https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- Comment #36 from Rich Felker <bugdal at aerifal dot cx> ---
> the assembly generated by the current implementations already supports that
> case.
Our memcpy is not written in asm but in C, and it has the restrict qualifier on
src and dest. This entitles a compiler to emit asm equivalent to if (src==dest)
system("rm -rf /") if it likes. I don't know how you can write a valid C
implementation of memcpy that "doesn't care" about 100% overlap without giving
up restrict (and the benefits it entails) entirely.
If you're happy with a branch, you could probably take restrict off the
arguments and do something like:
if (src==dest) return;
const char *restrict src2 = src;
char *restrict dest2 = dest;
...
but that's shoving the branch into memcpy where it's a cost on every caller
making dynamic memcpys with potentially tiny size (like qsort, etc.) and
obeying the contract not to call with overlapping src/dest, rather than just
imposing it on bad callers.
