android / platform / external / musl / refs/heads/android13-d4-release / . / src / string / memset.c

#include <string.h> | |

#include <stdint.h> | |

void *memset(void *dest, int c, size_t n) | |

{ | |

unsigned char *s = dest; | |

size_t k; | |

/* Fill head and tail with minimal branching. Each | |

* conditional ensures that all the subsequently used | |

* offsets are well-defined and in the dest region. */ | |

if (!n) return dest; | |

s[0] = c; | |

s[n-1] = c; | |

if (n <= 2) return dest; | |

s[1] = c; | |

s[2] = c; | |

s[n-2] = c; | |

s[n-3] = c; | |

if (n <= 6) return dest; | |

s[3] = c; | |

s[n-4] = c; | |

if (n <= 8) return dest; | |

/* Advance pointer to align it at a 4-byte boundary, | |

* and truncate n to a multiple of 4. The previous code | |

* already took care of any head/tail that get cut off | |

* by the alignment. */ | |

k = -(uintptr_t)s & 3; | |

s += k; | |

n -= k; | |

n &= -4; | |

#ifdef __GNUC__ | |

typedef uint32_t __attribute__((__may_alias__)) u32; | |

typedef uint64_t __attribute__((__may_alias__)) u64; | |

u32 c32 = ((u32)-1)/255 * (unsigned char)c; | |

/* In preparation to copy 32 bytes at a time, aligned on | |

* an 8-byte bounary, fill head/tail up to 28 bytes each. | |

* As in the initial byte-based head/tail fill, each | |

* conditional below ensures that the subsequent offsets | |

* are valid (e.g. !(n<=24) implies n>=28). */ | |

*(u32 *)(s+0) = c32; | |

*(u32 *)(s+n-4) = c32; | |

if (n <= 8) return dest; | |

*(u32 *)(s+4) = c32; | |

*(u32 *)(s+8) = c32; | |

*(u32 *)(s+n-12) = c32; | |

*(u32 *)(s+n-8) = c32; | |

if (n <= 24) return dest; | |

*(u32 *)(s+12) = c32; | |

*(u32 *)(s+16) = c32; | |

*(u32 *)(s+20) = c32; | |

*(u32 *)(s+24) = c32; | |

*(u32 *)(s+n-28) = c32; | |

*(u32 *)(s+n-24) = c32; | |

*(u32 *)(s+n-20) = c32; | |

*(u32 *)(s+n-16) = c32; | |

/* Align to a multiple of 8 so we can fill 64 bits at a time, | |

* and avoid writing the same bytes twice as much as is | |

* practical without introducing additional branching. */ | |

k = 24 + ((uintptr_t)s & 4); | |

s += k; | |

n -= k; | |

/* If this loop is reached, 28 tail bytes have already been | |

* filled, so any remainder when n drops below 32 can be | |

* safely ignored. */ | |

u64 c64 = c32 | ((u64)c32 << 32); | |

for (; n >= 32; n-=32, s+=32) { | |

*(u64 *)(s+0) = c64; | |

*(u64 *)(s+8) = c64; | |

*(u64 *)(s+16) = c64; | |

*(u64 *)(s+24) = c64; | |

} | |

#else | |

/* Pure C fallback with no aliasing violations. */ | |

for (; n; n--, s++) *s = c; | |

#endif | |

return dest; | |

} |