By reviewing the glibc sources it turns out that memmove() only does word copy.
Thus, the function will not work correctly when |dst-src|<|word|.

The C99 standard (section 7.21.2.2 "The memmove function") mandates:
---
#include <string.h>
void *memmove(void *s1, const void *s2, size_t n);

The memmove function copies n characters from the object pointed to by
s2 into the object pointed to by s1. Copying takes place as if the n
characters from the object pointed to by s2 are first copied into a
temporary array of n characters that does not overlap the objects
pointed to by s1 and s2, and then the n characters from the temporary
array are copied into the object pointed to by s1.
---