1 | #pragma once |
2 | |
3 | #include <string.h> |
4 | |
5 | #ifdef __SSE2__ |
6 | #include <emmintrin.h> |
7 | |
8 | |
9 | /** memcpy function could work suboptimal if all the following conditions are met: |
10 | * 1. Size of memory region is relatively small (approximately, under 50 bytes). |
11 | * 2. Size of memory region is not known at compile-time. |
12 | * |
13 | * In that case, memcpy works suboptimal by following reasons: |
14 | * 1. Function is not inlined. |
15 | * 2. Much time/instructions are spend to process "tails" of data. |
16 | * |
17 | * There are cases when function could be implemented in more optimal way, with help of some assumptions. |
18 | * One of that assumptions - ability to read and write some number of bytes after end of passed memory regions. |
19 | * Under that assumption, it is possible not to implement difficult code to process tails of data and do copy always by big chunks. |
20 | * |
21 | * This case is typical, for example, when many small pieces of data are gathered to single contiguous piece of memory in a loop. |
22 | * - because each next copy will overwrite excessive data after previous copy. |
23 | * |
24 | * Assumption that size of memory region is small enough allows us to not unroll the loop. |
25 | * This is slower, when size of memory is actually big. |
26 | * |
27 | * Use with caution. |
28 | */ |
29 | |
30 | namespace detail |
31 | { |
32 | inline void memcpySmallAllowReadWriteOverflow15Impl(char * __restrict dst, const char * __restrict src, ssize_t n) |
33 | { |
34 | while (n > 0) |
35 | { |
36 | _mm_storeu_si128(reinterpret_cast<__m128i *>(dst), |
37 | _mm_loadu_si128(reinterpret_cast<const __m128i *>(src))); |
38 | |
39 | dst += 16; |
40 | src += 16; |
41 | n -= 16; |
42 | } |
43 | } |
44 | } |
45 | |
46 | /** Works under assumption, that it's possible to read up to 15 excessive bytes after end of 'src' region |
47 | * and to write any garbage into up to 15 bytes after end of 'dst' region. |
48 | */ |
49 | inline void memcpySmallAllowReadWriteOverflow15(void * __restrict dst, const void * __restrict src, size_t n) |
50 | { |
51 | detail::memcpySmallAllowReadWriteOverflow15Impl(reinterpret_cast<char *>(dst), reinterpret_cast<const char *>(src), n); |
52 | } |
53 | |
54 | /** NOTE There was also a function, that assumes, that you could read any bytes inside same memory page of src. |
55 | * This function was unused, and also it requires special handling for Valgrind and ASan. |
56 | */ |
57 | |
58 | #else /// Implementation for other platforms. |
59 | |
60 | inline void memcpySmallAllowReadWriteOverflow15(void * __restrict dst, const void * __restrict src, size_t n) |
61 | { |
62 | memcpy(dst, src, n); |
63 | } |
64 | |
65 | #endif |
66 | |