Fix a nasty dag combiner bug that caused nondeterminstic crashes (MY FAVORITE!):

SimplifySelectOps would eliminate a Select, delete it, then return true.

The clients would see that it did something and return null.

The top level would see a null return, and decide that nothing happened,
proceeding to process the node in other ways: boom.

The fix is simple: clients of SimplifySelectOps should return the select
node itself.

In order to catch really obnoxious boogs like this in the future, add an
assert that nodes are not deleted.  We do this by checking for a sentry node
type that the SDNode dtor sets when a node is destroyed.

llvm-svn: 28514

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -392,6 +392,8 @@ void DAGCombiner::Run(bool RunningAfterLegalize) {
     
     // If nothing happened, try a target-specific DAG combine.
     if (RV.Val == 0) {
+      assert(N->getOpcode() != ISD::DELETED_NODE &&
+             "Node was deleted but visit returned NULL!");
       if (N->getOpcode() >= ISD::BUILTIN_OP_END ||
           TLI.hasTargetDAGCombine((ISD::NodeType)N->getOpcode()))
         RV = TLI.PerformDAGCombine(N, DagCombineInfo);
@@ -404,6 +406,10 @@ void DAGCombiner::Run(bool RunningAfterLegalize) {
       // CombineTo was used.  Since CombineTo takes care of the worklist 
       // mechanics for us, we have no work to do in this case.
       if (RV.Val != N) {
+        assert(N->getOpcode() != ISD::DELETED_NODE &&
+               RV.Val->getOpcode() != ISD::DELETED_NODE &&
+               "Node was deleted but visit returned new node!");
+
         DEBUG(std::cerr << "\nReplacing "; N->dump();
               std::cerr << "\nWith: "; RV.Val->dump(&DAG);
               std::cerr << '\n');
@@ -1573,9 +1579,11 @@ SDOperand DAGCombiner::visitSELECT(SDNode *N) {
   // fold X ? Y : X --> X ? Y : 0 --> X & Y
   if (MVT::i1 == VT && N0 == N2)
     return DAG.getNode(ISD::AND, VT, N0, N1);
+  
   // If we can fold this based on the true/false value, do so.
   if (SimplifySelectOps(N, N1, N2))
-    return SDOperand();
+    return SDOperand(N, 0);  // Don't revisit N.
+  
   // fold selects based on a setcc into other things, such as min/max/abs
   if (N0.getOpcode() == ISD::SETCC)
     // FIXME:
@@ -1611,7 +1619,7 @@ SDOperand DAGCombiner::visitSELECT_CC(SDNode *N) {
   
   // If we can fold this based on the true/false value, do so.
   if (SimplifySelectOps(N, N2, N3))
-    return SDOperand();
+    return SDOperand(N, 0);  // Don't revisit N.
   
   // fold select_cc into other things, such as min/max/abs
   return SimplifySelectCC(N0, N1, N2, N3, CC);
@@ -2814,7 +2822,10 @@ SDOperand DAGCombiner::SimplifySelect(SDOperand N0, SDOperand N1, SDOperand N2){
 
 /// SimplifySelectOps - Given a SELECT or a SELECT_CC node, where LHS and RHS
 /// are the two values being selected between, see if we can simplify the
-/// select.
+/// select.  Callers of this should assume that TheSelect is deleted if this
+/// returns true.  As such, they should return the appropriate thing (e.g. the
+/// node) back to the top-level of the DAG combiner loop to avoid it being
+/// looked at.
 ///
 bool DAGCombiner::SimplifySelectOps(SDNode *TheSelect, SDOperand LHS, 
                                     SDOperand RHS) {
@@ -3247,7 +3258,7 @@ SDOperand DAGCombiner::SimplifySetCC(MVT::ValueType VT, SDOperand N0,
             // Perform the xform if C1 is a single bit.
             if ((C1 & (C1-1)) == 0) {
               return DAG.getNode(ISD::SRL, VT, N0,
-                             DAG.getConstant(Log2_64(C1),TLI.getShiftAmountTy()));
+                          DAG.getConstant(Log2_64(C1),TLI.getShiftAmountTy()));
             }
           }
         }