Allow searching for prebuilt implicit modules.

[arames]

Summary

The behavior is controlled by the -fprebuilt-implicit-modules option, and
allows searching for implicit modules in the prebuilt module cache paths.

The current command-line options for prebuilt modules do not allow to easily
maintain and use multiple versions of modules. Both the producer and users of
prebuilt modules are required to know the relationships between compilation
options and module file paths. Using a particular version of a prebuilt module
requires passing a particular option on the command line (e.g.
-fmodule-file=[<name>=]<file> or -fprebuilt-module-path=<directory>).

However the compiler already knows how to distinguish and automatically locate
implicit modules. Hence this proposal to introduce the
-fprebuilt-implicit-modules option. When set, it enables searching for
implicit modules in the prebuilt module paths (specified via
-fprebuilt-module-path). To not modify existing behavior, this search takes
place after the standard search for prebuilt modules. If not

Here is a workflow illustrating how both the producer and consumer of prebuilt
modules would need to know what versions of prebuilt modules are available and
where they are located.

clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v1 <config 1 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v2 <config 2 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules_v3 <config 3 options>

clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules_v1 <config 1 options>
clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap <non-prebuilt config options>

With prebuilt implicit modules, the producer can generate prebuilt modules as
usual, all in the same output directory. The same mechanisms as for implicit
modules take care of incorporating hashes in the path to distinguish between
module versions.

Note that we do not specify the output module filename, so -o implicit modules are generated in the cache path prebuilt_modules.

clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 1 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 2 options>
clang -cc1 -x c modulemap -fmodules -emit-module -fmodule-name=foo -fmodules-cache-path=prebuilt_modules <config 3 options>

The user can now simply enable prebuilt implicit modules and point to the
prebuilt modules cache. No need to "parse" command-line options to decide
what prebuilt modules (paths) to use.

clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <config 1 options>
clang -cc1 -x c use.c -fmodules fmodule-map-file=modulemap -fprebuilt-module-path=prebuilt_modules -fprebuilt-implicit-modules <non-prebuilt config options>

This is for example particularly useful in a use-case where compilation is
expensive, and the configurations expected to be used are predictable, but not
controlled by the producer of prebuilt modules. Modules for the set of
predictable configurations can be prebuilt, and using them does not require
"parsing" the configuration (command-line options).

[arames]

Rebase.

[arames]

Ping.

[arames]

Ping.
Still looking for someone to take a look. Happy to answer any questions.

[arames]

Ping.

[arames]

Rebase on top of tree.

[arames]

Rebase on top-of-tree.

[arames]

Rebase.

[Bigcheese]

clang/docs/Modules.rst

231

295

I also think it's important to point out here that disabling the module hash means you are responsible for making sure the modules are compatible. How to do this is mentioned below, but it's never stated why you would want to do this.

clang/test/Modules/prebuilt-implicit-modules.m

10

Is this actually supposed to be module_e? I would expect this to be verifying that it didn't add module_a to %t1.

[arames]

clang/docs/Modules.rst

295

Fixed the typo.

Good point.
I am refactoring the examples to make everything clearer.

303

Fixed a few other typos in the examples as well.

clang/test/Modules/prebuilt-implicit-modules.m

10

That was a typo. Fixed.

[arames]

Addressed review comments.

• Fixed typos in the doc.
• Added doc about module compatibility.
• Cleaned and tested commands in the doc.
• Reworked module hash code to not require -fmodules-cache-path when using -fprebuilt-implicit-modules.

[arames]

Fix a typo.

[arames]

clang/include/clang/Lex/HeaderSearch.h

187

In the previous version of this patch, this value was derived from the stem of ModuleCachePath.
However we do not want to depend on ModuleCachePath being set to be able to retrieve the module hash. See the update in the test.

clang/test/Modules/prebuilt-implicit-modules.m

12

For this to be possible, the computation of the paths to (potential) implicit prebuilt modules cannot depend on the cache path being set.

