Index: include/llvm/Bitcode/LLVMBitCodes.h
===================================================================
--- include/llvm/Bitcode/LLVMBitCodes.h
+++ include/llvm/Bitcode/LLVMBitCodes.h
@@ -34,7 +34,8 @@
     CONSTANTS_BLOCK_ID,
     FUNCTION_BLOCK_ID,
 
-    UNUSED_ID1,
+    // Valid inside vendor specific toolchains only, discarded by open-source
+    VENDOR_RESERVED_ID,
 
     VALUE_SYMTAB_BLOCK_ID,
     METADATA_BLOCK_ID,