AMD 250 manual 204

Software Optimization Guide for AMD64 Processors 25112 Rev. 3.06 September 2005

//again to the top of the column for the next dot-product of

//4 rows of A upon B-transpose's row vector.

Btr_ptr -= 32;

//The addresses Aptr0, Aptr1, Aptr2, and Aptr3 need to be

//incremented to the next block of 4 rows of A to be multiplied

//upon B's column. 4 rows of A are 128 doubles in size, and in

//the n-loop above they were incremented by 32 already, so they

//must be incremented an additional 96 to point to the next

//4 rows of A to be dotted.

Aptr0 += 96; Aptr1 += 96; Aptr2 += 96; Aptr3 += 96;

_mm_prefetch(&Ctr_prefptr[0], 2);

//This loop below "dots" 4 consecutive rows of A upon a row

//of B-transpose by looping 8 times through code that

//multiplies and accumulates the products of 4 elements of A's

//rows with 4 elements of B-transpose's column.

for (n = 0; n	< 8; n++) {
Ctr_ptr[0]	+= Aptr0[0]*Btr_ptr[0] +	Aptr0[1]*Btr_ptr[1] +
	Aptr0[2]*Btr_ptr[2] +	Aptr0[3]*Btr_ptr[3];
Ctr_ptr[1]	+= Aptr1[0]*Btr_ptr[0] +	Aptr1[1]*Btr_ptr[1] +
	Aptr1[2]*Btr_ptr[2] +	Aptr1[3]*Btr_ptr[3];
Ctr_ptr[2]	+= Aptr2[0]*Btr_ptr[0] +	Aptr2[1]*Btr_ptr[1] +
	Aptr2[2]*Btr_ptr[2] +	Aptr2[3]*Btr_ptr[3];
Ctr_ptr[3]	+= Aptr3[0]*Btr_ptr[0] +	Aptr3[1]*Btr_ptr[1] +
	Aptr3[2]*Btr_ptr[2] +	Aptr3[3]*Btr_ptr[3];

//Increment pointers to B transpose's column and A's rows to

//the next 4 elements to be multiplied and accumulated.

Btr_ptr += 4; Aptr0 += 4; Aptr1 += 4; Aptr2 += 4; Aptr3 += 4;

}

//The addresses to prefetch in B-transpose and C-transpose

//are incremented by 8 doubles, or 64 bytes, or 1 cache line.

//Each loop of the 4 loops of Ctr_8col_blck above brings in a

//new set of 8 doubles and after 4 loops the full column of the

//next column of B and the next set of 8 elements of C to be

//determined are also brought into the cache.

Btr_prefptr += 8;

Ctr_prefptr += 8;

//The pointer to C-transpose is incremented by 4 doubles

//to address the next 4 elements of C-transpose's row to be

//determined.

Ctr_ptr += 4;

//The pointer to B-transpose points to the end of the present

//row. We need to subtract 32 doubles so Btr_ptr points again

//to the top of the column for the next dot-product of 4 rows of A

//upon B-transpose's row vector

204 Optimizing with SIMD Instructions Chapter 9