diff --git a/clang/include/clang/Frontend/ASTUnit.h b/clang/include/clang/Frontend/ASTUnit.h
--- a/clang/include/clang/Frontend/ASTUnit.h
+++ b/clang/include/clang/Frontend/ASTUnit.h
@@ -686,6 +686,8 @@
   /// creating modules.
   /// \param Diags - The diagnostics engine to use for reporting errors; its
   /// lifetime is expected to extend past that of the returned ASTUnit.
+  /// \param PrebuiltModuleFiles - A map for module names to prebuilt module
+  /// paths.
   ///
   /// \returns - The initialized ASTUnit or null if the AST failed to load.
   static std::unique_ptr<ASTUnit> LoadFromASTFile(
@@ -695,7 +697,8 @@
       bool OnlyLocalDecls = false, ArrayRef<RemappedFile> RemappedFiles = None,
       CaptureDiagsKind CaptureDiagnostics = CaptureDiagsKind::None,
       bool AllowPCHWithCompilerErrors = false,
-      bool UserFilesAreVolatile = false);
+      bool UserFilesAreVolatile = false,
+      const std::map<std::string, std::string> &PrebuiltModuleFiles = {});
 
 private:
   /// Helper function for \c LoadFromCompilerInvocation() and
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -759,7 +759,8 @@
     const FileSystemOptions &FileSystemOpts, bool UseDebugInfo,
     bool OnlyLocalDecls, ArrayRef<RemappedFile> RemappedFiles,
     CaptureDiagsKind CaptureDiagnostics, bool AllowPCHWithCompilerErrors,
-    bool UserFilesAreVolatile) {
+    bool UserFilesAreVolatile,
+    const std::map<std::string, std::string> &PrebuiltModuleFiles) {
   std::unique_ptr<ASTUnit> AST(new ASTUnit(true));
 
   // Recover resources if we crash before exiting this method.
@@ -785,6 +786,7 @@
   AST->ModuleCache = new InMemoryModuleCache;
   AST->HSOpts = std::make_shared<HeaderSearchOptions>();
   AST->HSOpts->ModuleFormat = PCHContainerRdr.getFormat();
+  AST->HSOpts->PrebuiltModuleFiles = PrebuiltModuleFiles;
   AST->HeaderInfo.reset(new HeaderSearch(AST->HSOpts,
                                          AST->getSourceManager(),
                                          AST->getDiagnostics(),
diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -565,7 +565,9 @@
 
     std::unique_ptr<ASTUnit> AST = ASTUnit::LoadFromASTFile(
         InputFile, CI.getPCHContainerReader(), ASTUnit::LoadPreprocessorOnly,
-        ASTDiags, CI.getFileSystemOpts(), CI.getCodeGenOpts().DebugTypeExtRefs);
+        ASTDiags, CI.getFileSystemOpts(), CI.getCodeGenOpts().DebugTypeExtRefs,
+        false, None, CaptureDiagsKind::None, false, false,
+        CI.getHeaderSearchOpts().PrebuiltModuleFiles);
     if (!AST)
       goto failure;
 
@@ -631,7 +633,9 @@
 
     std::unique_ptr<ASTUnit> AST = ASTUnit::LoadFromASTFile(
         InputFile, CI.getPCHContainerReader(), ASTUnit::LoadEverything, Diags,
-        CI.getFileSystemOpts(), CI.getCodeGenOpts().DebugTypeExtRefs);
+        CI.getFileSystemOpts(), CI.getCodeGenOpts().DebugTypeExtRefs, false,
+        None, CaptureDiagsKind::None, false, false,
+        CI.getHeaderSearchOpts().PrebuiltModuleFiles);
 
     if (!AST)
       goto failure;
diff --git a/clang/test/Modules/Inputs/explicit-build-pcm/a.h b/clang/test/Modules/Inputs/explicit-build-pcm/a.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/explicit-build-pcm/a.h
@@ -0,0 +1,5 @@
+#if !__building_module(a) && !BUILDING_A_PCH
+#error "should only get here when building module a"
+#endif
+
+const int a = 1;
diff --git a/clang/test/Modules/Inputs/explicit-build-pcm/b.h b/clang/test/Modules/Inputs/explicit-build-pcm/b.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/explicit-build-pcm/b.h
@@ -0,0 +1,7 @@
+#include "a.h"
+
+#if !__building_module(b)
+#error "should only get here when building module b"
+#endif
+
+const int b = 2;
diff --git a/clang/test/Modules/Inputs/explicit-build-pcm/c.h b/clang/test/Modules/Inputs/explicit-build-pcm/c.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/explicit-build-pcm/c.h
@@ -0,0 +1,7 @@
+#include "b.h"
+
+#if !__building_module(c)
+#error "should only get here when building module c"
+#endif
+
+const int c = 3;
diff --git a/clang/test/Modules/Inputs/explicit-build-pcm/d.h b/clang/test/Modules/Inputs/explicit-build-pcm/d.h
new file mode 100644
diff --git a/clang/test/Modules/Inputs/explicit-build-pcm/module.modulemap b/clang/test/Modules/Inputs/explicit-build-pcm/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/explicit-build-pcm/module.modulemap
@@ -0,0 +1,4 @@
+module a { header "a.h" }
+module b { header "b.h" export * }
+module c { header "c.h" export * }
+module d { header "d.h" }
diff --git a/clang/test/Modules/explicit-build-pcm.cpp b/clang/test/Modules/explicit-build-pcm.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/explicit-build-pcm.cpp
@@ -0,0 +1,26 @@
+// RUN: rm -rf %t
+// expected-no-diagnostics
+
+// RUN: %clang_cc1 -x c++ -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Rmodule-build -fno-modules-error-recovery \
+// RUN:            -fmodule-name=a -emit-module %S/Inputs/explicit-build/module.modulemap -o %t/a.pcm
+
+// RUN: %clang_cc1 -x c++ -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Rmodule-build -fno-modules-error-recovery \
+// RUN:            -fmodule-file=a=%t/a.pcm \
+// RUN:            -fmodule-name=b -emit-module %S/Inputs/explicit-build/module.modulemap -o %t/b.pcm
+
+// RUN: %clang_cc1 -x c++ -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Rmodule-build -fno-modules-error-recovery \
+// RUN:            -fmodule-file=a=%t/a.pcm \
+// RUN:            -fmodule-file=b=%t/b.pcm \
+// RUN:            -fmodule-name=c -emit-module %S/Inputs/explicit-build/module.modulemap -o %t/c.pcm
+
+// RUN: mv %t/a.pcm %t/a.moved.pcm
+// RUN: mv %t/b.pcm %t/b.moved.pcm
+// RUN: mv %t/c.pcm %t/c.moved.pcm
+//
+// RUN: %clang_cc1 -x pcm -std=c++11 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -Rmodule-build -fno-modules-error-recovery \
+// RUN:            -fmodule-file=a=%t/a.moved.pcm \
+// RUN:            -fmodule-file=b=%t/b.moved.pcm \
+// RUN:            %t/c.moved.pcm -o %t/c.o
+// expected-no-diagnostics
+
+#include <stdio.h>