Good question. I preface this by the fact that I am not any expert on gcc compiler internals, I am assuming how I'd do it.

1. This implementation tests every byte, as discussed in other posts here. That makes it slow.

2. This implementation is likely not used - the gcc compiler probably has an internal code sequence it emits for a strcpy.

Seeing that the implementation relies on CHECK_BOUND_(HIGH/LOW) it's using bounded pointers, which are implemented as structs holding 3 fields. The value, the upper bound and the lower bound. That's one extra field compared to D arrays.

The function still checks every byte for \0.

And if i'm reading it correctly, only checks the upper bound after copying the data. And checking it before copying would require a call to strlen.

I would've assumed glibc has a bunch of different implementations and then picks the fastest one at runtime depending on CPU capabilities, but maybe they just scrapped that since GCC has built-in versions of most string functions anyways, since it can often know or at least guess the characteristics of the input and output (length, alignment) and emit different code depending on that.