Index: include/llvm/Target/TargetLowering.h
===================================================================
--- include/llvm/Target/TargetLowering.h
+++ include/llvm/Target/TargetLowering.h
@@ -1462,10 +1462,16 @@
     return false;
   }
 
-  /// Return true if it's free to truncate a value of type Ty1 to type
-  /// Ty2. e.g. On x86 it's free to truncate a i32 value in register EAX to i16
+  /// Return true if it's free to truncate a value of type FromTy to type
+  /// ToTy. e.g. On x86 it's free to truncate a i32 value in register EAX to i16
   /// by referencing its sub-register AX.
-  virtual bool isTruncateFree(Type * /*Ty1*/, Type * /*Ty2*/) const {
+  virtual bool isTruncateFree(Type *FromTy, Type *ToTy) const {
+    unsigned FromNumBits = FromTy->getPrimitiveSizeInBits();
+    unsigned ToNumBits = ToTy->getPrimitiveSizeInBits();
+    assert((FromNumBits > ToNumBits)
+            && "Can't truncate from small to large.");
+    (void)FromNumBits; // silence -Wunused
+    (void)ToNumBits;
     return false;
   }
 
@@ -1478,7 +1484,13 @@
     return false;
   }
 
-  virtual bool isTruncateFree(EVT /*VT1*/, EVT /*VT2*/) const {
+  virtual bool isTruncateFree(EVT FromVT, EVT ToVT) const {
+    unsigned FromNumBits = FromVT.getSizeInBits();
+    unsigned ToNumBits = ToVT.getSizeInBits();
+    assert((FromNumBits > ToNumBits)
+            && "Can't truncate from small to large.");
+    (void)FromNumBits; // silence -Wunused
+    (void)ToNumBits;
     return false;
   }
 
Index: lib/CodeGen/SelectionDAG/DAGCombiner.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -9149,8 +9149,9 @@
     void addSliceGain(const LoadedSlice &LS) {
       // Each slice saves a truncate.
       const TargetLowering &TLI = LS.DAG->getTargetLoweringInfo();
-      if (!TLI.isTruncateFree(LS.Inst->getValueType(0),
-                              LS.Inst->getOperand(0).getValueType()))
+      // isTruncateFree(FromType,ToType)
+      if (!TLI.isTruncateFree(LS.Inst->getOperand(0).getValueType(),
+                                            LS.Inst->getValueType(0)))
         ++Truncates;
       // If there is a shift amount, this slice gets rid of it.
       if (LS.Shift)