diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -1840,6 +1840,8 @@
   assert(Operands.size() == 1 && "Wrong number of operands.");
 
   if (IntrinsicID == Intrinsic::is_constant) {
+    if (isa<UndefValue>(Operands[0]))
+      return ConstantInt::getFalse(Ty->getContext());
     // We know we have a "Constant" argument. But we want to only
     // return true for manifest constants, not those that depend on
     // constants with unknowable values, e.g. GlobalValue or BlockAddress.
diff --git a/llvm/lib/Transforms/Scalar/LowerConstantIntrinsics.cpp b/llvm/lib/Transforms/Scalar/LowerConstantIntrinsics.cpp
--- a/llvm/lib/Transforms/Scalar/LowerConstantIntrinsics.cpp
+++ b/llvm/lib/Transforms/Scalar/LowerConstantIntrinsics.cpp
@@ -45,10 +45,18 @@
           "Number of 'objectsize' intrinsic calls handled");
 
 static Value *lowerIsConstantIntrinsic(IntrinsicInst *II) {
-  Value *Op = II->getOperand(0);
-
-  return isa<Constant>(Op) ? ConstantInt::getTrue(II->getType())
-                           : ConstantInt::getFalse(II->getType());
+  SmallVector<Value *> WorkList;
+  WorkList.push_back(II->getOperand(0));
+  while (WorkList.size()) {
+    Value *V = WorkList.back();
+    WorkList.pop_back();
+    if (auto *CA = dyn_cast<ConstantAggregate>(V))
+      for (Value *Op : CA->operands())
+        WorkList.push_back(Op);
+    else if (!isa<ConstantData>(V) || isa<UndefValue>(V))
+      return ConstantInt::getFalse(II->getType());
+  }
+  return ConstantInt::getTrue(II->getType());
 }
 
 static bool replaceConditionalBranchesOnConstant(Instruction *II,
diff --git a/llvm/test/Transforms/EarlyCSE/is_constant.ll b/llvm/test/Transforms/EarlyCSE/is_constant.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/EarlyCSE/is_constant.ll
@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -passes=early-cse %s -S -o - | FileCheck %s
+
+; Test that EarlyCSE doesn't turn this into a return of true.
+declare i1 @llvm.is.constant.i32(i32) #0
+define dso_local i32 @foo() {
+; CHECK-LABEL: @foo(
+; CHECK-NEXT:    ret i32 0
+;
+
+  %1 = call i1 @llvm.is.constant.i32(i32 undef)
+  %2 = zext i1 %1 to i32
+  ret i32 %2
+}
+attributes #0 = { convergent nofree nosync nounwind readnone willreturn }
diff --git a/llvm/test/Transforms/InstCombine/intrinsics.ll b/llvm/test/Transforms/InstCombine/intrinsics.ll
--- a/llvm/test/Transforms/InstCombine/intrinsics.ll
+++ b/llvm/test/Transforms/InstCombine/intrinsics.ll
@@ -19,6 +19,7 @@
 declare double @llvm.trunc.f64(double %Val) nounwind readonly
 declare double @llvm.rint.f64(double %Val) nounwind readonly
 declare double @llvm.nearbyint.f64(double %Val) nounwind readonly
+declare i1 @llvm.is.constant.i32(i32) nounwind readonly
 
