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POhow to use movntdqa to avoid cache pollution?
 

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  1. POhow to use movntdqa to avoid cache pollution?
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    <p>i am trying to write a memcpy function that does not load the source memory to the cpu cache. The purpose is to avoid cache pollution. The memcpy function below works, but pollutes the cache like the standard memcpy does. i am using P8700 proccesoor with visual C++ 2008 express. i see the cpu cache usage with intel vtune.</p> <pre><code>void memcpy(char *dst,char*src,unsigned size){ char *dst_end=dst+size; while(dst!=dst_end){ __m128i res = _mm_stream_load_si128((__m128i *)src); *((__m128i *)dst)=res; src+=16; dst+=16; } } </code></pre> <p>i have another version, that have the same results - works but pollutes the cache.</p> <pre><code>void memcpy(char *dst,char*src,unsigned size){ char *dst_end = dst+size; __asm{ mov edi, dst mov edx, dst_end mov esi,src inner_start: LFENCE MOVNTDQA xmm0, [esi ] MOVNTDQA xmm1, [esi+16] MOVNTDQA xmm2, [esi+32] MOVNTDQA xmm3, [esi+48] //19. ; Copy data to buffer MOVDQA [edi], xmm0 MOVDQA [edi+16], xmm1 MOVDQA [edi+32], xmm2 MOVDQA [edi+48], xmm3 // 25. ; Increment pointers by cache line size and test for end of loop add esi, 040h add edi, 040h cmp edi, edx jne inner_start } } </code></pre> <p>update: this is the test program</p> <pre><code> void test(int table_size,int num_iter,int item_size){ char *src_table=alloc_aligned(table_size*item_size);//return value is aligned on 64 bytes char *dst=alloc_aligned(item_size); //destination is always the same buffer for (int i=0;i&lt;num_iter;i++){ int location=my_rand()%table_size; char *src=src_table+location*item_size;//selecting a different src every time memcpy(dst,src,item_size); } } main(){ test(1024*32,1024*1024,1024*32) } </code></pre>
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    1. CONote that if you actually study several large pieces of software, you'll find that most memory copies actually copy less than 32 bytes and copying more than 4 KiB of data almost never happens. What this means is that optimising `memcpy()` for massive amounts of data (at the expense of startup overhead that makes it slower for small memory copies) is silly and your optimisation just makes performance worse for real world software.
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