[PATCH] [ms-cxxabi] Emit linkonce complete dtors in TUs that need them

[John McCall]

On Jul 15, 2013, at 10:03 AM, Reid Kleckner <rnk at google.com> wrote:
> http://llvm-reviews.chandlerc.com/D1066

Okay, so what I'm understanding here is that, when MSVC emits a destructor
(for example, when it's out of line), it emits a global symbol which evaluates
the destructor body and also destroys the non-virtual-base subobjects, which
is basically the behavior of Itanium base destructors.  If it needs a function
that destroys the entire thing (the behavior of the Itanium complete destructor),
it emits it ad hoc, using a symbol that'll be uniqued within the module.

Meanwhile, the vf-table for a virtual destructor has entrypoints for what,
exactly?  There seems to be a vector deleting destructor, presumably used
for delete[] (which doesn't actually need to be virtually dispatched, but that
decision may post-date MSVC's decision).  There's also a scalar deleting
destructor, used for delete?  How do they generate direct calls to the destructor,
i.e. foo->~Foo()?

Anyway, this should all be in a nice, detailed comment somewhere in
MSCXXABI.cpp.

-bool CodeGenModule::MayDeferGeneration(const ValueDecl *Global) {
+bool CodeGenModule::MayDeferGeneration(GlobalDecl GD) {

Instead of doing this, you should just make non-deferred generation only emit
the variants that it actually guarantees, which seems to be just the base
variant.

+  // XXX: In the Microsoft ABI, we want to emit a delegating complete dtor
+  // without a definition, which means we won't be able to tell if the
+  // definition is a try body.  In this case, MSVC simply delegates, so we do
+  // the same.

What is this XXX about?  Is this a FIXME?  It looks like we're actually doing it.

+  // If the class has no virtual bases, then the complete and base destructors
+  // are equivalent, for all C++ ABIs supported by clang.  We can save on code
+  // size by calling the base dtor directly, especially if we'd have to emit a
+  // thunk otherwise.
+  // FIXME: We could do this for Itanium, but we should consult with John first.

Okay, you can't put this in a comment. :)

I'm fine with unconditionally emitting calls to base dtors in this case.

+  if (getCXXABI().useThunkForDtorVariant(dtorType) &&
+      dtorType == Dtor_Complete && dtor->getParent()->getNumVBases() == 0)

Checking dtorType and getNumVBases() are basically free because they
don't require a real function call.  You should do those first.  Or actually, I
guess you don't need the check.

Otherwise this looks good.

John.