diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -2271,6 +2271,24 @@
   ThreadEdge(BB, PredsToFactor, SuccBB);
 }
 
+namespace {
+
+/// IsUsedBy - Returns 'true' if the instruction is used based on the predicate.
+template <typename UnaryPredicate>
+bool IsUsedBy(const Instruction *I, UnaryPredicate Predicate) {
+  for (const Use &U : I->uses()) {
+    const Instruction *User = cast<Instruction>(U.getUser());
+    if (Predicate(User))
+      return true;
+    if (isa<CastInst>(User))
+      return IsUsedBy(User, Predicate);
+  }
+
+  return false;
+}
+
+}
+
 /// TryThreadEdge - Thread an edge if it's safe and profitable to do so.
 bool JumpThreadingPass::TryThreadEdge(
     BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
@@ -2304,6 +2322,46 @@
     return false;
   }
 
+  // If the PHI node has a constant value and the PHI is used by an is.constant
+  // intrinsic, then don't thread the edge. Otherwise, we could end up in a
+  // situation where the intrinsic will resolve to 'true' when it should resolve
+  // to 'false'. For example, %z should be 'false' in this:
+  //
+  //   A:
+  //     %v = phi i32 [ 3, %X ], [ %v, %Y ]
+  //     %z = call i1 @llvm.is.constant(i32 %v)
+  //
+  // but when jump threading happens we get two different values:
+  //
+  //   A.thread:
+  //     %v.thread = phi i32 [ 3, %X ]
+  //     %z.thread = call i1 @llvm.is.constant(i32 %v.thread)
+  //     br ...
+  //
+  //   A:
+  //     %v = phi i32 [ %v, %Y ]
+  //     %z = call i1 @llvm.is.constant(i32 %v)
+  //     br ...
+  //
+  // One consequence, if %z and %z.thread are used in a PHI, we cannot perform
+  // DCE on any dead paths. This is an issue if the programmer relies on DCE
+  // occurring.
+  for (PHINode &PN : BB->phis()) {
+    if (llvm::all_of(PN.incoming_values(), [](const Value *V){
+      return !isa<Constant>(V);
+    }))
+      // We are only concerned about PHI nodes with a constant incoming value.
+      continue;
+
+    for (Use &U : PN.uses())
+      if (IsUsedBy(cast<Instruction>(U.getUser()),
+                   [](const Instruction *I){
+                     using namespace PatternMatch;
+                     return match(I, m_Intrinsic<Intrinsic::is_constant>());
+                   }))
+        return false;
+  }
+
   ThreadEdge(BB, PredBBs, SuccBB);
   return true;
 }
diff --git a/llvm/test/Transforms/JumpThreading/is_constant.ll b/llvm/test/Transforms/JumpThreading/is_constant.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/JumpThreading/is_constant.ll
@@ -0,0 +1,35 @@
+; RUN: opt -jump-threading -S -verify < %s | FileCheck %s
+
+; CHECK-LABEL: define void @test1(
+; CHECK-LABEL: if.end18:
+; CHECK-NOT:   call i1 @llvm.is.constant.i64
+define void @test1() {
+entry:
+  %conv = trunc i64 undef to i32
+  %conv8 = ashr exact i64 undef, 32
+  %cmp9 = icmp ugt i64 %conv8, 24
+  br i1 %cmp9, label %if.then11, label %if.end18
+
+if.then11:                                        ; preds = %entry
+  br label %if.end18
+
+if.end18:                                         ; preds = %if.then11, %entry
+  %len.0 = phi i32 [ 24, %if.then11 ], [ %conv, %entry ]
+  %cmp3.i.i70 = icmp ugt i32 %len.0, 24
+  %0 = call i1 @llvm.is.constant.i64(i64 undef)
+  br i1 %cmp3.i.i70, label %if.then.i.i71, label %if.end12.i.i74
+
+if.then.i.i71:                                    ; preds = %if.end18
+  unreachable
+
+if.end12.i.i74:                                   ; preds = %if.end18
+  br i1 %0, label %if.else.i.i, label %if.then7.i.i
+
+if.then7.i.i:                                     ; preds = %if.end12.i.i74
+  ret void
+
+if.else.i.i:                                      ; preds = %if.end12.i.i74
+  unreachable
+}
+
+declare i1 @llvm.is.constant.i64(i64)