diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -246,7 +246,9 @@
 .. option:: -ffreestanding
 
  Indicate that the file should be compiled for a freestanding, not a hosted,
- environment.
+ environment. Note that it is assumed that a freestanding environment will
+ additionally provide `memcpy`, `memmove`, `memset` and `memcmp`
+ implementations, as these are needed for efficient codegen for many programs.
 
 .. option:: -fno-builtin