diff --git a/include/swift/AST/DiagnosticsFrontend.def b/include/swift/AST/DiagnosticsFrontend.def
index 113542b215a9b..250fd5b726e5d 100644
--- a/include/swift/AST/DiagnosticsFrontend.def
+++ b/include/swift/AST/DiagnosticsFrontend.def
@@ -24,6 +24,13 @@
 WARNING(warning_no_such_sdk,none,
         "no such SDK: '%0'", (StringRef))
 
+WARNING(warning_no_resource_dir_in_sdk, none,
+        "    You passed in an external -sdk without a Swift runtime.\n"
+        "    Either specify a directory containing the runtime libraries with\n"
+        "    the -resource-dir flag, or use -sysroot instead to point at a C/C++\n"
+        "    sysroot alone. Falling back to this path for the Swift runtime modules\n"
+        "    and libraries:\n"
+        "    %0", (StringRef))
 ERROR(error_no_frontend_args, none,
       "no arguments provided to '-frontend'", ())
 
diff --git a/lib/ClangImporter/ClangImporter.cpp b/lib/ClangImporter/ClangImporter.cpp
index fa3e771bf9adc..c1132518dc176 100644
--- a/lib/ClangImporter/ClangImporter.cpp
+++ b/lib/ClangImporter/ClangImporter.cpp
@@ -761,23 +761,11 @@ void importer::getNormalInvocationArguments(
   invocationArgStrs.push_back("-fapinotes-swift-version=" +
                               languageVersion.asAPINotesVersionString());
 
-  // Prefer `-sdk` paths.
-  if (!searchPathOpts.getSDKPath().empty()) {
-    llvm::SmallString<261> path{searchPathOpts.getSDKPath()};
-    llvm::sys::path::append(path, "usr", "lib", "swift", "apinotes");
-
-    invocationArgStrs.push_back("-iapinotes-modules");
-    invocationArgStrs.push_back(path.str().str());
-  }
-
-  // Fallback to "legacy" `-resource-dir` paths.
-  {
     llvm::SmallString<261> path{searchPathOpts.RuntimeResourcePath};
     llvm::sys::path::append(path, "apinotes");
 
     invocationArgStrs.push_back("-iapinotes-modules");
     invocationArgStrs.push_back(path.str().str());
-  }
 }
 
 static void
diff --git a/lib/Frontend/CompilerInvocation.cpp b/lib/Frontend/CompilerInvocation.cpp
index 01222304fc5e0..8643c6042b65d 100644
--- a/lib/Frontend/CompilerInvocation.cpp
+++ b/lib/Frontend/CompilerInvocation.cpp
@@ -270,7 +270,7 @@ static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
     RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
   }
 
-  if (!SearchPathOpts.SkipSDKImportPaths && !SearchPathOpts.getSDKPath().empty()) {
+  if (!SearchPathOpts.SkipSDKImportPaths && Triple.isOSDarwin() && !SearchPathOpts.getSDKPath().empty()) {
     const char *swiftDir = FrontendOpts.UseSharedResourceFolder
       ? "swift" : "swift_static";
 
@@ -283,18 +283,6 @@ static void updateRuntimeLibraryPaths(SearchPathOptions &SearchPathOpts,
 
     LibPath = SearchPathOpts.getSDKPath();
     llvm::sys::path::append(LibPath, "usr", "lib", swiftDir);
-    if (!Triple.isOSDarwin()) {
-      // Use the non-architecture suffixed form with directory-layout
-      // swiftmodules.
-      llvm::sys::path::append(LibPath, getPlatformNameForTriple(Triple));
-      RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
-
-      // Compatibility with older releases - use the architecture suffixed form
-      // for pre-directory-layout multi-architecture layout.  Note that some
-      // platforms (e.g. Windows) will use this even with directory layout in
-      // older releases.
-      llvm::sys::path::append(LibPath, swift::getMajorArchitectureName(Triple));
-    }
     RuntimeLibraryImportPaths.push_back(std::string(LibPath.str()));
   }
   SearchPathOpts.setRuntimeLibraryImportPaths(RuntimeLibraryImportPaths);
@@ -2281,6 +2269,7 @@ static bool validateSwiftModuleFileArgumentAndAdd(const std::string &swiftModule
 
