So I know when you do x mod 2^n for unsigned operations, the compiler will simply convert that operation into x & (2^n - 1).

But when I look at the compiler implementation using signed numbers, e.g.

int signed_rem8(int x) { return x %8; }

I get something like this (https://godbolt.org/z/xY3Ef6WEc):

 movl    -4(%rbp),