[Bug c++/82899] *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

--- Comment #12 from Antony Polukhin <antoshkka at gmail dot com> ---
(In reply to Marc Glisse from comment #10)
> This seems fixed in 8.1 (at least we don't generate the extra mov anymore),
> can you check?

Actually it still does not work for subobjects. For example
https://godbolt.org/g/zPha3U

Code 

struct array {
    int d[2];
};

struct test {
    array data1;
    array data2;

    test(const array& t);
};

test::test(const array& t)
    : data1{t}
    , data2{t}
{}

produces assembly

test::test(array const&):
  mov rax, QWORD PTR [rsi]
  mov QWORD PTR [rdi], rax
  mov rax, QWORD PTR [rsi]   <== Not required. Could not alias
  mov QWORD PTR [rdi+8], rax
  ret

[class.ctor] paragraph 14 also covers this case:

"During the construction of an object, if the value of the object *or any of
its subobjects* is accessed through a glvalue that is not obtained, directly or
indirectly, from the constructor’s this pointer, the value of the object or
subobject thus obtained is unspecified."

Looks like not only `this` should be marked with __restrict, but also all the
subobjects of the type.