[PATCH] [ms-cxxabi] Implement member data pointers for non-dynamic classes

[John McCall]

On Mar 20, 2013, at 3:59 PM, Reid Kleckner <rnk at google.com> wrote:
> CHANGE SINCE LAST DIFF
>  http://llvm-reviews.chandlerc.com/D558?vs=1333&id=1338#toc


+  // It's a little silly for us to cache this.
+  llvm::IntegerType *getPtrDiffTy() {

It's actually CGM.PtrDiffTy, so there was never a good reason to cache it
separately.

+llvm::Constant *
+MicrosoftCXXABI::getNullMemberDataPointer(const MemberPointerType *MPT) {
+  assert(MPT->isMemberDataPointer());
+  llvm::Constant *Zero = llvm::ConstantInt::get(getPtrDiffTy(), 0);
+  llvm::Constant *AllOnes = llvm::Constant::getAllOnesValue(getPtrDiffTy());
+  const CXXRecordDecl *RD =
+    cast<CXXRecordDecl>(MPT->getClass()->getAs<RecordType>()->getDecl());

MPT->getClass()->getAsCXXRecordDecl()

+  if (RD->getNumVBases() == 0) {

I don't think you can safely call this on a forward-declaration.

Why don't you make helper functions to create the all-ones and all-zero
ptrdiff_t values?  That would make this code much more succinct.

+  // FIXME: Emit a pair of {vbtable offset, field offset} when we know vbtable
+  // layout.
+  return GetBogusMemberPointer(QualType(MPT, 0));

I don't think this ever actually comes up here.  If it did, you'd need a
totally different interface anyway.

I believe vb-table offsets are only introduced when you convert across a
virtual base boundary.

+  // For member data pointers, this is just a check against -1 or 0.
+  if (MPT->isMemberDataPointer()) {
+    assert(MemPtr->getType() == getPtrDiffTy());
+    llvm::Constant *Val = getNullMemberDataPointer(MPT);
+    return Builder.CreateICmpNE(MemPtr, Val, "memptr.tobool");
+  }

You forgot the virtual inheritance case here.

+  if (MemPtr->getType() != getPtrDiffTy()) {
+    // If it's not a ptrdiff_t, then it's an aggregate.
+    ErrorUnsupportedABI(CGF, "non-scalar member pointers");
+    return llvm::Constant::getNullValue(PType);
+  }
+

Shouldn't this be pretty straightforward to implement?  We may not know
how to lay out a vbtbl yet, so the conversion case will still elude us, but we
do know the offset of the vb-table and how to do an offsetted load out of it.

John.