[Issue 18928] extern(C++) bad codegen, wrong calling convention

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18928

--- Comment #3 from Manu <turkeyman at gmail.com> ---
You'll have to relate these 2 lines to the .cpp file (attached) in main()

Calling code in main():

// static call...
    auto x = addUser(test, "test");
00007FF74843C587  lea         rdx,[string "test" (07FF748448D44h)]  
00007FF74843C58E  mov         rcx,qword ptr [test]  
00007FF74843C592  call        addUser (07FF74840C735h)  
00007FF74843C597  mov         dword ptr [rbp+104h],eax  
00007FF74843C59D  mov         eax,dword ptr [rbp+104h]  
00007FF74843C5A3  mov         dword ptr [x],eax  

// virtual call...
        x = test->addUser("test");
00007FF74843C5A6  mov         rax,qword ptr [test]  
00007FF74843C5AA  mov         rax,qword ptr [rax]  
00007FF74843C5AD  lea         r8,[string "test" (07FF748448D44h)]  
00007FF74843C5B4  lea         rdx,[rbp+124h]  // what is? RVO return address?
00007FF74843C5BB  mov         rcx,qword ptr [test]  
00007FF74843C5BF  call        qword ptr [rax]  
00007FF74843C5C1  mov         eax,dword ptr [rax]  
00007FF74843C5C3  mov         dword ptr [x],eax  

This is the C++ code calling into D; it appears C++ makes a static call
differently to a virtual call? I suspect RDX in the virtual call is the RVO
return address? I wonder why the static-call doesn't follow the same call
convention?


I can attach the DMD disassembly? But it's probably easier to build that
yourself.


This might reveal that the VC ABI has RVO rules that we don't understand?

--