[Bug c++/60367] Default argument object is not getting constructed

[rob.desbois at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60367

--- Comment #5 from rob.desbois at gmail dot com ---
The following is a side-by-side diff of the disassembly of the incorrect
version vs. a correct version (defaulting the parameter with = foo{}). The
object foo has a single member of type char initialized in default, move, and
copy ctors.

 incorrect version                         | correct version
-------------------------------------------|-----------------------------------
push   %rbp                                   push   %rbp
mov    %rsp,%rbp                              mov    %rsp,%rbp
push   %rbx                                   push   %rbx
sub    $0x28,%rsp                          |  sub    $0x18,%rsp
lea    -0x11(%rbp),%rax                       lea    -0x11(%rbp),%rax
mov    %rax,%rdi                              mov    %rax,%rdi
callq  0x400bbe <_ZN3fooC2Ev>              |  callq  0x400bb8 <_ZN3fooC2Ev>
movzbl -0x11(%rbp),%eax                    |  lea    -0x11(%rbp),%rax
mov    %al,-0x30(%rbp)                     <   
lea    -0x30(%rbp),%rax                    <   
mov    %rax,%rdi                              mov    %rax,%rdi
callq  0x4009f0 <_Z6test_a3foo>            |  callq  0x400a50 <_Z6test_b3foo>
lea    -0x11(%rbp),%rax                       lea    -0x11(%rbp),%rax
mov    %rax,%rdi                              mov    %rax,%rdi
callq  0x400bfe <_ZN3fooD2Ev>              |  callq  0x400bf8 <_ZN3fooD2Ev>
mov    $0x0,%eax                              mov    $0x0,%eax
jmp    0x400b65 <main+85>                  |  jmp    0x400b5e <main+78>
mov    %rax,%rbx                              mov    %rax,%rbx
lea    -0x11(%rbp),%rax                       lea    -0x11(%rbp),%rax
mov    %rax,%rdi                              mov    %rax,%rdi
callq  0x400bfe <_ZN3fooD2Ev>              |  callq  0x400bf8 <_ZN3fooD2Ev>
mov    %rbx,%rax                              mov    %rbx,%rax
mov    %rax,%rdi                              mov    %rax,%rdi
callq  0x4008b0 <_Unwind_Resume@plt>          callq  0x4008b0
<_Unwind_Resume@plt>
add    $0x28,%rsp                          |  add    $0x18,%rsp
pop    %rbx                                   pop    %rbx
pop    %rbp                                   pop    %rbp
retq                                          retq



It does look like the incorrect version on the left could be bitwise-copying
the default-constructed object.
As an aside - both versions have a single constructor call but TWO destructor
calls...