 define void @powi(double %V, double *%P) {
 ; CHECK-LABEL: @powi(
@@ -59,7 +60,7 @@
 ; Make sure we don't add range metadata to i1 cttz.
 define i1 @cttz_i1(i1 %arg) {
 ; CHECK-LABEL: @cttz_i1(
-; CHECK-NEXT:    [[CNT:%.*]] = call i1 @llvm.cttz.i1(i1 [[ARG:%.*]], i1 false) #2
+; CHECK-NEXT:    [[CNT:%.*]] = call i1 @llvm.cttz.i1(i1 [[ARG:%.*]], i1 false) #3
 ; CHECK-NEXT:    ret i1 [[CNT]]
 ;
   %cnt = call i1 @llvm.cttz.i1(i1 %arg, i1 false) nounwind readnone
@@ -89,7 +90,7 @@
 define i32 @cttz_knownbits2(i32 %arg) {
 ; CHECK-LABEL: @cttz_knownbits2(
 ; CHECK-NEXT:    [[OR:%.*]] = or i32 [[ARG:%.*]], 4
-; CHECK-NEXT:    [[CNT:%.*]] = call i32 @llvm.cttz.i32(i32 [[OR]], i1 true) #2, !range ![[$CTTZ_RANGE:[0-9]+]]
+; CHECK-NEXT:    [[CNT:%.*]] = call i32 @llvm.cttz.i32(i32 [[OR]], i1 true) #3, !range ![[$CTTZ_RANGE:[0-9]+]]
 ; CHECK-NEXT:    ret i32 [[CNT]]
 ;
   %or = or i32 %arg, 4
@@ -151,7 +152,7 @@
 ; Make sure we don't add range metadata to i1 ctlz.
 define i1 @ctlz_i1(i1 %arg) {
 ; CHECK-LABEL: @ctlz_i1(
-; CHECK-NEXT:    [[CNT:%.*]] = call i1 @llvm.ctlz.i1(i1 [[ARG:%.*]], i1 false) #2
+; CHECK-NEXT:    [[CNT:%.*]] = call i1 @llvm.ctlz.i1(i1 [[ARG:%.*]], i1 false) #3
 ; CHECK-NEXT:    ret i1 [[CNT]]
 ;
   %cnt = call i1 @llvm.ctlz.i1(i1 %arg, i1 false) nounwind readnone
@@ -181,7 +182,7 @@
 define i8 @ctlz_knownbits2(i8 %arg) {
 ; CHECK-LABEL: @ctlz_knownbits2(
 ; CHECK-NEXT:    [[OR:%.*]] = or i8 [[ARG:%.*]], 32
-; CHECK-NEXT:    [[CNT:%.*]] = call i8 @llvm.ctlz.i8(i8 [[OR]], i1 true) #2, !range ![[$CTLZ_RANGE:[0-9]+]]
+; CHECK-NEXT:    [[CNT:%.*]] = call i8 @llvm.ctlz.i8(i8 [[OR]], i1 true) #3, !range ![[$CTLZ_RANGE:[0-9]+]]
 ; CHECK-NEXT:    ret i8 [[CNT]]
 ;
   %or = or i8 %arg, 32
@@ -425,5 +426,13 @@
   ret void
 }
 
+define i32 @is_constant() {
+; CHECK-LABEL: @is_constant(
+; CHECK-NEXT: ret i32 0
+  %1 = call i1 @llvm.is.constant.i32(i32 undef) nounwind readonly
+  %2 = zext i1 %1 to i32
+  ret i32 %2
+}
+
 ; CHECK: [[$CTTZ_RANGE]] = !{i32 0, i32 3}
 ; CHECK: [[$CTLZ_RANGE]] = !{i8 0, i8 3}
diff --git a/llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll b/llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll
--- a/llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll
+++ b/llvm/test/Transforms/LowerConstantIntrinsics/constant-intrinsics.ll
@@ -112,3 +112,26 @@
 
   ret i1 %res6
 }
+
+@real_mode_blob_end = external dso_local global [0 x i8], align 1
+define i1 @global_array() {
+; CHECK-LABEL: @global_array(
+; CHECK-NEXT: ret i1 false
+  %1 = call i1 @llvm.is.constant.i64(i64 ptrtoint ([0 x i8]* @real_mode_blob_end to i64))
+  ret i1 %1
+}
+
+define i1 @undef1() {
+; CHECK-LABEL: undef1(
+; CHECK-NEXT: ret i1 false
+  %1 = call i1 @llvm.is.constant.i64(i64 undef)
+  ret i1 %1
+}
+
+@undef_array = dso_local global [2 x i8] [i8 42, i8 undef]
+define i1 @undef2() {
+; CHECK-LABEL: undef2(
+; CHECK-NEXT: ret i1 false
+  %1 = call i1 @llvm.is.constant.i64(i64 ptrtoint ([2 x i8]* @undef_array to i64))
+  ret i1 %1
+}