[llvm] r305481 - [BasicAA] Don't call isKnownNonEqual if we might be have gone through a PHINode.

[Craig Topper via llvm-commits]

Author: ctopper
Date: Thu Jun 15 12:16:56 2017
New Revision: 305481

URL: http://llvm.org/viewvc/llvm-project?rev=305481&view=rev
Log:
[BasicAA] Don't call isKnownNonEqual if we might be have gone through a PHINode.

This is a fix for the test case in PR32314.

Basic Alias Analysis can ask if two nodes are known non-equal after looking through a phi node to find a GEP. isAddOfNonZero saw an add of a constant from the same phi and said that its output couldn't be equal. But Basic Alias Analysis was really asking about the value from the previous loop iteration.

This patch at least makes that case not happen anymore, I'm not sure if there were still other ways this can fail. As was discussed in the bug, it looks like fixing BasicAA would be difficult so this patch seemed like a possible workaround

Differential Revision: https://reviews.llvm.org/D33136

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=305481&r1=305480&r2=305481&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp Thu Jun 15 12:16:56 2017
@@ -1011,10 +1011,24 @@ static AliasResult aliasSameBasePointerG
     // equal each other so we can exit early.
     if (C1 && C2)
       return NoAlias;
-    if (isKnownNonEqual(GEP1->getOperand(GEP1->getNumOperands() - 1),
-                        GEP2->getOperand(GEP2->getNumOperands() - 1),
-                        DL))
-      return NoAlias;
+    {
+      Value *GEP1LastIdx = GEP1->getOperand(GEP1->getNumOperands() - 1);
+      Value *GEP2LastIdx = GEP2->getOperand(GEP2->getNumOperands() - 1);
+      if (isa<PHINode>(GEP1LastIdx) || isa<PHINode>(GEP2LastIdx)) {
+        // If one of the indices is a PHI node, be safe and only use
+        // computeKnownBits so we don't make any assumptions about the
+        // relationships between the two indices. This is important if we're
+        // asking about values from different loop iterations. See PR32314.
+        // TODO: We may be able to change the check so we only do this when
+        // we definitely looked through a PHINode.
+        KnownBits Known1 = computeKnownBits(GEP1LastIdx, DL);
+        KnownBits Known2 = computeKnownBits(GEP2LastIdx, DL);
+        if (Known1.Zero.intersects(Known2.One) ||
+            Known1.One.intersects(Known2.Zero))
+          return NoAlias;
+      } else if (isKnownNonEqual(GEP1LastIdx, GEP2LastIdx, DL))
+        return NoAlias;
+    }
     return MayAlias;
   } else if (!LastIndexedStruct || !C1 || !C2) {
     return MayAlias;