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67 lines
2.4 KiB
C++
67 lines
2.4 KiB
C++
#pragma once
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#include <string.h>
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#include <DB/Core/Defines.h>
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#if __SSE2__
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#include <emmintrin.h>
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/** memcpy function could work suboptimal if all the following conditions are met:
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* 1. Size of memory region is relatively small (approximately, under 50 bytes).
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* 2. Size of memory region is not known at compile-time.
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*
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* In that case, memcpy works suboptimal by following reasons:
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* 1. Function is not inlined.
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* 2. Much time/instructions are spend to process "tails" of data.
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*
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* There are cases when function could be implemented in more optimal way, with help of some assumptions.
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* One of that assumptions - ability to read and write some number of bytes after end of passed memory regions.
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* Under that assumption, it is possible not to implement difficult code to process tails of data and do copy always by big chunks.
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*
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* This case is typical, for example, when many small pieces of data are gathered to single contiguous piece of memory in a loop.
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* - because each next copy will overwrite excessive data after previous copy.
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*
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* Assumption that size of memory region is small enough allows us to not unroll the loop.
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* This is slower, when size of memory is actually big.
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*
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* Use with caution.
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*/
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namespace detail
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{
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inline void memcpySmallAllowReadWriteOverflow15Impl(char * __restrict dst, const char * __restrict src, ssize_t n)
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{
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while (n > 0)
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{
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_mm_storeu_si128(reinterpret_cast<__m128i *>(dst),
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_mm_loadu_si128(reinterpret_cast<const __m128i *>(src)));
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dst += 16;
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src += 16;
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n -= 16;
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}
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}
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}
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/** Works under assumption, that it's possible to read up to 15 excessive bytes after end of 'src' region
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* and to write any garbage into up to 15 bytes after end of 'dst' region.
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*/
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inline void memcpySmallAllowReadWriteOverflow15(void * __restrict dst, const void * __restrict src, size_t n)
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{
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detail::memcpySmallAllowReadWriteOverflow15Impl(reinterpret_cast<char *>(dst), reinterpret_cast<const char *>(src), n);
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}
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/** NOTE There was also a function, that assumes, that you could read any bytes inside same memory page of src.
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* This function was unused, and also it requires special handling for Valgrind and ASan.
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*/
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#else /// Implementation for other platforms.
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inline void memcpySmallAllowReadWriteOverflow15(void * __restrict dst, const void * __restrict src, size_t n)
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{
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memcpy(dst, src, n);
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}
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#endif
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