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IA-32 Intel® Architecture Optimization
6-48
Using the 8-byte Streaming Stores and Software Prefetch
Example 6-11 presents the copy algorithm that uses second level cache.
The algorithm performs the following steps:
1. Uses blocking technique to transfer 8-byte data from memory into
second-level cache using the
_mm_prefetch intrinsic, 128 bytes at
a time to fill a block. The size of a block should be less than one
half of the size of the second-level cache, but large enough to
amortize the cost of the loop.
2. Loads the data into an
xmm register using the _mm_load_ps intrinsic.
3. Transfers the 8-byte data to a different memory location via the
_mm_stream intrinsics, bypassing the cache. For this operation, it is
important to ensure that the page table entry prefetched for the
memory is preloaded in the TLB.
Example 6-11 A Memory Copy Routine Using Software Prefetch
#define PAGESIZE 4096;
#define NUMPERPAGE 512 // # of elements to fit a page
double a[N], b[N], temp;
for (kk=0; kk<N; kk+=NUMPERPAGE) {
temp = a[kk+NUMPERPAGE]; // TLB priming
// use block size = page size,
// prefetch entire block, one cache line per loop
for (j=kk+16; j<kk+NUMPERPAGE; j+=16) {
_mm_prefetch((char*)&a[j], _MM_HINT_NTA);
}
continued