[llvm-commits] [llvm] r61010 - /llvm/trunk/docs/AliasAnalysis.html

[Nick Lewycky]

Author: nicholas
Date: Sun Dec 14 15:08:48 2008
New Revision: 61010

URL: http://llvm.org/viewvc/llvm-project?rev=61010&view=rev
Log:
Clarify the meaning of the NoAlias response. The plan is to refer to this from
a future version of LangRef.

Modified:
    llvm/trunk/docs/AliasAnalysis.html

Modified: llvm/trunk/docs/AliasAnalysis.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/AliasAnalysis.html?rev=61010&r1=61009&r2=61010&view=diff

==============================================================================
--- llvm/trunk/docs/AliasAnalysis.html (original)
+++ llvm/trunk/docs/AliasAnalysis.html Sun Dec 14 15:08:48 2008
@@ -191,16 +191,20 @@
 </div>
 
 <div class="doc_text">
-
-<p>An Alias Analysis implementation can return one of three responses:
-MustAlias, MayAlias, and NoAlias.  The No and May alias results are obvious: if
-the two pointers can never equal each other, return NoAlias, if they might,
-return MayAlias.</p>
-
-<p>The MustAlias response is trickier though.  In LLVM, the Must Alias response
-may only be returned if the two memory objects are guaranteed to always start at
-exactly the same location.  If two memory objects overlap, but do not start at
-the same location, return MayAlias.</p>
+<p>The NoAlias response is used when the two pointers refer to distinct objects,
+even regardless of whether the pointers compare equal.  For example, freed
+pointers don't alias any pointers that were allocated afterwards.  As a
+degenerate case, pointers returned by malloc(0) have no bytes for an object,
+and are considered NoAlias even when malloc returns the same pointer.  The same
+rule applies to NULL pointers.</p>
+
+<p>The MayAlias response is used whenever the two pointers might refer to the
+same object.  If the two memory objects overlap, but do not start at the same
+location, return MayAlias.</p>
+
+<p>The MustAlias response may only be returned if the two memory objects are
+guaranteed to always start at exactly the same location. A MustAlias response
+implies that the pointers compare equal.</p>
 
 </div>