[Bug middle-end/82899] New: *this in constructors could not alias with reference input parameters of the same type

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82899

            Bug ID: 82899
           Summary: *this in constructors could not alias with reference
                    input parameters of the same type
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: antoshkka at gmail dot com
  Target Milestone: ---

Following code 
struct test {
    char data[2] = {1, 2};

    test(const test& v);
    ~test(){} // non trivial destructor!
};
test::test(const test& v)
    : data{ v.data[0], v.data[0] }
{}


Produces the following assembly:

test::test(test const&):
        movzx   eax, BYTE PTR [rsi]
        mov     BYTE PTR [rdi], al
        movzx   eax, BYTE PTR [rsi]   <=== This is not required
        mov     BYTE PTR [rdi+1], al
        ret


`*this` could not alias with `v` because `v` is already constructed while *this
is still being constructed. Any attempt to make `*this` alias `v` is a UB:

#1
test t; // let's assume that `test` is default constructible
new (&t) test(t); // UB: destructor was not called

#2
std::aligned_storage_t<test> t;
new (&t) test(reinterpret_cast<test&>(t)); // UB: test was not constructed on t

#3 (doubtful)
test t; // let's assume that `test` has trivial destructor
// t.~test(); // users may skip this call
new (&t) test(t); // extremely wired


Please, either assume that *this never alias input parameter of the same type
OR
assume that *this never alias input parameter of the same type if the
destructor is not trivial.

This aliasing issue affects performance of all the copy and move constructors.