[PATCH] D99790: [CGCall] Annotate `this` argument with alignment

[James Y Knight via Phabricator via cfe-commits]

jyknight added a comment.

It seems like there's a bug with vtable thunks getting the wrong information. This appears to be a pre-existing bug, but this change has caused it to start actively breaking code.

Test case:

  class Base1 {
      virtual void Foo1();
  };
  
  class Base2 {
      virtual void Foo2();
  };
  
  class alignas(16) Obj : public Base1, public Base2 {
     void Foo1() override;
     void Foo2() override;
     ~Obj();
  };
  
  void Obj::Foo1() {}
  void Obj::Foo2() {}

emits three method definitions:

  define dso_local void @_ZN3Obj4Foo1Ev(%class.Obj* nonnull align 16 dereferenceable(16) %0) unnamed_addr #0 align 2 !dbg !7 {
  define dso_local void @_ZN3Obj4Foo2Ev(%class.Obj* nonnull align 16 dereferenceable(16) %0) unnamed_addr #0 align 2 !dbg !25 {
  define dso_local void @_ZThn8_N3Obj4Foo2Ev(%class.Obj* nonnull align 16 dereferenceable(16) %0) unnamed_addr #2 align 2 !dbg !29 {

(See https://godbolt.org/z/MxhYMe1q7, for now at least)

That third method declaration is bogus -- its argument is _not_ an `Obj*` at all! In fact, it's pointing at `Obj + 8` -- at the embedded `Base2` object! As such, `align 16` is incorrect, as is `dereferenceable(16)`. The additional 8 bytes of dereferenceable claim apparently hasn't broken anything noticeable, but the alignment claim is causing actual trouble.

As such, I suggest to revert this change, separately commit a fix to that underlying bug, and then re-submit this change after that.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D99790/new/

https://reviews.llvm.org/D99790