Comment by Pannoniae

1. Better memory locality by knowing what you load and when, exactly.

2. The ability to "cheat" on calling conventions.

3. The ability for techniques like threaded code, and in general, better cache-awareness.

4. Less mov's.

5. Guaranteeing no spilling in important loops.

6. Compilers don't do well with flags registers and you can't read/write them in high level languages. You're hoping your `if (result < a) { carry = 1; }` becomes a direct flag test. Especially important in bignum, you can't really utilise adcx/adox directly from high-level code.

7. Hot/cold layout without PGO. Yes PGO is good but sometimes you know better and PGO isn't very suitable for "configurable" code.

8. Computed goto. See https://github.com/python/cpython/issues/128563 , who doesn't like 10% free performance?

9. Exploiting uninitialised memory for classic party tricks like not initialising a buffer fully (let's say you have a library function with a return buffer. You don't want dynamic allocations for some reason. You can simulate this with a pointer return into a let's say a static 4KB buffer and a count return, you only initialise it until the count. Caller has the responsibility not to overread.)