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

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #9 from Richard Biener <rguenth at gcc dot gnu.org> ---
Created attachment 34402
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34402&action=edit
patch to pattern-detect the load/store

This pattern matches real/imagpart uses and single-use complex stores and
transforms them to component-wise accesses in forwprop.  Together we vectorize
the loop now and produce:

.L28:
        movupd  (%rbx,%rax), %xmm1
        movupd  (%r15,%rax), %xmm0
        addpd   %xmm1, %xmm0
        movups  %xmm0, 0(%r13,%rax)
        addq    $16, %rax
        cmpq    %rax, %rdx
        jne     .L28

note that we need a runtime alias check to disambiguate things (because
std::vector memory cannot be disambiguated statically) and similarly we
cannot prove sufficent alignment to use aligned loads/stores.