Manual Processor Dispatch: Example

This example shows multiple optimized functions in a single binary file. You can use manual processor dispatch to support up to seven different versions of any one function, targeting up to seven different processors.

#include <mmintrin.h>


/* Pentium processor function does not use intrinsics to add two arrays. */

__declspec(cpu_specific(pentium))

void array_sum(int *r, int *a, int *b,size_t l)

{

   for (; length > 0; l--)
   *result++ = *a++ + *b++;

}

/* Implementation for a Pentium processor with MMX technology uses
an MMX instruction intrinsic to add four elements simultaneously. */

__declspec(cpu_specific(pentium_MMX)) 
void array_sum(int *r,int const *a, int *b, size_t l)

{

   __m64 *mmx_result = (__m64 *)result;
   __m64 const *mmx_a = (__m64 const *)a;
   __m64 const *mmx_b = (__m64 const *)b;

   for (; length > 3; length -= 4)
   *mmx_result++ = _mm_add_pi16(*mmx_a++, *mmx_b++);

   /* The following code, which takes care of excess elements, is not
   needed if the array sizes passed are known to be multiples of four. */

   result = (unsigned short *)mmx_r;
   a = (unsigned short const *)mmx_a;
   b = (unsigned short const *)mmx_b;

   for (; length > 0; l--)
   *result++ = *a++ + *b++;

}

/* The function stub informs the compiler to generate the
   CPU-dispatch function listed in the cpu_dispatch clause. */

__declspec(cpu_dispatch(pentium, pentium_MMX))
void array_sum (int *r,int const *a, int *b, size_t l) )

{

   /* The function body is empty. */

}