Why are these 8 byte-writes not optimized into a MOV?

Question

My colleague and myself are unsuccessful in explaining why GCC, ICC and Clang do not optimize this function

void f(std::uint64_t a, void * p) {
    std::uint8_t *x = reinterpret_cast<std::uint8_t *>(p);
    x[7] = a >> 56;
    x[6] = a >> 48;
    x[5] = a >> 40;
    x[4] = a >> 32;
    x[3] = a >> 24;
    x[2] = a >> 16;
    x[1] = a >> 8;
    x[0] = a;
}

Into this

mov     QWORD PTR [rsi], rdi

If we formulate f in terms of memcpy, it emits just that mov. Why does it not happen if we do the seemingly trivial sequence of byte writes?

Answer 1

我不是专家，但是gcc只能在gcc 7中为立即常量合并相邻商店：

• 关闭立即常量的错误：https://gcc.gnu.org/bugzilla/show_bug.cgi?id=23684
• 打开错误以分配小结构：https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78821
• 商店合并密码：https://github.com/gcc-mirror/gcc/blob/master/gcc/gimple-ssa-store-merging.c

如果我不得不猜测，第二个错误，等待可能不会太长。