Reapply r131781, now that the GVN bug with partially-aliasing loads

[oota-llvm.git] / lib / Analysis / BasicAliasAnalysis.cpp

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 24297d4a0f3cf39c9465c04552c3df9c40ca9b6f..3d10a5fd4d76fc6a0dcd776346049729d0e5b937 100644 (file)
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -944,7 +944,17 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
       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;
 }
 
 static AliasAnalysis::AliasResult