[DAG] Avoid using deleted node in rebuildSetCC

Summary:
The combine in rebuildSetCC may be combined to another
node leaving our references stale. Keep a handle on
it to avoid stale references.

Fixes PR36602.

Reviewers: dbabokin, RKSimon, eli.friedman, davide

Subscribers: hiraditya, uabelho, JesperAntonsson, qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D46404

llvm-svn: 331985

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11524,17 +11524,30 @@ SDValue DAGCombiner::rebuildSetCC(SDValue N) {
   // Transform br(xor(x, y)) -> br(x != y)
   // Transform br(xor(xor(x,y), 1)) -> br (x == y)
   if (N.getOpcode() == ISD::XOR) {
-    SDNode *TheXor = N.getNode();
-
-    // Avoid missing important xor optimizations.
-    while (SDValue Tmp = visitXOR(TheXor)) {
-      // We don't have a XOR anymore, bail.
-      if (Tmp.getOpcode() != ISD::XOR)
-        return Tmp;
-
-      TheXor = Tmp.getNode();
+    // Because we may call this on a speculatively constructed
+    // SimplifiedSetCC Node, we need to simplify this node first.
+    // Ideally this should be folded into SimplifySetCC and not
+    // here. For now, grab a handle to N so we don't lose it from
+    // replacements interal to the visit.
+    HandleSDNode XORHandle(N);
+    while (N.getOpcode() == ISD::XOR) {
+      SDValue Tmp = visitXOR(N.getNode());
+      // No simplification done.
+      if (!Tmp.getNode())
+        break;
+      // Returning N is form in-visit replacement that may invalidated
+      // N. Grab value from Handle.
+      if (Tmp.getNode() == N.getNode())
+        N = XORHandle.getValue();
+      else // Node simplified. Try simplifying again.
+        N = Tmp;
     }
 
+    if (N.getOpcode() != ISD::XOR)
+      return N;
+
+    SDNode *TheXor = N.getNode();
+
     SDValue Op0 = TheXor->getOperand(0);
     SDValue Op1 = TheXor->getOperand(1);
 

llvm/test/CodeGen/X86/pr36602.ll

@@ -0,0 +1,30 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
+
+
+define i32 @fn2() {
+; CHECK-LABEL: fn2:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    testb %al, %al
+; CHECK-NEXT:    jne .LBB0_2
+; CHECK-NEXT:  # %bb.1: # %bb1
+; CHECK-NEXT:    xorl %eax, %eax
+; CHECK-NEXT:    retq
+; CHECK-NEXT:  .LBB0_2: # %bb2
+; CHECK-NEXT:    movl $1, %eax
+; CHECK-NEXT:    retq
+  %_tmp10 = icmp eq i8 0, 0
+  %_tmp13 = icmp slt i8 undef, 1
+  %_tmp151 = or i1 %_tmp10, %_tmp13
+  %_tmp15 = zext i1 %_tmp151 to i8
+  br i1 %_tmp151, label %bb1, label %bb2
+
+bb1:                                              ; preds = %0, %0
+  ret i32 0
+
+bb2:                                              ; preds = %0, %0
+  ret i32 1
+
+
+}