[Bug c++/64410] gcc 25% slower than clang 3.5 for adding complex numbers

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64410

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
There are a number of things that make it complicated.
1) gcc doesn't like to vectorize when the number of iterations is not known at
compile time.
2) gcc doesn't vectorize anything already involving complex or vector
operations.
3) the ABI for complex uses 2 separate double instead of a vector of 2 double.

I believe there are dups at least for 2).