[Bug middle-end/56888] memcpy implementation optimized as a call to memcpy

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56888

--- Comment #42 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Rich Felker from comment #41)
> > Josef Wolf mentioned that he ran into this on the gcc-help mailing list here: https://gcc.gnu.org/ml/gcc-help/2019-10/msg00079.html
> 
> I don't think that's an instance of this issue.

Well ok, maybe not THAT message specifically; see the rest of the thread
though.

> It's normal/expected that __builtin_foo compiles to a call to foo in the
> absence of factors that lead to it being optimized to something simpler.
> The idiom of using __builtin_foo to get the compiler to emit an optimized
> implementation of foo for you, to serve as the public definition of foo, is
> simply not valid. That's kinda a shame because it would be nice to be able to
> do it for lots of math library functions, but of course in order for this to be
> able to work gcc would have to promise it can generate code for the operation
> for all targets, which is unlikely to be reasonable.