Added comments to explain why we treat offsets in shadow memory opposite te to other accesses

Author: gbMattN

compiler-rt/lib/tysan/tysan.cpp

@@ -102,6 +102,7 @@ static tysan_type_descriptor *getRootTD(tysan_type_descriptor *TD) {
   return RootTD;
 }
 
+// Walk up TDA to see if it reaches TDB
 bool walkAliasTree(tysan_type_descriptor *TDA, tysan_type_descriptor *TDB,
                    uptr OffsetA, uptr OffsetB) {
   do {
@@ -158,24 +159,29 @@ static bool isAliasingLegalUp(tysan_type_descriptor *TDA,
   return walkAliasTree(TDA, TDB, OffsetA, OffsetB);
 }
 
-static bool isAliasingLegalWithOffset(tysan_type_descriptor *AccessTD,
-                                      tysan_type_descriptor *ShadowTD,
-                                      int OffsetInShadow) {
+static bool isAliasingLegalWithOffset(tysan_type_descriptor *TDA,
+                                      tysan_type_descriptor *TDB,
+                                      int OffsetB) {
   // This is handled in the other cases
-  if (OffsetInShadow == 0)
+  if (OffsetB == 0)
     return false;
 
   // You can't have an offset into a member
-  if (ShadowTD->Tag == TYSAN_MEMBER_TD)
+  if (TDB->Tag == TYSAN_MEMBER_TD)
     return false;
 
-  int OffsetInAccess = 0;
-  if (AccessTD->Tag == TYSAN_MEMBER_TD) {
-    OffsetInAccess = AccessTD->Member.Offset;
-    AccessTD = AccessTD->Member.Base;
+  int OffsetA = 0;
+  if (TDA->Tag == TYSAN_MEMBER_TD) {
+    OffsetA = TDA->Member.Offset;
+    TDA = TDA->Member.Base;
   }
 
-  return walkAliasTree(ShadowTD, AccessTD, OffsetInShadow, OffsetInAccess);
+  // Since the access was partially inside TDB (the shadow), it can be assumed
+  // that we are accessing a member in an object. This means that rather than
+  // walk up the scalar access TDA to reach an object, we should walk up the
+  // object TBD to reach the scalar we are accessing it with. The offsets will
+  // still be checked at the end to make sure this alias is legal.
+  return walkAliasTree(TDB, TDA, OffsetB, OffsetA);
 }
 
 static bool isAliasingLegal(tysan_type_descriptor *TDA,