diff --git a/clang/test/CodeGen/stackrealign-main.c b/clang/test/CodeGen/stackrealign-main.c
--- a/clang/test/CodeGen/stackrealign-main.c
+++ b/clang/test/CodeGen/stackrealign-main.c
@@ -18,5 +18,5 @@
 // CHECK-NOT: "stackrealign"
 // CHECK: }
 // CHECK: attributes [[MAIN]] = { noinline nounwind optnone {{.*}}"stackrealign"{{.*}} }
-// CHECK: !{i32 1, !"override-stack-alignment", i32 64}
+// CHECK: !{i32 8, !"override-stack-alignment", i32 64}
 // DEFAULT-NOT: "override-stack-alignment"
diff --git a/llvm/include/llvm-c/Core.h b/llvm/include/llvm-c/Core.h
--- a/llvm/include/llvm-c/Core.h
+++ b/llvm/include/llvm-c/Core.h
@@ -398,7 +398,8 @@
 typedef enum {
   /**
    * Emits an error if two values disagree, otherwise the resulting value is
-   * that of the operands.
+   * that of the operands. Does not error if one module has a value but the
+   * other does not.
    *
    * @see Module::ModFlagBehavior::Error
    */
@@ -444,6 +445,14 @@
    * @see Module::ModFlagBehavior::AppendUnique
    */
   LLVMModuleFlagBehaviorAppendUnique,
+
+  /**
+   * Emits an error if two values disagree, or when a value is specified for
+   * one module, but not another.
+   *
+   * @see Module::ModFlagBehavior::ErrorWhenMissing
+   */
+  LLVMModuleFlagBehaviorErrorWhenMissing,
 } LLVMModuleFlagBehavior;
 
 /**
diff --git a/llvm/include/llvm/IR/Module.h b/llvm/include/llvm/IR/Module.h
--- a/llvm/include/llvm/IR/Module.h
+++ b/llvm/include/llvm/IR/Module.h
@@ -116,7 +116,8 @@
   /// This enumeration defines the supported behaviors of module flags.
   enum ModFlagBehavior {
     /// Emits an error if two values disagree, otherwise the resulting value is
-    /// that of the operands.
+    /// that of the operands. Does not produce an error if one module is
+    /// missing a value.
     Error = 1,
 
     /// Emits a warning if two values disagree. The result value will be the
@@ -148,9 +149,13 @@
     /// Takes the max of the two values, which are required to be integers.
     Max = 7,
 
+    /// Emits an error if two values disagree, or when a value is specified for
+    /// one module, but not another.
+    ErrorWhenMissing = 8,
+
     // Markers:
     ModFlagBehaviorFirstVal = Error,
-    ModFlagBehaviorLastVal = Max
+    ModFlagBehaviorLastVal = ErrorWhenMissing
   };
 
   /// Checks if Metadata represents a valid ModFlagBehavior, and stores the
diff --git a/llvm/lib/IR/Core.cpp b/llvm/lib/IR/Core.cpp
--- a/llvm/lib/IR/Core.cpp
+++ b/llvm/lib/IR/Core.cpp
@@ -322,6 +322,8 @@
     return Module::ModFlagBehavior::Append;
   case LLVMModuleFlagBehaviorAppendUnique:
     return Module::ModFlagBehavior::AppendUnique;
+  case LLVMModuleFlagBehaviorErrorWhenMissing:
+    return Module::ModFlagBehavior::ErrorWhenMissing;
   }
   llvm_unreachable("Unknown LLVMModuleFlagBehavior");
 }
@@ -341,6 +343,8 @@
     return LLVMModuleFlagBehaviorAppend;
   case Module::ModFlagBehavior::AppendUnique:
     return LLVMModuleFlagBehaviorAppendUnique;
+  case Module::ModFlagBehavior::ErrorWhenMissing:
+    return LLVMModuleFlagBehaviorErrorWhenMissing;
   default:
     llvm_unreachable("Unhandled Flag Behavior");
   }
diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -729,7 +729,8 @@
 }
 
 void Module::setOverrideStackAlignment(unsigned Align) {
-  addModuleFlag(ModFlagBehavior::Error, "override-stack-alignment", Align);
+  addModuleFlag(ModFlagBehavior::ErrorWhenMissing, "override-stack-alignment",
+                Align);
 }
 
 void Module::setSDKVersion(const VersionTuple &V) {
diff --git a/llvm/lib/IR/Verifier.cpp b/llvm/lib/IR/Verifier.cpp
--- a/llvm/lib/IR/Verifier.cpp
+++ b/llvm/lib/IR/Verifier.cpp
@@ -1561,6 +1561,7 @@
   case Module::Error:
   case Module::Warning:
   case Module::Override:
+  case Module::ErrorWhenMissing:
     // These behavior types accept any value.
     break;
 
diff --git a/llvm/lib/Linker/IRMover.cpp b/llvm/lib/Linker/IRMover.cpp
--- a/llvm/lib/Linker/IRMover.cpp
+++ b/llvm/lib/Linker/IRMover.cpp
@@ -1216,17 +1216,41 @@
 