 static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
                                 DiagnosticEngine &Diags,
+                                const llvm::Triple &Triple,
                                 const CASOptions &CASOpts,
                                 const FrontendOptions &FrontendOpts,
                                 StringRef workingDirectory) {
@@ -2415,6 +2404,18 @@ static bool ParseSearchPathArgs(SearchPathOptions &Opts, ArgList &Args,
 
   if (const Arg *A = Args.getLastArg(OPT_resource_dir))
     Opts.RuntimeResourcePath = A->getValue();
+  else if (!Triple.isOSDarwin() && Args.hasArg(OPT_sdk)) {
+    llvm::SmallString<128> SDKResourcePath(Opts.getSDKPath());
+    llvm::sys::path::append(
+        SDKResourcePath, "usr", "lib",
+        FrontendOpts.UseSharedResourceFolder ? "swift" : "swift_static");
+    // Check for eg <sdkRoot>/usr/lib/swift/
+    if (llvm::sys::fs::exists(SDKResourcePath))
+      Opts.RuntimeResourcePath = SDKResourcePath.str();
+    else
+      Diags.diagnose(SourceLoc(), diag::warning_no_resource_dir_in_sdk,
+                     Opts.RuntimeResourcePath);
+  }
 
   Opts.SkipAllImplicitImportPaths |= Args.hasArg(OPT_nostdimport);
   Opts.SkipSDKImportPaths |= Args.hasArg(OPT_nostdlibimport);
@@ -4060,7 +4061,7 @@ bool CompilerInvocation::parseArgs(
 
   ParseSymbolGraphArgs(SymbolGraphOpts, ParsedArgs, Diags, LangOpts);
 
-  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags,
+  if (ParseSearchPathArgs(SearchPathOpts, ParsedArgs, Diags, LangOpts.Target,
                           CASOpts, FrontendOpts, workingDirectory)) {
     return true;
   }
diff --git a/test/Inputs/clang-importer-sdk/usr/lib/swift/apinotes/cfuncs.apinotes b/test/ClangImporter/Inputs/cfuncs.apinotes
similarity index 100%
rename from test/Inputs/clang-importer-sdk/usr/lib/swift/apinotes/cfuncs.apinotes
rename to test/ClangImporter/Inputs/cfuncs.apinotes
diff --git a/test/ClangImporter/sdk-apinotes.swift b/test/ClangImporter/sdk-apinotes.swift
index 8e53a0564569b..5c47d7781946c 100644
--- a/test/ClangImporter/sdk-apinotes.swift
+++ b/test/ClangImporter/sdk-apinotes.swift
@@ -1,4 +1,10 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck %s -verify
+// RUN: %empty-directory(%t/sdk/usr/lib/swift/apinotes)
+// RUN: %empty-directory(%t/sdk/usr/lib/swift/%target-sdk-name)
+// RUN: cp -r %clang-importer-sdk-path/usr/include %t/sdk/usr
+// RUN: cp -r %test-resource-dir/shims %t/sdk/usr/lib/swift
+// RUN: cp %S/Inputs/cfuncs.apinotes %t/sdk/usr/lib/swift/apinotes
+// RUN: cp -r %platform-module-dir/Swift.swiftmodule %t/sdk/usr/lib/swift/%target-sdk-name/
+// RUN: %target-swift-frontend(mock-sdk: -sdk %t/sdk) -typecheck %s -verify -disable-implicit-concurrency-module-import -disable-implicit-string-processing-module-import
 
