[Docs] Try to clarify the concept of domains for noalias scope

Summary:
This tries to anchor down the concept of domains a bit better.  I had
trouble initially relating this to anything.  Also talking to David
Majnemer on IRC suggested that I wasn't the only one.

Reviewers: hfinkel

Subscribers: llvm-commits, majnemer

Differential Revision: http://reviews.llvm.org/D18799

llvm-svn: 267647

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 0612d36..197439f 100644 (file)
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -4412,12 +4412,20 @@ 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. When evaluating an aliasing query, if for some domain, the set
+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
 subset of (or equal to) the set of scopes for that domain in another
 instruction's ``noalias`` list, then the two memory accesses are assumed not to
 alias.
 
+Because scopes in one domain don't affect scopes in other domains, separate
+domains can be used to compose multiple independent noalias sets.  This is
+used for example during inlining.  As the noalias function parameters are
+turned into noalias scope metadata, a new domain is used every time the
+function is inlined.
+
 The metadata identifying each domain is itself a list containing one or two
 entries. The first entry is the name of the domain. Note that if the name is a
 string then it can be combined across functions and translation units. A