1 | #include <string.h>
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2 | #include <stdint.h>
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3 |
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4 | void *memset(void *dest, int c, size_t n)
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5 | {
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6 | unsigned char *s = dest;
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7 | size_t k;
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8 |
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9 | /* Fill head and tail with minimal branching. Each
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10 | * conditional ensures that all the subsequently used
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11 | * offsets are well-defined and in the dest region. */
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12 |
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13 | if (!n) return dest;
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14 | s[0] = c;
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15 | s[n-1] = c;
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16 | if (n <= 2) return dest;
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17 | s[1] = c;
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18 | s[2] = c;
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19 | s[n-2] = c;
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20 | s[n-3] = c;
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21 | if (n <= 6) return dest;
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22 | s[3] = c;
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23 | s[n-4] = c;
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24 | if (n <= 8) return dest;
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25 |
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26 | /* Advance pointer to align it at a 4-byte boundary,
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27 | * and truncate n to a multiple of 4. The previous code
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28 | * already took care of any head/tail that get cut off
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29 | * by the alignment. */
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30 |
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31 | k = -(uintptr_t)s & 3;
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32 | s += k;
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33 | n -= k;
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34 | n &= -4;
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35 |
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36 | #ifdef __GNUC__
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37 | typedef uint32_t __attribute__((__may_alias__)) u32;
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38 | typedef uint64_t __attribute__((__may_alias__)) u64;
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39 |
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40 | u32 c32 = ((u32)-1)/255 * (unsigned char)c;
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41 |
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42 | /* In preparation to copy 32 bytes at a time, aligned on
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43 | * an 8-byte bounary, fill head/tail up to 28 bytes each.
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44 | * As in the initial byte-based head/tail fill, each
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45 | * conditional below ensures that the subsequent offsets
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46 | * are valid (e.g. !(n<=24) implies n>=28). */
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47 |
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48 | *(u32 *)(s+0) = c32;
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49 | *(u32 *)(s+n-4) = c32;
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50 | if (n <= 8) return dest;
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51 | *(u32 *)(s+4) = c32;
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52 | *(u32 *)(s+8) = c32;
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53 | *(u32 *)(s+n-12) = c32;
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54 | *(u32 *)(s+n-8) = c32;
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55 | if (n <= 24) return dest;
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56 | *(u32 *)(s+12) = c32;
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57 | *(u32 *)(s+16) = c32;
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58 | *(u32 *)(s+20) = c32;
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59 | *(u32 *)(s+24) = c32;
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60 | *(u32 *)(s+n-28) = c32;
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61 | *(u32 *)(s+n-24) = c32;
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62 | *(u32 *)(s+n-20) = c32;
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63 | *(u32 *)(s+n-16) = c32;
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64 |
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65 | /* Align to a multiple of 8 so we can fill 64 bits at a time,
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66 | * and avoid writing the same bytes twice as much as is
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67 | * practical without introducing additional branching. */
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68 |
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69 | k = 24 + ((uintptr_t)s & 4);
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70 | s += k;
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71 | n -= k;
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72 |
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73 | /* If this loop is reached, 28 tail bytes have already been
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74 | * filled, so any remainder when n drops below 32 can be
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75 | * safely ignored. */
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76 |
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77 | u64 c64 = c32 | ((u64)c32 << 32);
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78 | for (; n >= 32; n-=32, s+=32) {
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79 | *(u64 *)(s+0) = c64;
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80 | *(u64 *)(s+8) = c64;
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81 | *(u64 *)(s+16) = c64;
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82 | *(u64 *)(s+24) = c64;
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83 | }
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84 | #else
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85 | /* Pure C fallback with no aliasing violations. */
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86 | for (; n; n--, s++) *s = c;
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87 | #endif
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88 |
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89 | return dest;
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90 | }
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