"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?"
You need an architecture that supports your algorithms
What I mean, is that people tend to micro optimise, but then go use CORBA, JSON, XML, HTLM, REST (name your web service) architecture for inter-process communication or data access, when it's not appropriate. Sometimes, mbufs, dmesg, SQL or TCP/UDP are perfectly appropriate means of passing data around.
No compiler can help, if you include inappropriate frameworks.