[Analysis] Generate more precise TBAA tags when one access encloses the other

There are cases when two tags with different base types denote
accesses to the same direct or indirect member of a structure
type. Currently, merging of such tags results in a tag that
represents an access to an object that has the type of that
member. This patch changes this so that if one of the accesses
encloses the other, then the generic tag is the one of the
enclosed access.

Differential Revision: https://reviews.llvm.org/D39557

llvm-svn: 321019

1 files changed, 17 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp b/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
index c9ed026a1e3..173db399b9d 100644
--- a/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
@@ -544,21 +544,32 @@ static bool matchAccessTags(const MDNode *A, const MDNode *B,
   TBAAStructTagNode TagA(A), TagB(B);
   const MDNode *CommonType = getLeastCommonType(TagA.getAccessType(),
                                                 TagB.getAccessType());
-  if (GenericTag)
-    *GenericTag = createAccessTag(CommonType);
 
   // TODO: We need to check if AccessType of TagA encloses AccessType of
   // TagB to support aggregate AccessType. If yes, return true.
 
   // Climb the type DAG from base type of A to see if we reach base type of B.
   uint64_t OffsetA;
-  if (findAccessType(TagA, TagB.getBaseType(), OffsetA))
-    return OffsetA == TagB.getOffset();
+  if (findAccessType(TagA, TagB.getBaseType(), OffsetA)) {
+    bool SameMemberAccess = OffsetA == TagB.getOffset();
+    if (GenericTag)
+      *GenericTag = SameMemberAccess ? TagB.getNode() :
+                                       createAccessTag(CommonType);
+    return SameMemberAccess;
+  }
 
   // Climb the type DAG from base type of B to see if we reach base type of A.
   uint64_t OffsetB;
-  if (findAccessType(TagB, TagA.getBaseType(), OffsetB))
-    return OffsetB == TagA.getOffset();
+  if (findAccessType(TagB, TagA.getBaseType(), OffsetB)) {
+    bool SameMemberAccess = OffsetB == TagA.getOffset();
+    if (GenericTag)
+      *GenericTag = SameMemberAccess ? TagA.getNode() :
+                                       createAccessTag(CommonType);
+    return SameMemberAccess;
+  }
+
+  if (GenericTag)
+    *GenericTag = createAccessTag(CommonType);
 
   // If the final access types have different roots, they're part of different
   // potentially unrelated type systems, so we must be conservative.