[Bigcheese]

LGTM

[arames]

Rebased on ToT.

[dexonsmith]

@arames, I see you've had a number of commits. I suggest you request commit access: https://llvm.org/docs/DeveloperPolicy.html#obtaining-commit-access

clang/docs/Modules.rst

358

Nit: please remove the stray space at the end of this line.

[arames]

Fixed the trailing whitespace.

I also just got commit rights, so I will commit it myself.

[thakis]

clang/lib/Driver/ToolChains/Clang.cpp

3279

You need to use a fno_ flag as 2nd param if you want to use hasFlag, the default value is the third param: http://45.33.8.238/win/27354/step_4.txt

`
../../clang/lib/Driver/ToolChains/Clang.cpp(3382,63): error: conversion from 'bool' to 'llvm::opt::OptSpecifier' is ambiguous
    if (Args.hasFlag(options::OPT_fprebuilt_implicit_modules, false))
                                                              ^~~~~
../../llvm/include\llvm/Option/OptSpecifier.h(24,16): note: candidate constructor
  /*implicit*/ OptSpecifier(unsigned ID) : ID(ID) {}
               ^
../../llvm/include\llvm/Option/OptSpecifier.h(25,16): note: candidate constructor
  /*implicit*/ OptSpecifier(const Option *Opt);
               ^
../../llvm/include\llvm/Option/ArgList.h(295,47): note: passing argument to parameter 'Neg' here
  bool hasFlag(OptSpecifier Pos, OptSpecifier Neg, bool Default=true) const;
                                              ^
1 error generated.

[stella.stamenova]

This change broke the windows lldb bot:

http://lab.llvm.org:8011/#/builders/83/builds/570

Can you please fix this or revert?

[arames]

In D68997#2377641, @stella.stamenova wrote:

This change broke the windows lldb bot:

http://lab.llvm.org:8011/#/builders/83/builds/570

Can you please fix this or revert?

Looking at this now.

clang/docs/Modules.rst

``-fmodule-file=[<name>=]<file>``
   prebuilt module search mechanism (in addition to ``-fprebuilt-module-path``)
   and the module is only loaded if required. Note that in this case the
   specified file also overrides this module's paths that might be embedded
   in other precompiled module files.
 
 ``-fprebuilt-module-path=<directory>``
   Specify the path to the prebuilt modules. If specified, we will look for modules in this directory for a given top-level module name. We don't need a module map for loading prebuilt modules in this directory and the compiler will not try to rebuild these modules. This can be specified multiple times.
 
