diff --git a/llvm/bindings/ocaml/debuginfo/debuginfo_ocaml.c b/llvm/bindings/ocaml/debuginfo/debuginfo_ocaml.c
--- a/llvm/bindings/ocaml/debuginfo/debuginfo_ocaml.c
+++ b/llvm/bindings/ocaml/debuginfo/debuginfo_ocaml.c
@@ -33,7 +33,7 @@
   i_DIFlagPublic,
   i_DIFlagFwdDecl,
   i_DIFlagAppleBlock,
-  i_DIFlagReservedBit4,
+  i_DIFlagIsZeroSize,
   i_DIFlagVirtual,
   i_DIFlagArtificial,
   i_DIFlagExplicit,
@@ -78,8 +78,8 @@
     return LLVMDIFlagFwdDecl;
   case i_DIFlagAppleBlock:
     return LLVMDIFlagAppleBlock;
-  case i_DIFlagReservedBit4:
-    return LLVMDIFlagReservedBit4;
+  case i_DIFlagIsZeroSize:
+    return LLVMDIFlagIsZeroSize;
   case i_DIFlagVirtual:
     return LLVMDIFlagVirtual;
   case i_DIFlagArtificial:
diff --git a/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.ml b/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.ml
--- a/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.ml
+++ b/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.ml
@@ -65,7 +65,7 @@
     | Public
     | FwdDecl
     | AppleBlock
-    | ReservedBit4
+    | IsZeroSize
     | Virtual
     | Artificial
     | Explicit
diff --git a/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.mli b/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.mli
--- a/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.mli
+++ b/llvm/bindings/ocaml/debuginfo/llvm_debuginfo.mli
@@ -65,7 +65,7 @@
     | Public
     | FwdDecl
     | AppleBlock
-    | ReservedBit4
+    | IsZeroSize
     | Virtual
     | Artificial
     | Explicit
diff --git a/llvm/include/llvm-c/DebugInfo.h b/llvm/include/llvm-c/DebugInfo.h
--- a/llvm/include/llvm-c/DebugInfo.h
+++ b/llvm/include/llvm-c/DebugInfo.h
@@ -31,7 +31,7 @@
   LLVMDIFlagPublic = 3,
   LLVMDIFlagFwdDecl = 1 << 2,
   LLVMDIFlagAppleBlock = 1 << 3,
-  LLVMDIFlagReservedBit4 = 1 << 4,
+  LLVMDIFlagIsZeroSize = 1 << 4,
   LLVMDIFlagVirtual = 1 << 5,
   LLVMDIFlagArtificial = 1 << 6,
   LLVMDIFlagExplicit = 1 << 7,
@@ -59,8 +59,8 @@
   LLVMDIFlagLittleEndian = 1 << 28,
   LLVMDIFlagAllCallsDescribed = 1 << 29,
   LLVMDIFlagIndirectVirtualBase = (1 << 2) | (1 << 5),
-  LLVMDIFlagAccessibility = LLVMDIFlagPrivate | LLVMDIFlagProtected |
-                            LLVMDIFlagPublic,
+  LLVMDIFlagAccessibility =
+      LLVMDIFlagPrivate | LLVMDIFlagProtected | LLVMDIFlagPublic,
   LLVMDIFlagPtrToMemberRep = LLVMDIFlagSingleInheritance |
                              LLVMDIFlagMultipleInheritance |
                              LLVMDIFlagVirtualInheritance
diff --git a/llvm/include/llvm/IR/DebugInfoFlags.def b/llvm/include/llvm/IR/DebugInfoFlags.def
--- a/llvm/include/llvm/IR/DebugInfoFlags.def
+++ b/llvm/include/llvm/IR/DebugInfoFlags.def
@@ -36,8 +36,10 @@
 HANDLE_DI_FLAG(3, Public)
 HANDLE_DI_FLAG((1 << 2), FwdDecl)
 HANDLE_DI_FLAG((1 << 3), AppleBlock)
-// Used to be BlockByRef, can be reused for anything except DICompositeType.
-HANDLE_DI_FLAG((1 << 4), ReservedBit4)
+// IsZeroSize is valid only for DIDerivedType.
+// This bit was used as BlockByrefStruct for DICompositeType (D67453), do not
+// reuse it for DICompositeType so that Verifier can detect old bitcode.
+HANDLE_DI_FLAG((1 << 4), IsZeroSize)
 HANDLE_DI_FLAG((1 << 5), Virtual)
 HANDLE_DI_FLAG((1 << 6), Artificial)
 HANDLE_DI_FLAG((1 << 7), Explicit)
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1102,7 +1102,9 @@
            N.getRawVTableHolder());
   AssertDI(!hasConflictingReferenceFlags(N.getFlags()),
            "invalid reference flags", &N);
-  unsigned DIBlockByRefStruct = 1 << 4;
+  const unsigned DIBlockByRefStruct = 1 << 4;
+  static_assert(DIBlockByRefStruct == DINode::FlagIsZeroSize,
+                "DI Flag BlockByRefStruct == IsZeroSize");
   AssertDI((N.getFlags() & DIBlockByRefStruct) == 0,
            "DIBlockByRefStruct on DICompositeType is no longer supported", &N);
 
diff --git a/llvm/test/Verifier/blockbyref.ll b/llvm/test/Verifier/blockbyref.ll
--- a/llvm/test/Verifier/blockbyref.ll
+++ b/llvm/test/Verifier/blockbyref.ll
@@ -16,4 +16,4 @@
 !0 = !{i32 2, !"Debug Info Version", i32 3}
 !1 = distinct !DISubprogram()
 !2 = !DILocalVariable(scope: !1, type: !3)
-!3 = !DICompositeType(tag: DW_TAG_structure_type, flags: DIFlagReservedBit4)
+!3 = !DICompositeType(tag: DW_TAG_structure_type, flags: DIFlagIsZeroSize)