diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -1198,12 +1198,23 @@
     function, returning a pointer to allocated storage disjoint from the
     storage for any other object accessible to the caller.
 
+.. _nocapture:
+
 ``nocapture``
-    This indicates that the callee does not make any copies of the
-    pointer that outlive the callee itself in any form such as a pointer stored
-    in the memory or as a return value. This is not a valid
-    attribute for return values.  Addresses used in volatile operations
-    are considered to be captured.
+    This indicates that the callee does not :ref:`capture <pointercapture>` the
+    pointer. This is not a valid attribute for return values.
+    This attribute applies only to the particular copy of the pointer passed in
+    this argument. A caller could pass two copies of the same pointer with one
+    being annotated nocapture and the other not, and the callee could validly
+    capture through the non annotated parameter.
+
+.. code-block:: llvm
+
+    define void @f(i8* nocapture %a, i8* %b) {
+      ; (capture %b)
+    }
+
+    call void @f(i8* @glb, i8* @glb) ; well-defined
 
 ``nofree``
     This indicates that callee does not free the pointer argument. This is not
@@ -2648,6 +2659,79 @@
 which specialized optimization passes may use to implement type-based
 alias analysis.
 
+.. _pointercapture:
+
+Pointer Capture
+---------------
+
+Given a function call and a pointer that is passed as an argument or stored in
+the memory before the call, a pointer is *captured* by the call if it makes a
+copy of any part of the pointer that outlives the call.
+To be precise, a pointer is captured if one or more of the following conditions
+hold:
+
+1. The call stores any bit of the pointer carrying information into a place,
+and the stored bits can be read from the place by the caller after this call
+exits.
+
+.. code-block:: llvm
+
+    @glb  = global i8* null
+    @glb2 = global i8* null
+    @glb3 = global i8* null
+    @glbi = global i32 0
+
+    define i8* @f(i8* %a, i8* %b, i8* %c, i8* %d, i8* %e) {
+      store i8* %a, i8** @glb ; %a is captured by this call
+
+      store i8* %b,   i8** @glb2 ; %b isn't captured because the stored value is overwritten by the store below
+      store i8* null, i8** @glb2
+
+      store i8* %c,   i8** @glb3
+      call void @g() ; If @g makes a copy of %c that outlives this call (@f), %c is captured
+      store i8* null, i8** @glb3
+
+      %i = ptrtoint i8* %d to i64
+      %j = trunc i64 %i to i32
+      store i32 %j, i32* @glbi ; %d is captured
+
+      ret i8* %e ; %e is captured
+    }
+
+2. The call stores any bit of the pointer carrying information into a place,
+and the stored bits can be safely read from the place by another thread via
+synchronization.
+
+.. code-block:: llvm
+
+    @lock = global i1 true
+
+    define void @f(i8* %a) {
+      store i8* %a, i8** @glb
+      store atomic i1 false, i1* @lock release ; %a is captured because another
+thread can safely read @glb
+      store i8* null, i8** @glb
+      ret void
+    }
+
+3. The call's behavior depends on any bit of the pointer carrying information.
+
+.. code-block:: llvm
+
+    @glb  = global i8 0
+    define void @f(i8* %a) {
+      %c = icmp eq i8* %a, @glb
+      br i1 %c, BB_EXIT, BB_CONTINUE ; escapes %a
+    BB_EXIT:
+      call void exit()
+      unreachable
+    BB_CONTINUE:
+      ret void
+    }
+
+4. The pointer is used in a volatile access as its address.
+
+
 .. _volatile:
 
 Volatile Memory Accesses