diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -2534,6 +2534,44 @@
 code for the proper architecture. It's possible to override this on the
 command line with the ``-mtriple`` command line option.
 
+.. _objectlifetime:
+
+Object Lifetime
+----------------------
+
+A memory object, or simply object, is a region of a memory space that is
+reserved by a memory allocation such as :ref:`alloca <i_alloca>`, heap
+allocation calls, and global variable definitions.
+Once it is allocated, the bytes stored in the region can only be read or written
+through a pointer that is :ref:`based on <_pointeraliasing>` the allocation
+value.
+If a pointer that is not based on the object tries to read or write to the
+object, it is undefined behavior.
+
+A lifetime of a memory object is a property that decides its accessibility.
+Unless stated otherwise, a memory object is alive since its allocation, and
+dead after its deallocation.
+It is undefined behavior to access a memory object that isn't alive, but
+operations that don't dereference it such as
+:ref:`getelementptr <i_getelementptr>` or :ref:`ptrtoint <i_ptrtoint>` return
+a valid result.
+This explains code motion of these instructions across instructions that
+deallocate the object or impact it's lifetime.
+A memory object's lifetime can be explicitly specified using
+:ref:`llvm.lifetime.start <_int_lifestart>` and
+:ref:`llvm.lifetime.end <_int_lifeend>` intrinsic function calls.
+
+Two memory objects that are both alive at some program execution point have
+addresses that don't overlap.
+If their addresses can be represented in integers, two integer ranges
+``[a1, a1+sz1)`` and ``[a2, a2+sz2)`` where a1/sz1 is the begin address/size of
+the first object and a2/sz2 is the size of the second object respectively
+are disjoint.
+It is allowed for the two objects to be adjacent and share the same integral
+address at the beginning of one object and at the end of another object.
+A dead memory object can overlap any other memory object, unless stated
+otherwise.
+
 .. _pointeraliasing:
 
 Pointer Aliasing Rules
@@ -17742,8 +17780,9 @@
 Memory Use Markers
 ------------------
 
-This class of intrinsics provides information about the lifetime of
-memory objects and ranges where variables are immutable.
+This class of intrinsics provides information about the
+:ref:`lifetime of memory objects <_objectlifetime>` and ranges where variables
+are immutable.
 
 .. _int_lifestart:
 
@@ -17760,8 +17799,8 @@
 Overview:
 """""""""
 
-The '``llvm.lifetime.start``' intrinsic specifies the start of a memory
-object's lifetime.
+The '``llvm.lifetime.start``' intrinsic specifies the start of
+:ref:`a memory object's lifetime <_objectlifetime>`.
 
 Arguments:
 """"""""""
@@ -17773,10 +17812,31 @@
 Semantics:
 """"""""""
 
-This intrinsic indicates that before this point in the code, the value
-of the memory pointed to by ``ptr`` is dead. This means that it is known
-to never be used and has an undefined value. A load from the pointer
-that precedes this intrinsic can be replaced with ``'undef'``.
+If ``ptr`` is syntactically pointing to a stack-allocated object, the object is
+initially marked as dead. The syntactic use is determined by following use-def
+chains of operations. If ``ptr`` syntactically points to more than one
+stack-allocated object via e.g., ``phi``, all of those are dead. This explains
+merging multiple ``lifetime.start`` calls into one.
+
+After '``llvm.lifetime.start``', the stack object that ``ptr`` points to in
+runtime is marked as alive and it is initialized with '``undef``'.
+The stack object is dead again when either
+:ref:`llvm.lifetime.end <int_lifeend>` to the alloca is executed or the
+function returns.
+
+After :ref:`llvm.lifetime.end <int_lifeend>` is called,
+'``llvm.lifetime.start``' on the stack object can be called again.
+In this case, the stack object has multiple lifetimes.
+The second '``llvm.lifetime.start``' call does not relocate the object.
+
+The behavior of calling '``llvm.lifetime.start``' after another
+'``llvm.lifetime.start``' to the same alive stack object is
+implementation-defined and depends on the backend that is enabled.
+
+If ``ptr`` is syntactically pointing to at least one non-stack-allocated object,
+or the object it points to is unknown, it is equivalent to
+'``memset(ptr, undef, size)``'. If ``size`` is -1, it is equivalent to
+memset undef from the begin to the end of the object.
 
 .. _int_lifeend:
 
@@ -17793,8 +17853,8 @@
 Overview:
 """""""""
 
-The '``llvm.lifetime.end``' intrinsic specifies the end of a memory
-object's lifetime.
+The '``llvm.lifetime.end``' intrinsic specifies the end of
+:ref:`a memory object's lifetime <_objectlifetime>`.
 
 Arguments:
 """"""""""
@@ -17806,10 +17866,16 @@
 Semantics:
 """"""""""
 
-This intrinsic indicates that after this point in the code, the value of
-the memory pointed to by ``ptr`` is dead. This means that it is known to
-never be used and has an undefined value. Any stores into the memory
-object following this intrinsic may be removed as dead.
+If ``ptr`` is syntactically pointing to a stack-allocated object(s),
+the object that ``ptr`` points to is dead. See
+:ref:`lifetime.start <int_lifestart>` for the rules for syntactically finding
+the object(s).
+Calling ``llvm.lifetime.end`` on a dead alloca is no-op.
+
+If ``ptr`` is syntactically pointing to at least one non-stack-allocated object,
+or the object it points to is unknown, it is equivalent to
+'``memset(ptr, undef, size)``'. If ``size`` is -1, it is equivalent to
+memset undef from the begin to the end of the object.
 
 '``llvm.invariant.start``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^