https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32667
--- Comment #30 from post+gcc at ralfj dot de --- There have been several assertions above that a certain way to solve this either has no performance cost at all or severe performance cost. That sounds like we are missing data -- ideally, someone would benchmark the actual cost of emitting that branch. It seems kind of pointless to just make assertions about the impact of this change without real data. > On the other hand, expecting the libc memcpy to make this check greatly > pessimizes every reasonable small use of memcpy with a gratuitous branch for > what is undefined behavior and should never appear in any valid program. I don't think this is true. As far as I can see, the performance impact of having memcpy support the src==dest case is zero -- the assembly generated by the current implementations already supports that case. (At least I have not seen any evidence to the contrary.) No new check in memcpy is required.
