[llvm] r179062 - Redo the fix Benjamin Kramer committed in r178793 about iterator invalidation in Reassociate.

[Shuxin Yang]

Author: shuxin_yang
Date: Mon Apr  8 17:00:43 2013
New Revision: 179062

URL: http://llvm.org/viewvc/llvm-project?rev=179062&view=rev
Log:
Redo the fix Benjamin Kramer committed in r178793 about iterator invalidation in Reassociate.

I brazenly think this change is slightly simpler than r178793 because: 
  - no "state" in functor
  - "OpndPtrs[i]" looks simpler than "&Opnds[OpndIndices[i]]" 

  While I can reproduce the probelm in Valgrind, it is rather difficult to come up
a standalone testing case. The reason is that when an iterator is invalidated,
the stale invalidated elements are not yet clobbered by nonsense data, so the
optimizer can still proceed successfully. 

  Thank Benjamin for fixing this bug and generously providing the test case.

Modified:
    llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp?rev=179062&r1=179061&r2=179062&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/Reassociate.cpp Mon Apr  8 17:00:43 2013
@@ -143,13 +143,9 @@ namespace {
     //   So, if Rank(X) < Rank(Y) < Rank(Z), it means X is defined earlier 
     //   than Y which is defined earlier than Z. Permute "x | 1", "Y & 2",
     //   "z" in the order of X-Y-Z is better than any other orders.
-    class PtrSortFunctor {
-      ArrayRef<XorOpnd> A;
-
-    public:
-      PtrSortFunctor(ArrayRef<XorOpnd> Array) : A(Array) {}
-      bool operator()(unsigned LHSIndex, unsigned RHSIndex) {
-        return A[LHSIndex].getSymbolicRank() < A[RHSIndex].getSymbolicRank();
+    struct PtrSortFunctor {
+      bool operator()(XorOpnd * const &LHS, XorOpnd * const &RHS) {
+        return LHS->getSymbolicRank() < RHS->getSymbolicRank();
       }
     };
   private:
@@ -1274,7 +1270,7 @@ Value *Reassociate::OptimizeXor(Instruct
     return 0;
 
   SmallVector<XorOpnd, 8> Opnds;
-  SmallVector<unsigned, 8> OpndIndices;
+  SmallVector<XorOpnd*, 8> OpndPtrs;
   Type *Ty = Ops[0].Op->getType();
   APInt ConstOpnd(Ty->getIntegerBitWidth(), 0);
 
@@ -1285,23 +1281,29 @@ Value *Reassociate::OptimizeXor(Instruct
       XorOpnd O(V);
       O.setSymbolicRank(getRank(O.getSymbolicPart()));
       Opnds.push_back(O);
-      OpndIndices.push_back(Opnds.size() - 1);
     } else
       ConstOpnd ^= cast<ConstantInt>(V)->getValue();
   }
 
+  // NOTE: From this point on, do *NOT* add/delete element to/from "Opnds".
+  //  It would otherwise invalidate the "Opnds"'s iterator, and hence invalidate
+  //  the "OpndPtrs" as well. For the similar reason, do not fuse this loop
+  //  with the previous loop --- the iterator of the "Opnds" may be invalidated
+  //  when new elements are added to the vector.
+  for (unsigned i = 0, e = Opnds.size(); i != e; ++i)
+    OpndPtrs.push_back(&Opnds[i]);
+
   // Step 2: Sort the Xor-Operands in a way such that the operands containing
   //  the same symbolic value cluster together. For instance, the input operand
   //  sequence ("x | 123", "y & 456", "x & 789") will be sorted into:
   //  ("x | 123", "x & 789", "y & 456").
-  std::sort(OpndIndices.begin(), OpndIndices.end(),
-            XorOpnd::PtrSortFunctor(Opnds));
+  std::sort(OpndPtrs.begin(), OpndPtrs.end(), XorOpnd::PtrSortFunctor());
 
   // Step 3: Combine adjacent operands
   XorOpnd *PrevOpnd = 0;
   bool Changed = false;
   for (unsigned i = 0, e = Opnds.size(); i < e; i++) {
-    XorOpnd *CurrOpnd = &Opnds[OpndIndices[i]];
+    XorOpnd *CurrOpnd = OpndPtrs[i];
     // The combined value
     Value *CV;