DAG Combiner: Fold SelectCC When Cond is UNDEF

[mehdi_amini]

Summary

In case folding a node end up with a NaN as operand for the select, the folding here fails because the comparison is "UNDEF". This patch handles the UNDEF accordingly.

There is no test case because I can find a situation in an open source backend where we would generate a NaN in DAGCombine (the folding is performed earlier during DAG creation usually). Yet it seems that generating a NaN is not invalid by itself.

[mehdi_amini]

+llvm-commit

[resistor]

LGTM.

llvm/trunk/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitSELECT_CC(SDNode *N) {
   if (SCC.getNode()) {
     AddToWorklist(SCC.getNode());
 
     if (ConstantSDNode *SCCC = dyn_cast<ConstantSDNode>(SCC.getNode())) {
       if (!SCCC->isNullValue())
         return N2;    // cond always true -> true val
       else
         return N3;    // cond always false -> false val
-    }
-
+    } else if (SCC->getOpcode() == ISD::UNDEF) {
+      // When the condition is UNDEF, just return the first operand. This is
+      // coherent the DAG creation, no setcc node is created in this case
+      return N2;
+    } else if (SCC.getOpcode() == ISD::SETCC) {
       // Fold to a simpler select_cc
-    if (SCC.getOpcode() == ISD::SETCC)
       return DAG.getNode(ISD::SELECT_CC, SDLoc(N), N2.getValueType(),
                          SCC.getOperand(0), SCC.getOperand(1), N2, N3,
                          SCC.getOperand(2));
     }
+  }
 
   // If we can fold this based on the true/false value, do so.
   if (SimplifySelectOps(N, N2, N3))
     return SDValue(N, 0);  // Don't revisit N.
 
   // fold select_cc into other things, such as min/max/abs
   return SimplifySelectCC(SDLoc(N), N0, N1, N2, N3, CC);
 }