Writing string.h functions using string instructions in asm x86-64

Introduction The C standard library offers a bunch of functions (whose declarations can be found in the string.h header) to manage NULL-terminated strings and arrays. These are some of the most used C functions, often implemented as builtin by the C compiler, as they are crucial to the speed of programs.
On the other hand, the x86 architecture contains “string instructions”, aimed at implementing operations on strings at the hardware level. Moreover, the x86 architecture was incrementally enhanced with SIMD instructions over the years, allowing for the processing of multiple bytes of data in a single instruction.