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...
