When BasicAA can determine that two pointers have the same base but
differ by a dynamic offset, return PartialAlias instead of MayAlias.
See the comment in the code for details. This fixes PR9971.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131781 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index f1bb8a3..f506608 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -940,7 +940,17 @@
return NoAlias;
}
- return MayAlias;
+ // Statically, we can see that the base objects are the same, but the
+ // pointers have dynamic offsets which we can't resolve. And none of our
+ // little tricks above worked.
+ //
+ // TODO: Returning PartialAlias instead of MayAlias is a mild hack; the
+ // practical effect of this is protecting TBAA in the case of dynamic
+ // indices into arrays of unions. An alternative way to solve this would
+ // be to have clang emit extra metadata for unions and/or union accesses.
+ // A union-specific solution wouldn't handle the problem for malloc'd
+ // memory however.
+ return PartialAlias;
}
/// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select