Index: include/llvm/Target/TargetLowering.h
===================================================================
--- include/llvm/Target/TargetLowering.h
+++ include/llvm/Target/TargetLowering.h
@@ -2678,6 +2678,15 @@
                                              const SelectionDAG &DAG,
                                              unsigned Depth = 0) const;
 
+  /// Determine which of the bits of FrameIndex \p FIOp are known to be 0.
+  /// Default implementation computes low bits based on alignment
+  /// information. This should preserve known bits passed into it.
+  virtual void computeKnownBitsForFrameIndex(const SDValue FIOp,
+                                             KnownBits &Known,
+                                             const APInt &DemandedElts,
+                                             const SelectionDAG &DAG,
+                                             unsigned Depth = 0) const;
+
   /// This method can be implemented by targets that want to expose additional
   /// information about sign bits to the DAG Combiner. The DemandedElts
   /// argument allows us to only collect the minimum sign bits that are shared
Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2893,11 +2893,7 @@
   }
   case ISD::FrameIndex:
   case ISD::TargetFrameIndex:
-    if (unsigned Align = InferPtrAlignment(Op)) {
-      // The low bits are known zero if the pointer is aligned.
-      Known.Zero.setLowBits(Log2_32(Align));
-      break;
-    }
+    TLI->computeKnownBitsForFrameIndex(Op, Known, DemandedElts, *this, Depth);
     break;
 
   default:
Index: lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -1288,6 +1288,19 @@
   Known.resetAll();
 }
 
+void TargetLowering::computeKnownBitsForFrameIndex(const SDValue Op,
+                                                   KnownBits &Known,
+                                                   const APInt &DemandedElts,
+                                                   const SelectionDAG &DAG,
+                                                   unsigned Depth) const {
+  assert(isa<FrameIndexSDNode>(Op) && "expected FrameIndex");
+
+  if (unsigned Align = DAG.InferPtrAlignment(Op)) {
+    // The low bits are known zero if the pointer is aligned.
+    Known.Zero.setLowBits(Log2_32(Align));
+  }
+}
+
 /// This method can be implemented by targets that want to expose additional
 /// information about sign bits to the DAG Combiner.
 unsigned TargetLowering::ComputeNumSignBitsForTargetNode(SDValue Op,