[MS] Emit vftable thunks for functions with incomplete prototypes

Summary:
The following class hierarchy requires that we be able to emit a
this-adjusting thunk for B::foo in C's vftable:

  struct Incomplete;
  struct A {
    virtual A* foo(Incomplete p) = 0;
  };
  struct B : virtual A {
    void foo(Incomplete p) override;
  };
  struct C : B { int c; };

This TU is valid, but lacks a definition of 'Incomplete', which makes it
hard to build a thunk for the final overrider, B::foo.

Before this change, Clang gives up attempting to emit the thunk, because
it assumes that if the parameter types are incomplete, it must be
emitting the thunk for optimization purposes. This is untrue for the MS
ABI, where the implementation of B::foo has no idea what thunks C's
vftable may require. Clang needs to emit the thunk without necessarily
having access to the complete prototype of foo.

This change makes Clang emit a musttail variadic call when it needs such
a thunk. I call these "unprototyped" thunks, because they only prototype
the "this" parameter, which must always come first in the MS C++ ABI.

These thunks work, but they create ugly LLVM IR. If the call to the
thunk is devirtualized, it will be a call to a bitcast of a function
pointer. Today, LLVM cannot inline through such a call, but I want to
address that soon, because we also use this pattern for virtual member
pointer thunks.

This change also implements an old FIXME in the code about reusing the
thunk's computed CGFunctionInfo as much as possible. Now we don't end up
computing the thunk's mangled name and arranging it's prototype up to
around three times.

Fixes PR25641

Reviewers: rjmccall, rsmith, hans

Subscribers: Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D45112

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@329009 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 4 insertions, 6 deletions

diff --git a/lib/CodeGen/CGVTables.h b/lib/CodeGen/CGVTables.h
index b92212c368..a11474a15e 100644
--- a/lib/CodeGen/CGVTables.h
+++ b/lib/CodeGen/CGVTables.h
@@ -57,12 +57,10 @@ class CodeGenVTables {
   /// Cache for the deleted virtual member call function.
   llvm::Constant *DeletedVirtualFn = nullptr;
 
-  /// emitThunk - Emit a single thunk.
-  void emitThunk(GlobalDecl GD, const ThunkInfo &Thunk, bool ForVTable);
-
-  /// maybeEmitThunkForVTable - Emit the given thunk for the vtable if needed by
-  /// the ABI.
-  void maybeEmitThunkForVTable(GlobalDecl GD, const ThunkInfo &Thunk);
+  /// Get the address of a thunk and emit it if necessary.
+  llvm::Constant *maybeEmitThunk(GlobalDecl GD,
+                                 const ThunkInfo &ThunkAdjustments,
+                                 bool ForVTable);
 
   void addVTableComponent(ConstantArrayBuilder &builder,
                           const VTableLayout &layout, unsigned idx,