[AST] Improve support of external layouts in `MicrosoftRecordLayoutBuilder`

Summary:
This patch fixes several small problems with external layouts support in
`MicrosoftRecordLayoutBuilder`:
- aligns properly the size of a struct that ends with a bit field. It was
  aligned on byte before, not on the size of the field, so the struct size was
  smaller than it should be;
- adjusts the struct size when injecting a vbptr in the case when there were no
  bases or fields allocated after the vbptr. Similarly, without the adjustment
  the struct was smaller than it should be;
- the same fix as above for the vfptr.
All these fixes affect the non-virtual size of a struct, so they are tested
through non-virtual inheritance.

Reviewers: rnk, zturner, rsmith

Reviewed By: rnk

Subscribers: jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58544

llvm-svn: 356047

1 files changed, 17 insertions, 5 deletions

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 99b7cbd0224..2d112c45222 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -2692,7 +2692,8 @@ void MicrosoftRecordLayoutBuilder::layoutBitField(const FieldDecl *FD) {
     auto FieldBitOffset = External.getExternalFieldOffset(FD);
     placeFieldAtBitOffset(FieldBitOffset);
     auto NewSize = Context.toCharUnitsFromBits(
-        llvm::alignTo(FieldBitOffset + Width, Context.getCharWidth()));
+        llvm::alignDown(FieldBitOffset, Context.toBits(Info.Alignment)) +
+        Context.toBits(Info.Size));
     Size = std::max(Size, NewSize);
     Alignment = std::max(Alignment, Info.Alignment);
   } else if (IsUnion) {
@@ -2741,12 +2742,17 @@ void MicrosoftRecordLayoutBuilder::injectVBPtr(const CXXRecordDecl *RD) {
   CharUnits InjectionSite = VBPtrOffset;
   // But before we do, make sure it's properly aligned.
   VBPtrOffset = VBPtrOffset.alignTo(PointerInfo.Alignment);
+  // Determine where the first field should be laid out after the vbptr.
+  CharUnits FieldStart = VBPtrOffset + PointerInfo.Size;
   // Shift everything after the vbptr down, unless we're using an external
   // layout.
-  if (UseExternalLayout)
+  if (UseExternalLayout) {
+    // It is possible that there were no fields or bases located after vbptr,
+    // so the size was not adjusted before.
+    if (Size < FieldStart)
+      Size = FieldStart;
     return;
-  // Determine where the first field should be laid out after the vbptr.
-  CharUnits FieldStart = VBPtrOffset + PointerInfo.Size;
+  }
   // Make sure that the amount we push the fields back by is a multiple of the
   // alignment.
   CharUnits Offset = (FieldStart - InjectionSite)
@@ -2771,8 +2777,14 @@ void MicrosoftRecordLayoutBuilder::injectVFPtr(const CXXRecordDecl *RD) {
   if (HasVBPtr)
     VBPtrOffset += Offset;
 
-  if (UseExternalLayout)
+  if (UseExternalLayout) {
+    // The class may have no bases or fields, but still have a vfptr
+    // (e.g. it's an interface class). The size was not correctly set before
+    // in this case.
+    if (FieldOffsets.empty() && Bases.empty())
+      Size += Offset;
     return;
+  }
 
   Size += Offset;