[llvm-bugs] [Bug 37329] New: Missing optimization: *this in constructors should not alias with reference input parameters of the same type

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37329

            Bug ID: 37329
           Summary: Missing optimization: *this in constructors should not
                    alias with reference input parameters of the same type
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: antoshkka at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

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

    test(const test& v);
};
test::test(const test& v)
    : data{ v.data[0], v.data[0] }
{}


Produces the following assembly:
test::test(test const&): # @test::test(test const&)
  mov al, byte ptr [rsi]
  mov byte ptr [rdi], al
  mov al, byte ptr [rsi]   <=== This line is not required
  mov byte ptr [rdi + 1], al
  ret

Line 'mov al, byte ptr [rsi]' is for cases when input argument `v` aliases
`this`. Clang already warns on code like `Y v(v)`.

warning: variable 'v' is uninitialized when used within its own initialization
[-Wuninitialized]
  Y v(v);
    ~ ^

C++ Standard covers the above case and allows to assume that argument do not
alias with this.

[class.ctor] paragraph 14:

"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."

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