Index: llvm/docs/LangRef.rst
===================================================================
--- llvm/docs/LangRef.rst
+++ llvm/docs/LangRef.rst
@@ -2489,6 +2489,19 @@
 ``fast``
    This flag implies all of the others.
 
+Forward-Progress Requirement
+----------------------------
+
+Every thread of execution is required to eventually cause a side-effect
+(including but not limited to accessing of inaccessible memory, volatile or
+atomic memory access, or other forms of thread synchronization). Failure to
+cause a side-effect in a finite amount of time results in undefined behavior.
+
+In particular, infinite loops and infinite recursion without side-effects
+result in undefined behavior. Language frontends that do not have a
+forward-progress requirement should insert a call to `@llvm.sideeffect` as part
+of every function definition and every loop.
+
 .. _uselistorder:
 
 Use-list Order Directives