[llvm-bugs] [Bug 36750] New: Meyers' singleton does not work with [[gnu::const]] and -O1 to -O3

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36750

            Bug ID: 36750
           Summary: Meyers' singleton does not work with [[gnu::const]]
                    and -O1 to -O3
           Product: clang
           Version: 5.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: antons.jelkins at bmw.de
                CC: llvm-bugs at lists.llvm.org

If Mayers’ singleton is defined with [[gnu::const]] attribute, the class
constructor is not called and the singleton getter returns an uninitialised
object.

Reproduction:

$ cat main.cpp 
#include <iostream>

class MyClass
{
public:
    MyClass() : m_val(42) {}
    void handle() const { std::cout << m_val << std::endl; }

private:
    int m_val;
};

[[ gnu::const ]]
static const MyClass &myClass() noexcept
{
    static const MyClass s_myClass;
    return s_myClass;
}

int main()
{
    myClass().handle();
    return 0;
}

$ clang++ -O0 -std=c++14 ./main.cpp && ./a.out 
42
$ clang++ -O1 -std=c++14 ./main.cpp && ./a.out 
0

Expectation:
In both cases 42 is printed.

Observation:
In the second case the MyClass object is not constructed. This assembly code is
generated by clang:

main: # @main
  push rax
  mov edi, offset myClass()::s_myClass
  call MyClass::handle() const
  xor eax, eax
  pop rcx
  ret

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