It's worth pointing out that the author was reading an outdated implementation of strlen() from glibc. The generic implementation is still here with the same code, and would be complied if no ARCH-specific implementation is written, as seen here, https://github.com/bminor/glibc/blob/master/string/strlen.c.

But it's probably irrelevant to most systems today, as the assembly version is almost always used, e.g.

* i386: https://github.com/bminor/glibc/blob/master/sysdeps/i386/str...

* i586/i686: https://github.com/bminor/glibc/blob/master/sysdeps/i386/i58...

* i686 w/ SSE2: https://github.com/bminor/glibc/blob/master/sysdeps/i386/i68...

* i686 w/ SSE2 + BSF: https://github.com/bminor/glibc/blob/master/sysdeps/i386/i68...

* x86_64 w/ SSE2: https://github.com/bminor/glibc/blob/master/sysdeps/x86_64/s...

* ARM: https://github.com/bminor/glibc/blob/master/sysdeps/arm/strl...

* ARMv6: https://github.com/bminor/glibc/blob/master/sysdeps/arm/armv...

* ARMv6T2: https://github.com/bminor/glibc/blob/master/sysdeps/arm/armv...

* ARMv8a/AArch64: https://github.com/bminor/glibc/blob/master/sysdeps/aarch64/...

* ARMv8a/AArch64 w/ ASIMD: https://github.com/bminor/glibc/blob/master/sysdeps/aarch64/...

etc.