[ARM64] PR19792: Fix cycle in DAG after performPostLD1Combine

Povray and dealII currently assert with "Overran sorted position" in
AssignTopologicalOrder.  The problem is that performPostLD1Combine can
introduce cycles.

Consider:

(insert_vector_elt (INSERT_SUBREG undef,
                                  (load (add %vreg0, Constant<8>), undef),  <= A
                                  TargetConstant<2>),
                   (load %vreg0, undef),                                    <= B
                   Constant<1>)

This is turned into a LD1LANEpost node.  However the address in A is not a
valid user of the post-incremented address of B in LD1LANEpost.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209242 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/Target/ARM64/ARM64ISelLowering.cpp b/lib/Target/ARM64/ARM64ISelLowering.cpp
index 538360cf39..385373116d 100644
--- a/lib/Target/ARM64/ARM64ISelLowering.cpp
+++ b/lib/Target/ARM64/ARM64ISelLowering.cpp
@@ -7298,6 +7298,7 @@ static SDValue performPostLD1Combine(SDNode *N,
   }
 
   SDValue Addr = LD->getOperand(1);
+  SDValue Vector = N->getOperand(0);
   // Search for a use of the address operand that is an increment.
   for (SDNode::use_iterator UI = Addr.getNode()->use_begin(), UE =
        Addr.getNode()->use_end(); UI != UE; ++UI) {
@@ -7310,6 +7311,10 @@ static SDValue performPostLD1Combine(SDNode *N,
     // would create a cycle.
     if (User->isPredecessorOf(LD) || LD->isPredecessorOf(User))
       continue;
+    // Also check that add is not used in the vector operand.  This would also
+    // create a cycle.
+    if (User->isPredecessorOf(Vector.getNode()))
+      continue;
 
     // If the increment is a constant, it must match the memory ref size.
     SDValue Inc = User->getOperand(User->getOperand(0) == Addr ? 1 : 0);
@@ -7324,7 +7329,7 @@ static SDValue performPostLD1Combine(SDNode *N,
     SmallVector<SDValue, 8> Ops;
     Ops.push_back(LD->getOperand(0));  // Chain
     if (IsLaneOp) {
-      Ops.push_back(N->getOperand(0)); // The vector to be inserted
+      Ops.push_back(Vector);           // The vector to be inserted
       Ops.push_back(N->getOperand(2)); // The lane to be inserted in the vector
     }
     Ops.push_back(Addr);