Implement part of the EmptySubobjectMap optimization described in PR6998. We still need to do this for bases.

llvm-svn: 105919

1 files changed, 16 insertions, 1 deletions

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 7c0c8088e83..6b8d7cf22dd 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -415,7 +415,14 @@ EmptySubobjectMap::CanPlaceFieldAtOffset(const FieldDecl *FD, uint64_t Offset) {
 void EmptySubobjectMap::UpdateEmptyFieldSubobjects(const CXXRecordDecl *RD, 
                                                    const CXXRecordDecl *Class,
                                                    uint64_t Offset) {
-  
+  // We know that the only empty subobjects that can conflict with empty
+  // field subobjects are subobjects empty bases that can be placed at offset
+  // zero. Because of this, we only need to keep track of empty field 
+  // subobjects with offsets less than the size of the largest empty
+  // subobject for our class.
+  if (Offset >= SizeOfLargestEmptySubobject)
+    return;
+
   AddSubobjectAtOffset(RD, Offset);
 
   const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
@@ -480,6 +487,14 @@ void EmptySubobjectMap::UpdateEmptyFieldSubobjects(const FieldDecl *FD,
     uint64_t ElementOffset = Offset;
     
     for (uint64_t I = 0; I != NumElements; ++I) {
+      // We know that the only empty subobjects that can conflict with empty
+      // field subobjects are subobjects empty bases that can be placed at 
+      // offset zero. Because of this, we only need to keep track of empty field
+      // subobjects with offsets less than the size of the largest empty
+      // subobject for our class.
+      if (ElementOffset >= SizeOfLargestEmptySubobject)
+        return;
+
       UpdateEmptyFieldSubobjects(RD, RD, ElementOffset);
       ElementOffset += Layout.getSize();
     }