b, long size) {
union { __m128i v; long l[2]; } temp1, temp2, temp3;
int index=0;
for( index=0; index < size; index+=2 ) {
temp1.l[0] = a[index];
temp1.l[1] = a[index+1];
temp2.l[0] = b[index];
temp2.l[1] = b[index + 1];
temp3.v = _mm_add_epi8( temp1.v, temp2.v );
result[index] = temp3.l[0];
result[index+1] = temp3.l[1];
printf("c0=%ld c1=%ld\n", result[index], result[index+1] );
}
}
int main( int count, char ** args ) {
// array of 4 8 byte ints
long a[] = { 1, 2, 3, 4};
long b[] = { 1, 2, 3, 4};
long result[] = {0,0,0,0};
test_add_long(result, a, b, 4);
return 0;
}
