Comment by gizmo686
15 hours ago
X86-64 has the REP prefix for string operation. When combined with the MOVS instruction, that is pretty much an instruction for strcpy.
15 hours ago
X86-64 has the REP prefix for string operation. When combined with the MOVS instruction, that is pretty much an instruction for strcpy.
No, it's an instruction for memcpy. You still need to compute the string length first, which means touching every byte individually because you can't use SIMD due to alignment assumptions (or lack thereof) and the potential to touch uninitialized or unmapped memory (when the string crosses a page boundary).
You do aligned reads, which can't crash.
Not even musl uses a scalar loop, if it can do aligned reads/writes: https://git.musl-libc.org/cgit/musl/tree/src/string/stpcpy.c
And you don't need to worry about C UB if you do it in ASM.