[Bug c++/78773] New: "Empty" const only class not optimized away

[jpakkane at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78773

            Bug ID: 78773
           Summary: "Empty" const only class not optimized away
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jpakkane at gmail dot com
  Target Milestone: ---

Suppose we have the following very simple class for accessing a memory mapped
register:

class MemoryMappedRegister {
  public:
  MemoryMappedRegister(const int value) : 
    ptr{reinterpret_cast<char*>(value)} {}
  void write(char v) const { *ptr = v; }

  private:
  volatile char* const ptr;
};

const MemoryMappedRegister MyReg(12345);

void set_value(char v) {
  MyReg.write(v);
}

That is, the only reason for this class to exist is to provide a nicer UI to
register poking than C macros. When compiled with GCC (up to the version of 7
in gcc.godbolt.org as of this writing) using -O2 -std=c++14 the output is this:

set_value(char):
        mov     rax, QWORD PTR MyReg[rip]
        mov     BYTE PTR [rax], dil
        ret
        mov     QWORD PTR MyReg[rip], 12345
        ret

That is, it reserves space for the object to store the always constant pointer
value and loads it indirectly. When compiled with Clang 3.9 the output is this:

set_value(char):                          # @set_value(char)
        mov     byte ptr [12345], dil
        ret

Here no space is reserved for the object and the write is using immediate mode.