[Bug c++/80373] New: non-optimal handling of copying a std::complex<double>

[noone@turm-lahnstein.de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80373

            Bug ID: 80373
           Summary: non-optimal handling of copying a std::complex<double>
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: noone@turm-lahnstein.de
  Target Milestone: ---

While copying a std::complex<double> from a memory location to another, four
movsd operations are used. However it is possible to use two movups, which are
faster (at least on some hardware) and need less memory (36 bytes for
movsd-version, but only 16 the the movups-version).

Consider the following example:

    #include <complex>  
    void get(std::complex<double> *res){
       res[1]=res[0];
    }

is compiled to:

        movsd   (%rdi), %xmm0
        movsd   %xmm0, 16(%rdi)
        movsd   8(%rdi), %xmm0
        movsd   %xmm0, 24(%rdi)
        ret

but could be:

        movups  (%rdi), %xmm0
        movups  %xmm0, 16(%rdi)
        ret

That is in fact, what clang and icc17 do.