diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -9327,6 +9327,8 @@
 value of a type like ``i20`` with a size that is not an integral number
 of bytes, it is unspecified what happens to the extra bits that do not
 belong to the type, but they will typically be overwritten.
+If ``<value>`` is of aggregate type, padding is filled with
+:ref:`undef <undefvalues>`.
 
 Example:
 """"""""