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[Bug target/55171] incorrect virtual thunk on mingw
- From: "mitza at ociweb dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Wed, 07 Nov 2012 21:10:26 +0000
- Subject: [Bug target/55171] incorrect virtual thunk on mingw
- Auto-submitted: auto-generated
- References: <bug-55171-4@http.gcc.gnu.org/bugzilla/>
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.