Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -2181,6 +2181,13 @@
 operations relative to non-volatile operations. This is not Java's
 "volatile" and has no cross-thread synchronization behavior.
 
+A volatile load or store may access addresses which do not point to memory
+(for example, MMIO registers). Unlike a non-volatile load or store, these
+operations may have side-effects. However, an access to an address which
+does not point to memory must not modify any memory that is accessible by
+the module being compiled, and the compiler may assume volatile operations
+do not trap.
+
 IR-level volatile loads and stores cannot safely be optimized into
 llvm.memcpy or llvm.memmove intrinsics even when those intrinsics are
 flagged volatile. Likewise, the backend should never split or merge