std::string_view constructor and UDL (g++7.1 -std=c++1z -O1)

output.s

.LC0:
        .string "ABCDEFGHIJKLMNOP"
main:
        subq    $8, %rsp
        movl    $16, %edi
        movl    $.LC0, %esi
        call    sink(std::basic_string_view<char, std::char_traits<char> >)
        movl    $16, %edi
        movl    $.LC0, %esi
        call    sink(std::basic_string_view<char, std::char_traits<char> >)
        movl    $0, %eax
        addq    $8, %rsp
        ret

source.cpp

#include <string_view>

void sink(std::string_view);

int main() {
  sink("ABCDEFGHIJKLMNOP");

  using namespace std::literals;
  sink("ABCDEFGHIJKLMNOP"sv);
}