[PATCH] D45112: [MS] Emit vftable thunks for functions with incomplete prototypes

[Reid Kleckner via Phabricator via cfe-commits]

rnk created this revision.
rnk added reviewers: rjmccall, rsmith, hans.
Herald added a subscriber: Prazek.

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 PR36952


https://reviews.llvm.org/D45112

Files:
  clang/lib/CodeGen/CGCall.cpp
  clang/lib/CodeGen/CGVTables.cpp
  clang/lib/CodeGen/CGVTables.h
  clang/lib/CodeGen/CodeGenFunction.h
  clang/lib/CodeGen/CodeGenModule.h
  clang/lib/CodeGen/CodeGenTypes.h
  clang/lib/CodeGen/MicrosoftCXXABI.cpp
  clang/test/CodeGenCXX/ms-thunks-unprototyped-return.cpp
  clang/test/CodeGenCXX/ms-thunks-unprototyped.cpp

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