"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?"
Are you better at writing a string routine than the standard library authors who have written platform-optimized code that has been worked on for two, three, four decades? I'd be wary of rewriting anything in the standard library. This code has been around a very long time and has been scrutinized by many programmers.