libc_support.cc

#include "common_macros.h"

__attribute__((weak, visibility("hidden")))
extern "C"
[[noreturn]] void __stack_chk_fail() {
    __builtin_trap();
}

// memset implementation from musl
__attribute__((weak, visibility("hidden")))
extern "C"
void* memset(void* dest, int c, size_t n) {
    auto* s = static_cast<unsigned char*>(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;

#if defined(__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;
}