[LangRef] Make lifetime intrinsic's semantics consistent with StackColoring's comment

This patch is an update to LangRef by describing lifetime intrinsics' behavior
by following the description of MIR's LIFETIME_START/LIFETIME_END markers
at StackColoring.cpp (eb44682d67/llvm/lib/CodeGen/StackColoring.cpp (L163)) and the discussion in llvm-dev.

In order to explicitly define the meaning of an object lifetime, I added 'Object Lifetime' subsection.

Reviewed By: nlopes

Differential Revision: https://reviews.llvm.org/D94002

docs/LangRef.rst

@@ -2544,6 +2544,33 @@ This information is passed along to the backend so that it generates
 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>`, :ref:`ptrtoint <i_ptrtoint>` and
+:ref:`icmp <i_icmp>` return a valid result.
+This explains code motion of these instructions across operations that
+impact the object's lifetime.
+A stack object's lifetime can be explicitly specified using
+:ref:`llvm.lifetime.start <_int_lifestart>` and
+:ref:`llvm.lifetime.end <_int_lifeend>` intrinsic function calls.
+
 .. _pointeraliasing:
 
 Pointer Aliasing Rules
@@ -17808,8 +17835,9 @@ Other targets may support this intrinsic differently, for example, by lowering i
 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:
 
@@ -17826,8 +17854,8 @@ Syntax:
 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:
 """"""""""
@@ -17839,10 +17867,23 @@ to the object.
 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 a stack-allocated object and its offset is zero, the object is
+initially marked as dead.
+After '``llvm.lifetime.start``', the stack object that ``ptr`` points is marked
+as alive and has an uninitialized value.
+The stack object is marked as dead 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.
+The second '``llvm.lifetime.start``' call marks the object as alive, but it
+does not change the address of the object.
+
+If ``ptr`` is a non-stack-allocated object, its offset is non-zero or it is
+a stack object that is already alive, it simply fills all bytes of the object
+with ``poison``.
+
 
 .. _int_lifeend:
 
@@ -17859,8 +17900,8 @@ Syntax:
 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:
 """"""""""
@@ -17872,10 +17913,13 @@ to the object.
 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 a stack-allocated object and its offset is zero, the object is
+dead.
+Calling ``llvm.lifetime.end`` on an already dead alloca is no-op.
+
+If ``ptr`` is a non-stack-allocated object or its offset is non-zero,
+it is equivalent to simply filling all bytes of the object with ``poison``.
+
 
 '``llvm.invariant.start``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^