"I've been writing C for quite some time, but I never followed good conventions I'm afraid, and I never payed much attention to the optimization tricks of the higher C programmers. Sure, I use const when I can, I use the pointer methods for manual string copying, I even use register for all the good that does with modern compilers, but now, I'm trying to write a C-string handling library for personal use, but I need speed, and I really don't want to use inline ASM. So, I am wondering, what would other Soylenters do to write efficient, pure, standards-compliant C?"
Unfortunately it is hard to diagnose why this was not properly optimized without seeing the rest of the code, but it looks like a compiler bug. GCC will produce well optimized code in both cases, either converting to a memcpy or at least a tight and minimal loop.
The problem is that once you start trying to second guess the compiler you end up with code that is both horrible and probably won't optimize so well in a year or two when the compiler has been improved. Fortunately we now have a couple of really good free C compilers so can for the most part just write portable code and not worry about it.
I write firmware for microcontrollers in C so optimization is a big deal for me, but most of the time it is better to let GCC worry about it.