BasicAA: fix bug where we would return partialalias instead of noalias

nunoplopes · nunoplopes · commit 4514930eb0a4 · 2017-11-08T10:59:00.000Z
My fix is conservative and will make us return may-alias instead. The test case is: check(gep(x, 0), n, gep(x, n), -1) with n == sizeof(x) Here, the first value accesses the whole object, but the second access doesn't access anything. The semantics of -1 is read until the end of the object, which in this case means read nothing. No test case, since isn't trivial to exploit this one, but I've proved it correct. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317680 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1672,9 +1672,9 @@ AliasResult BasicAAResult::aliasCheck(const Value *V1, uint64_t V1Size,
   // If both pointers are pointing into the same object and one of them
   // accesses the entire object, then the accesses must overlap in some way.
   if (O1 == O2)
-    if ((V1Size != MemoryLocation::UnknownSize &&
-         isObjectSize(O1, V1Size, DL, TLI)) ||
-        (V2Size != MemoryLocation::UnknownSize &&
+    if (V1Size != MemoryLocation::UnknownSize &&
+        V2Size != MemoryLocation::UnknownSize &&
+        (isObjectSize(O1, V1Size, DL, TLI) ||
          isObjectSize(O2, V2Size, DL, TLI)))
       return AliasCache[Locs] = PartialAlias;
 
