diff --git a/clang/docs/HLSL/HLSLSupport.rst b/clang/docs/HLSL/HLSLSupport.rst
--- a/clang/docs/HLSL/HLSLSupport.rst
+++ b/clang/docs/HLSL/HLSLSupport.rst
@@ -89,6 +89,14 @@
 ``ExternalSemaSource`` to lazily complete data types, which is a **huge**
 performance win for HLSL.
 
+In case PCH is include when compile HLSL in cc1 mode, the ``ExternalSemaSource``
+will be a ``MultiplexExternalSemaSource`` which include both the ``PCH`` and
+``HLSLExternalSemaSource``. For Built-in data/templates in PCH are already
+define completed, ``HLSLExternalSemaSource`` will just reuse them.
+If the Built-in data/templates are not define completed,
+``HLSLExternalSemaSource`` will create new decls and use the old decls as
+argument for setPreviousDecl.
+
 CodeGen
 -------