"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?"
Oh come on, this is just pure nonsense.
Don't trust the compiler? Don't trust stl?
What ends up happening is people will waste time rewriting basic things like string classes, containers, and you get a code base where you constantly have to interface between multiple implementations of the same thing.
You of course won't implement half of what the stl gives you, because you say you don't need it. But others will, and will reimplement another quarter of the stls.
You'll have bugs, that are hard to find, but will tout that your code runs 2.1% faster. You of course still won't support utf-8 or wchar, because those are stupid. Which will make your library work on one single architecture on one single operating system.
Then some 16 year old comes in, links mono to your game, writes stuff in c# and his code runs twice as fast as yours. Supports utf-8, wchar and a thousand utility functions you thought were stupid, hence producing better and more.
Now there is of course place for asm. But there is also one for the compiler, and the stl. Let's not fall for the extremes.
Measure your performance, and optimize where needed.
And a string class? Unless you are doing it for learning, just grab a well tested library.