http://llvm.org/bugs/show_bug.cgi?id=17738
Bug ID: 17738
Summary: [-cxx-abi microsoft] Support vtordisp thunks
Product: clang
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
This ABI tries hard to avoid "virtual this adjustment". It's not always
possible though.
Consider the following example:
-------------
struct A {
virtual void f(); // Expects "(A*)this" in ECX
};
struct B : virtual A {
virtual void f(); // Expects "(char*)(B*)this + 12" in ECX
virtual ~B(); // Might call f()
};
----
B::f assumes that the class layout is |BA| and uses static "-12" this
adjustment in the prologue.
However, if a class overrides a virtual function of its base and has a
non-trivial ctor/dtor that call this virtual function (or may escape "this" to
some code that might call it), a virtual adjustment might be needed, in case
the current class layout and the most derived class layout are different.
For example, in this class:
-------------
struct C : virtual B {
virtual void f();
};
-------------
the layout is |CAB|.
If f() is called during B ctor or dtor, the thunk that's stored in B's the
vftable should apply a "+4+12" this adjustment so B::f() gets the right value
of this in the prologue. The extra adjustment is not needed if B is the most
derived class, i.e. the exact adjustment should be determined dynamically. The
value of the dynamic adjustment is stored in the hidden vtordisp associated
with the A vbase, hence the thunk is called a "vtordisp thunk".
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