 import cfuncs
 
diff --git a/test/ModuleInterface/swift_build_sdk_interfaces/check-only-mode.swift b/test/ModuleInterface/swift_build_sdk_interfaces/check-only-mode.swift
index 3e499cae7a30e..09ccddedaf8a3 100644
--- a/test/ModuleInterface/swift_build_sdk_interfaces/check-only-mode.swift
+++ b/test/ModuleInterface/swift_build_sdk_interfaces/check-only-mode.swift
@@ -1,4 +1,7 @@
 // RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/sdk/usr/lib)
+// RUN: cp -r %test-resource-dir %t/sdk/usr/lib
+// RUN: rm -rf %t/sdk/usr/lib/swift/host
 // RUN: mkdir -p %t/sdk/usr/lib/swift/Normal.swiftmodule
 // RUN: mkdir -p %t/sdk/System/Library/Frameworks/FMWK.framework/Modules/FMWK.swiftmodule
 
diff --git a/test/Serialization/module_defining_interface.swift b/test/Serialization/module_defining_interface.swift
index fee71bb39c61f..69aaa009857fd 100644
--- a/test/Serialization/module_defining_interface.swift
+++ b/test/Serialization/module_defining_interface.swift
@@ -1,8 +1,9 @@
 // RUN: %empty-directory(%t)
 // RUN: %empty-directory(%t/inputs)
 // RUN: %empty-directory(%t/test-sdk)
-// RUN: %empty-directory(%t/test-sdk/usr/lib/swift)
-// RUN: cp -r %platform-module-dir/Swift.swiftmodule %t/test-sdk/usr/lib/swift/Swift.swiftmodule
+// RUN: %empty-directory(%t/test-sdk/usr/lib/swift/%target-sdk-name)
+// RUN: cp -r %test-resource-dir/shims %t/test-sdk/usr/lib/swift
+// RUN: cp -r %platform-module-dir/Swift{,OnoneSupport}.swiftmodule %t/test-sdk/usr/lib/swift/%target-sdk-name/
 
 // RUN: %empty-directory(%t/test-sdk/usr/lib/Foo.swiftmodule)
 // RUN: split-file %s %t
diff --git a/test/Serialization/module_defining_interface_client.swift b/test/Serialization/module_defining_interface_client.swift
index 6f075677be096..aa5abda30b377 100644
--- a/test/Serialization/module_defining_interface_client.swift
+++ b/test/Serialization/module_defining_interface_client.swift
@@ -1,12 +1,13 @@
 // RUN: %empty-directory(%t)
 // RUN: %empty-directory(%t/inputs)
 // RUN: %empty-directory(%t/test-sdk)
-// RUN: %empty-directory(%t/test-sdk/usr/lib/swift)
+// RUN: %empty-directory(%t/test-sdk/usr/lib/swift/%target-sdk-name)
 // RUN: %empty-directory(%t/test-sdk/usr/lib/Foo.swiftmodule)
 // RUN: %empty-directory(%t/test-sdk/usr/lib/Bar.swiftmodule)
 // RUN: %empty-directory(%t/test-sdk/usr/lib/_Foo_Bar.swiftmodule)
 // RUN: %empty-directory(%t/test-sdk/usr/lib/Foo.swiftcrossimport)
-// RUN: cp -r %platform-module-dir/Swift.swiftmodule %t/test-sdk/usr/lib/swift/Swift.swiftmodule
+// RUN: cp -r %test-resource-dir/shims %t/test-sdk/usr/lib/swift
+// RUN: cp -r %platform-module-dir/Swift{,OnoneSupport}.swiftmodule %t/test-sdk/usr/lib/swift/%target-sdk-name/
 
 // RUN: split-file %s %t
 
diff --git a/utils/build.ps1 b/utils/build.ps1
index 0592a4b362e4b..c4a7caf247d52 100644
--- a/utils/build.ps1
+++ b/utils/build.ps1
@@ -1422,7 +1422,7 @@ function Build-CMakeProject {
         switch ($Platform.OS) {
           Windows {
             if ($SwiftSDK -ne "") {
-              $SwiftArgs += @("-sdk", $SwiftSDK)
+              $SwiftArgs += @("-sysroot", $SwiftSDK)
             } else {
               $SwiftArgs += @(
                 "-vfsoverlay", "$RuntimeBinaryCache\stdlib\windows-vfs-overlay.yaml",