diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2480,6 +2480,22 @@
   if (Sel.getOpcode() != ISD::SELECT || !Sel.hasOneUse()) {
     SelOpNo = 1;
     Sel = BO->getOperand(1);
+
+    // Peek through any trunc/zext to shift amount type.
+    if ((BinOpcode == ISD::SHL || BinOpcode == ISD::SRA ||
+         BinOpcode == ISD::SRL) && Sel.hasOneUse()) {
+      // This is valid when the truncated bits of x are already zero.
+      SDValue Op;
+      KnownBits Known;
+      if (isTruncateOf(DAG, Sel, Op, Known)) {
+        APInt TruncatedBits =
+          APInt::getBitsSet(Op.getScalarValueSizeInBits(),
+                            Sel.getScalarValueSizeInBits(),
+                            Op.getScalarValueSizeInBits());
+        if (TruncatedBits.isSubsetOf(Known.Zero))
+          Sel = Op;
+      }
+    }
   }
 
   if (Sel.getOpcode() != ISD::SELECT || !Sel.hasOneUse())