[llvm] r317680 - BasicAA: fix bug where we would return partialalias instead of noalias

[Nuno Lopes via llvm-commits]

Author: nlopes
Date: Wed Nov  8 02:59:00 2017
New Revision: 317680

URL: http://llvm.org/viewvc/llvm-project?rev=317680&view=rev
Log:
BasicAA: fix bug where we would return partialalias instead of noalias
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.

Modified:
    llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp

Modified: llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp?rev=317680&r1=317679&r2=317680&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp Wed Nov  8 02:59:00 2017
@@ -1672,9 +1672,9 @@ AliasResult BasicAAResult::aliasCheck(co
   // 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;