diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -3452,7 +3452,36 @@
 location could clobber arbitrary memory, therefore, it has undefined
 behavior.
 
-**MemorySanitizer**, a detector of uses of uninitialized memory,
+Execution of a conditional branch is undefined behavior if its condition is
+a not-frozen expression containing undef and it can be evaluted into different
+values.
+This explains optimizations that depend on branch conditions to construct
+predicates, such as Correlated Value Propagation and Global Value Numbering.
+As it raises undefined behavior otherwise, we can assume that the branch
+condition is frozen at destination blocks.
+
+.. code-block:: text
+
+    Unsafe:
+      br undef, BB1, BB2 ; UB
+
+      %X = and i32 undef, 255
+      switch %X, label %ret [ .. ] ; UB
+
+      store undef, i8* %ptr
+      %X = load i8* %ptr ; %X is undef
+      switch i8 %X, label %ret [ .. ] ; UB
+
+    Safe:
+      %X = or i8 undef, 255 ; always 255
+      switch i8 %X, label %ret [ .. ] ; Well-defined
+
+      %X = freeze i1 undef
+      br %X, BB1, BB2 ; Well-defined (non-deterministic jump)
+
+
+This is also consistent with the behavior of MemorySanitizer.
+MemorySanitizer, detector of uses of uninitialized memory,
 defines a branch with condition that depends on an undef value (or
 certain other values, like e.g. a result of a load from heap-allocated
 memory that has never been stored to) to have an externally visible
@@ -7035,7 +7064,8 @@
 argument is evaluated. If the value is ``true``, control flows to the
 '``iftrue``' ``label`` argument. If "cond" is ``false``, control flows
 to the '``iffalse``' ``label`` argument.
-If '``cond``' is ``poison``, this instruction has undefined behavior.
+If '``cond``' is ``poison`` or ``undef``, this instruction has undefined
+behavior.
 
 Example:
 """"""""