[LangRef] Spell out alias attribute/metadata violations are UB. (#116220)

Update the documentation for the noalias attribute, !alias.scope and
!loop.parallel_accesses metadata to clarify they are UB on voilation the
noalias property.

PR: #116220
---------

Co-authored-by: Nuno Lopes <[email protected]>

Author: fhahn

llvm/docs/LangRef.rst

@@ -1360,11 +1360,13 @@ Currently, only the following parameter attributes are defined:
     accessed, during the execution of the function, via pointer values not
     *based* on the argument or return value. This guarantee only holds for
     memory locations that are *modified*, by any means, during the execution of
-    the function. The attribute on a return value also has additional semantics
-    described below. The caller shares the responsibility with the callee for
-    ensuring that these requirements are met.  For further details, please see
-    the discussion of the NoAlias response in :ref:`alias analysis <Must, May,
-    or No>`.
+    the function. If there are other accesses not based on the argument or
+    return value, the behavior is undefined. The attribute on a return value
+    also has additional semantics described below. The caller shares the
+    responsibility with the callee for described below. The caller shares the
+    responsibility with the callee for ensuring that these requirements are met.
+    For further details, please see the discussion of the NoAlias response in
+    :ref:`alias analysis <Must, May,  or No>`.
 
     Note that this definition of ``noalias`` is intentionally similar
     to the definition of ``restrict`` in C99 for function arguments.
@@ -6816,7 +6818,9 @@ tuples this way:
 A memory access with an access tag ``(BaseTy1, AccessTy1, Offset1)``
 aliases a memory access with an access tag ``(BaseTy2, AccessTy2,
 Offset2)`` if either ``(BaseTy1, Offset1)`` is reachable from ``(Base2,
-Offset2)`` via the ``Parent`` relation or vice versa.
+Offset2)`` via the ``Parent`` relation or vice versa. If memory accesses
+alias even though they are noalias according to ``!tbaa`` metadata, the
+behavior is undefined.
 
 As a concrete example, the type descriptor graph for the following program
 
@@ -6936,9 +6940,9 @@ does not carry useful data and need not be preserved.
 noalias memory-access sets. This means that some collection of memory access
 instructions (loads, stores, memory-accessing calls, etc.) that carry
 ``noalias`` metadata can specifically be specified not to alias with some other
-collection of memory access instructions that carry ``alias.scope`` metadata.
-Each type of metadata specifies a list of scopes where each scope has an id and
-a domain.
+collection of memory access instructions that carry ``alias.scope`` metadata. If
+accesses from different collections alias, the behavior is undefined. Each type
+of metadata specifies a list of scopes where each scope has an id and a domain.
 
 When evaluating an aliasing query, if for some domain, the set
 of scopes with that domain in one instruction's ``alias.scope`` list is a
@@ -7695,7 +7699,8 @@ If all memory-accessing instructions in a loop have
 ``llvm.access.group`` metadata that each refer to one of the access
 groups of a loop's ``llvm.loop.parallel_accesses`` metadata, then the
 loop has no loop carried memory dependencies and is considered to be a
-parallel loop.
+parallel loop. If there is a loop-carried dependency, the behavior is
+undefined.
 
 Note that if not all memory access instructions belong to an access
 group referred to by ``llvm.loop.parallel_accesses``, then the loop must