SIMDjson is another example that comes to mind. The conceit of C is that you do t have control over the underlying machine instructions without inlining it yourself. So how do people write C code with cpu optimizations like this?
1) Use intrinsics, if your platform provides intrinsics for the instructions you want to use.
2) Use inline assembly (which I suppose doesn't technically count as writing C code, but is very much part of the story).
3) Write C code carefully against a chosen compiler with chosen optimization flags, and inspect the assembly. Especially for small chunks of code, a tool like godbolt.org is indispensable. Basically, you're writing the assembly you want the compiler to generate (either explicitly, or just in your head), then writing C code that seems likely to generate that sequence of instructions, then tweaking the code until the output you want is generated. If this is a super important optimization, it's also reasonable to add a build step that inspects the generated assembly and fails the build if it doesn't match the desired pattern; but in that case, writing inline assembly is usually easier.
As a way to show the wonderful complexities of compilers check the TOC of https://shop.elsevier.com/books/optimizing-compilers-for-mod...
Resources sufficient for implementing simdjson or similar vectorized parsers: https://www.intel.com/content/www/us/en/docs/intrinsics-guid... https://lemire.me/blog/ http://0x80.pl/articles/