[PATCH] Add MicrosoftVFTableContext to AST

John McCall rjmccall at gmail.com
Mon Jul 29 10:04:15 PDT 2013

On Jul 29, 2013, at 3:43 AM, Timur Iskhodzhanov <timurrrr at google.com> wrote:
> 2013/7/26 John McCall <rjmccall at gmail.com>:
>> In both ABIs, the algorithm for performing a virtual function call is to
>> adjust
>> the base pointer to a subobject which contains the method in its primary
>> v-table, load a function pointer from the v-table, and call it.
>> In your example, the difference is just that, under MSVC, B doesn’t have
>> an entry for f() in its vf-table.  (In fact, it doesn’t have a vf-table.)
>> So the
>> caller has to adjust to something that actually does have an entry for f(),
>> namely A.
>> +  // See if this class expands a vftable of the base we look at, which is
>> either
>> +  // the one defined by the vfptr base path or the primary base.
>> Still not sure why you’re not just starting from the right base and then
>> walking up the primary base chain.
>> That's not enough even in the simple case of "B: A".
>> The vfptr is in the A layout, so the "right base" is A.
>> If we don't go to the most derived class (B) from "the right base"
>> (A), we forget to add the more derived class's own new methods (and
>> probably return-adjusting thunks).
>> My point is that you should just start recursing from B instead of this
>> weird
>> combination of walking the path and then falling back on climbing the
>> primary base chain.
>> Finding B (the most-derived subobject that A is in the primary-base chain
>> of)
>> should be really easy — it’s just a depth-first search through the complete
>> object’s layout, stopping at the first thing with the same offset as A.
>> Hm...
>> Let's consider
>> --------
>> struct A {
>>   virtual TYPE* f();
>> };
>> struct B {
>>   virtual TYPE* g();
>> };
>> struct C: A, B {  <something>  }
>> --------
>> We'll have two vfptrs: for A at offset 0 and for B at offset 4
>> (assuming 32-bit arch).
>> Currently, for the B's vftable we'll do this:
>> enter AddMethods(C)
>>   enter AddMethods(B)
>>   allocate a vftable slot for B::g
>>   leave AddMethods(B)
>> update the B::g slot with this/return adjustment if C overrides it
>> leave AddMethods(C)
>> [this somewhat reflects how Itanium's VTableContext works]
>> Basically what you want is
>> enter AddMethods(B)
>> allocate a vftable slot for B::g,
>>   if we have a (final) overrider for g() in C,
>>     calculate this/return adjustment right here*.
>> leave AddMethods(B)
>> Ok, so at the (*) stage we can probably find the FinalOverrider to
>> just write the adjustments immediately...
>> This implies rewriting ComputeThisOffset to just take the final
>> overrider and return the this offset in a complete object.
>> There were a few minor complexities in rewriting it (e.g. API being
>> not convenient) that have overloaded my brain on Friday evening
>> though.
>> Do you think this is important for the first version? If so - I'll
>> continue trying to do this next week.
>> I think it’s worth it, thanks.
>> I’ll wait to review that unless there’s something intermediate about
>> the current patch you’d like me to check out.
> Nope, it didn't work well.
> It turns out we should actually append slots to the vftable of a
> non-primary subobject if we have a covariant-return override. As a
> result, we have to visit the more-derived classes after we enumerate
> all the methods of the primary sub objects.

That’s really interesting!  Also I feel like that cannot possibly work in the
presence of virtual inheritance.  What does MSVC do if you have:

struct C0 { int x; };
struct C1 : C0 { virtual void f(); int y; }; // converting C0* to C1* is non-trivial

struct A {
  virtual C0 *foo();
  virtual C0 *bar();

struct B : virtual A {
  virtual C1 *foo(); // added to the A subobject’s vf-table?
struct C : virtual A {
  virtual C1 *bar(); // added to the A subobject’s vf-table?

strruct D : B, C {} // What’s at the end of the A subobject’s vf-table?

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