Index: llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
===================================================================
--- llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
+++ llvm/trunk/include/llvm/IR/DebugInfoMetadata.h
@@ -1229,12 +1229,21 @@
 
   unsigned Line;
   unsigned ScopeLine;
-  unsigned Virtuality;
   unsigned VirtualIndex;
-  unsigned Flags;
-  bool IsLocalToUnit;
-  bool IsDefinition;
-  bool IsOptimized;
+
+  // Virtuality can only assume three values, so we can pack
+  // in 2 bits (none/pure/pure_virtual).
+  unsigned Virtuality : 2;
+
+  unsigned Flags : 27;
+
+  // These are boolean flags so one bit is enough.
+  // MSVC starts a new container field every time the base
+  // type changes so we can't use 'bool' to ensure these bits
+  // are packed.
+  unsigned IsLocalToUnit : 1;
+  unsigned IsDefinition : 1;
+  unsigned IsOptimized : 1;
 
   DISubprogram(LLVMContext &C, StorageType Storage, unsigned Line,
                unsigned ScopeLine, unsigned Virtuality, unsigned VirtualIndex,
@@ -1242,9 +1251,13 @@
                bool IsOptimized, ArrayRef<Metadata *> Ops)
       : DILocalScope(C, DISubprogramKind, Storage, dwarf::DW_TAG_subprogram,
                      Ops),
-        Line(Line), ScopeLine(ScopeLine), Virtuality(Virtuality),
-        VirtualIndex(VirtualIndex), Flags(Flags), IsLocalToUnit(IsLocalToUnit),
-        IsDefinition(IsDefinition), IsOptimized(IsOptimized) {}
+        Line(Line), ScopeLine(ScopeLine), VirtualIndex(VirtualIndex),
+        Virtuality(Virtuality), Flags(Flags), IsLocalToUnit(IsLocalToUnit),
+        IsDefinition(IsDefinition), IsOptimized(IsOptimized) {
+    static_assert(dwarf::DW_VIRTUALITY_max < 4, "Virtuality out of range");
+    assert(Virtuality < 4 && "Virtuality out of range");
+    assert((Flags < (1 << 27)) && "Flags out of range");
+  }
   ~DISubprogram() = default;
 
   static DISubprogram *
Index: llvm/trunk/unittests/IR/MetadataTest.cpp
===================================================================
--- llvm/trunk/unittests/IR/MetadataTest.cpp
+++ llvm/trunk/unittests/IR/MetadataTest.cpp
@@ -1413,9 +1413,10 @@
   bool IsDefinition = true;
   unsigned ScopeLine = 3;
   DITypeRef ContainingType = getCompositeType();
-  unsigned Virtuality = 4;
+  unsigned Virtuality = 2;
   unsigned VirtualIndex = 5;
   unsigned Flags = 6;
+  unsigned NotFlags = (~Flags) & ((1 << 27) - 1);
   bool IsOptimized = false;
   MDTuple *TemplateParams = getTuple();
   DISubprogram *Declaration = getSubprogram();
@@ -1513,7 +1514,7 @@
   EXPECT_NE(N, DISubprogram::get(Context, Scope, Name, LinkageName, File, Line,
                                  Type, IsLocalToUnit, IsDefinition, ScopeLine,
                                  ContainingType, Virtuality, VirtualIndex,
-                                 ~Flags, IsOptimized, TemplateParams,
+                                 NotFlags, IsOptimized, TemplateParams,
                                  Declaration, Variables));
   EXPECT_NE(N, DISubprogram::get(Context, Scope, Name, LinkageName, File, Line,
                                  Type, IsLocalToUnit, IsDefinition, ScopeLine,