This is a notable differentiation - Writing assembly is a different skill to reading it from a disassembly. Reverse engineering, malware analysis etc. does not inherently require you to be able to write asm, although it certainly would help.

The pinning local variables to registers thing didn't work in llvm a couple of years ago (which seems consistent with the gcc docs) but does work at the boundaries of inline asm and that's generally enough. I like a pin-register intrinsic, something like `u64 pin(u64, enum reg)` where the compile time constant enum names the register and the semantics are a no-op other than constraining the register allocator, but that doesn't seem to be readily available in gcc/clang.

I don't have a good answer to constraining instruction scheduling.

On reflection it's all somewhat more horrible than it needs to be, perhaps inline compiler IR is a better idea.