 /// Merge the linker flags in Src into the Dest module.
 Error IRLinker::linkModuleFlagsMetadata() {
-  // If the source module has no module flags, we are done.
-  const NamedMDNode *SrcModFlags = SrcM->getModuleFlagsMetadata();
-  if (!SrcModFlags)
+  NamedMDNode *SrcModFlags = SrcM->getModuleFlagsMetadata();
+  NamedMDNode *DstModFlags = DstM.getOrInsertModuleFlagsMetadata();
+
+  // If the source module has no module flags, we still need to scan the
+  // destination module for any ErrorWhenMissing, otherwise we are done.
+  if (!SrcModFlags) {
+    for (MDNode *Op : DstModFlags->operands()) {
+      ConstantInt *Behavior = mdconst::extract<ConstantInt>(Op->getOperand(0));
+      if (Behavior->getZExtValue() == Module::ErrorWhenMissing) {
+        MDString *ID = cast<MDString>(Op->getOperand(1));
+        return stringErr("linking module flags '" + ID->getString() +
+                         "': IDs have conflicting behaviors in '" +
+                         SrcM->getModuleIdentifier() + "' and '" +
+                         DstM.getModuleIdentifier() + "'");
+      }
+    }
     return Error::success();
+  }
 
   // If the destination module doesn't have module flags yet, then just copy
-  // over the source module's flags.
-  NamedMDNode *DstModFlags = DstM.getOrInsertModuleFlagsMetadata();
+  // over the source module's flags. Make sure to check for any
+  // ErrorWhenMissing.
   if (DstModFlags->getNumOperands() == 0) {
-    for (unsigned I = 0, E = SrcModFlags->getNumOperands(); I != E; ++I)
-      DstModFlags->addOperand(SrcModFlags->getOperand(I));
+    for (MDNode *Op : SrcModFlags->operands()) {
+      ConstantInt *Behavior = mdconst::extract<ConstantInt>(Op->getOperand(0));
+      if (Behavior->getZExtValue() == Module::ErrorWhenMissing) {
+        MDString *ID = cast<MDString>(Op->getOperand(1));
+        // TODO: main case is now failing.
+        return stringErr("linking module flags '" + ID->getString() +
+                         "': IDs have conflicting behaviors in '" +
+                         SrcM->getModuleIdentifier() + "' and '" +
+                         DstM.getModuleIdentifier() + "'");
+      }
+      DstModFlags->addOperand(Op);
+    }
 
     return Error::success();
   }
@@ -1360,7 +1384,8 @@
     case Module::Require:
     case Module::Override:
       llvm_unreachable("not possible");
-    case Module::Error: {
+    case Module::Error:
+    case Module::ErrorWhenMissing: {
       // Emit an error if the values differ.
       if (SrcOp->getOperand(2) != DstOp->getOperand(2))
         return stringErr("linking module flags '" + ID->getString() +
diff --git a/llvm/test/Linker/stack-alignment.ll b/llvm/test/Linker/stack-alignment.ll
--- a/llvm/test/Linker/stack-alignment.ll
+++ b/llvm/test/Linker/stack-alignment.ll
@@ -1,22 +1,19 @@
 ; RUN: split-file %s %t
-; TODO: it seems that ModFlagBehavior::Error doesn't error when a module is
-;       missing a module level attribute. IRLinker::linkModuleFlagsMetadata
-;       seems to think this is ok. Debugging stack mismatch errors is not a fun
-;       time.
-; RUN not llvm-link %t/main.ll %t/none.ll 2>&1 | FileCheck --check-prefix=NONE %s
+; RUN: not llvm-link %t/main.ll %t/none.ll 2>&1 | FileCheck --check-prefix=NONE %s
+; RUN: not llvm-link %t/none.ll %t/main.ll 2>&1 | FileCheck --check-prefix=NONE %s
 ; RUN: llvm-link %t/main.ll %t/8.ll
 ; RUN: not llvm-link %t/main.ll %t/16.ll 2>&1 | FileCheck --check-prefix=CHECK-16 %s
 
 ;--- main.ll
-; NONE: error: linking module flags 'override-stack-alignment': IDs have conflicting values
-; CHECK-16: error: linking module flags 'override-stack-alignment': IDs have conflicting values
+; NONE: error: linking module flags 'override-stack-alignment': IDs have conflicting behaviors
+; CHECK-16: error: linking module flags 'override-stack-alignment': IDs have conflicting behaviors
 !llvm.module.flags = !{!0}
-!0 = !{i32 1, !"override-stack-alignment", i32 8}
+!0 = !{i32 8, !"override-stack-alignment", i32 8}
 
 ;--- none.ll
 ;--- 8.ll
 !llvm.module.flags = !{!0}
-!0 = !{i32 1, !"override-stack-alignment", i32 8}
+!0 = !{i32 8, !"override-stack-alignment", i32 8}
 ;--- 16.ll
 !llvm.module.flags = !{!0}
-!0 = !{i32 1, !"override-stack-alignment", i32 16}
+!0 = !{i32 8, !"override-stack-alignment", i32 16}