http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55171
--- Comment #4 from Adam Mitz <mitza at ociweb dot com> 2012-11-07 21:10:26 UTC --- The generated code for the thunk is incorrect in that it re-uses the %eax register, clobbering the original "this" value: Dump of assembler code for function _ZTv0_n16_NK7Derived3fooEv: => 0x004019d1 <+0>: mov 0x4(%esp),%eax 0x004019d5 <+4>: mov (%eax),%eax 0x004019d7 <+6>: add -0x10(%eax),%eax 0x004019da <+9>: mov %eax,0x4(%esp) 0x004019de <+13>: jmp 0x401932 <Derived::foo() const> vtable = *base_this derived_this = vtable + *(vtable - 16) The same thunk on Linux uses both %eax and %ecx: Dump of assembler code for function _ZTv0_n16_NK7Derived3fooEv: => 0x08048eab <+0>: mov 0x8(%esp),%eax 0x08048eaf <+4>: mov (%eax),%ecx 0x08048eb1 <+6>: add -0x10(%ecx),%eax 0x08048eb4 <+9>: mov %eax,0x8(%esp) 0x08048eb8 <+13>: jmp 0x8048e08 <Derived::foo() const> vtable = *base_this derived_this = base_this + (*vtable - 16) The MinGW version must either clobber a scratch register like %ecx or %edx, or if this isn't possible (fastcall/thiscall?), save to the stack and restore.
