Index: lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- lib/Transforms/Scalar/JumpThreading.cpp
+++ lib/Transforms/Scalar/JumpThreading.cpp
@@ -475,6 +475,26 @@
     return !Result.empty();
   }
 
+  // Handle Cast instructions.  Only see through Cast when the source operand is
+  // PHI or Cmp and the source type is i1 to save the compilation time.
+  if (CastInst *CI = dyn_cast<CastInst>(I)) {
+    Value *Source = CI->getOperand(0);
+    if (!Source->getType()->isIntegerTy(1))
+      return false;
+    if (!isa<PHINode>(Source) && !isa<CmpInst>(Source))
+      return false;
+    ComputeValueKnownInPredecessors(Source, BB, Result, Preference, Changed,
+                                    CxtI);
+    if (Result.empty())
+      return false;
+
+    // Convert the known values.
+    for (auto &R : Result)
+      R.first = ConstantExpr::getCast(CI->getOpcode(), R.first, CI->getType());
+
+    return true;
+  }
+
   PredValueInfoTy LHSVals, RHSVals;
 
   // Handle some boolean conditions.
Index: test/Transforms/JumpThreading/basic.ll
===================================================================
--- test/Transforms/JumpThreading/basic.ll
+++ test/Transforms/JumpThreading/basic.ll
@@ -511,9 +511,43 @@
 ; CHECK-NEXT: phi i32
 }
 
+;;; Verify that we can handle constraint propagation through cast.
+define i32 @test17(i1 %cond) {
+Entry:
+; CHECK-LABEL: @test17(
+	br i1 %cond, label %Merge, label %F1
+
+; CHECK: Entry:
+; CHECK-NEXT:  br i1 %cond, label %F2, label %Merge
+
+F1:
+	%v1 = call i32 @f1()
+	br label %Merge
+
+Merge:
+	%B = phi i32 [0, %Entry], [%v1, %F1]
+	%M = icmp eq i32 %B, 0 
+	%M1 = zext i1 %M to i32
+	%N = icmp eq i32 %M1, 0 
+	br i1 %N, label %T2, label %F2
+
+; CHECK: Merge:
+; CHECK-NOT: phi
+; CHECK-NEXT:   %v1 = call i32 @f1()
+
+T2:
+	%Q = call i32 @f2() 
+	ret i32 %Q
+
+F2:
+	ret i32 %B
+; CHECK: F2:
+; CHECK-NEXT: phi i32
+}
+
 ;;; Just check that ComputeValueKnownInPredecessors() does not return true with
 ;;; no values and triggers the assert in ProcessThreadableEdges().
-define i32 @test17() {
+define i32 @test18() {
 entry:
 	%A = add i32 0, 1
 	%B = icmp eq i32 %A, 0