+``-fprebuilt-implicit-modules``
+  Enable prebuilt implicit modules. If a prebuilt module is not found in the
+  prebuilt modules paths (specified via ``-fprebuilt-module-path``), we will
+  look for a matching implicit modules in the prebuilt modules paths.
+
 -cc1 Options
 ~~~~~~~~~~~~
 
 ``-fmodules-strict-context-hash``
   Enables hashing of all compiler options that could impact the semantics of a
   module in an implicit build. This includes things such as header search paths
   and diagnostics. Using this option may lead to an excessive number of modules
   being built if the command line arguments are not homogeneous across your
   build.
 
+Using Prebuilt Modules
+----------------------
+
+Below are a few examples illustrating uses of prebuilt modules via the different options.
+
+First, let's set up files for our examples.
+
+.. code-block:: c
+
+  /* A.h */
+  // empty
+
+.. code-block:: c
+
+  /* B.h */
+  #include "A.h"
+
+.. code-block:: c
+
+  /* use.c */
+  #include "B.h"
+
+.. code-block:: c
+
+  /* module.modulemap */
+  module A {
+    header "A.h"
+  }
+  module B {
+    header "B.h"
+  }
+
+In examples below, the compilation of ``use.c`` can be done without ``-cc1``, but the commands used to prebuild the modules would need to be updated to take into account the default options passed to ``clang -cc1``. (See ``clang use.c -v``)
+
+First let us use an explicit mapping from modules to files.
+
+.. code-block:: sh
+
+  rm -rf prebuilt ; mkdir prebuilt
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=A -o prebuilt/A.pcm
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -o prebuilt/B.pcm -fmodule-file=A=prebuilt/A.pcm
+  clang -cc1 -x c use.c -fmodules -o use.o -fmodule-file=A=prebuilt/A.pcm -fmodule-file=B=prebuilt/B.pcm
+
+Instead of of specifying the mappings manually, it can be convenient to use the ``-fprebuilt-module-path`` option.
+
+.. code-block:: sh
+
+  rm -rf prebuilt ; mkdir prebuilt
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=A -o prebuilt/A.pcm
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -o prebuilt/B.pcm -fmodule-file=A=prebuilt/A.pcm
+  clang -cc1 -x c use.c -fmodules -o use.o -fprebuilt-module-path=prebuilt
+
+A trick to prebuilt required modules in one command is to generate implicit modules using the ``-fdisable-module-hash`` option.
+
+.. code-block:: sh
+
+  rm -rf prebuilt ; mkdir prebuilt
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -fmodules-cache-path=prebuilt -fdisable-module-hash
+  ls prebuilt/*.pcm
+  # prebuilt/A.pcm  prebuilt/B.pcm
+  clang -cc1 -x c use.c -fmodules -fprebuilt-module-path=prebuilt -o /use.o
+
+Maintaining multiple versions of prebuilt modules requires using a different mapping or pointing to a different prebuilt modules cache path. For example:
+
+.. code-block:: sh
+
+  rm -rf prebuilt_1 ; rm -rf prebuilt_2 ; mkdir prebuilt_1 ; mkdir prebuilt_2
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -fmodules-cache-path=prebuilt_1 -D_CONFIG_1
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -fmodules-cache-path=prebuilt_2 -D_CONFIG_2
+  clang -cc1 -x c use.c -o use.o -fmodules -fprebuilt-module-path=prebuilt_1 -DCONFIG_1
+  clang -cc1 -x c use.c -o use.o -fmodules -fprebuilt-module-path=prebuilt_2 -DCONFIG_2
+
+Instead, one can build implicit modules in a given cache path (using
+``-fmodules-cache-path``, and reuse them as prebuilt implicit modules by
+passing ``-fprebuilt-module-path`` and ``-fprebuilt-implicit-modules``.
+
+.. code-block:: sh
+
+  rm -rf prebuilt; mkdir prebuilt
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -fmodules-cache-path=prebuilt -D_CONFIG_1
+  clang -cc1 -x c module.modulemap -fmodules -emit-module -fmodule-name=B -fmodules-cache-path=prebuilt -D_CONFIG_2
+  find prebuilt -name "*.pcm"
+  # prebuilt/W7OPMD3I127H/A-285BFOGQN82QX.pcm
+  # prebuilt/W7OPMD3I127H/B-285BFOGQN82QX.pcm
+  # prebuilt/Y4JCOK22SBQC/A-285BFOGQN82QX.pcm
+  # prebuilt/Y4JCOK22SBQC/B-285BFOGQN82QX.pcm
+  clang -cc1 -x c use.c -o /use.o -fmodules -fmodules-cache-path=cache -fprebuilt-module-path=prebuilt -fprebuilt-implicit-modules -D_CONFIG_1
+  clang -cc1 -x c use.c -o /use.o -fmodules -fmodules-cache-path=cache -fprebuilt-module-path=prebuilt -fprebuilt-implicit-modules -D_CONFIG_2
+
 Module Semantics
 ================
 
 Modules are modeled as if each submodule were a separate translation unit, and a module import makes names from the other translation unit visible. Each submodule starts with a new preprocessor state and an empty translation unit.
 
 .. note::
 
   This behavior is currently only approximated when building a module with submodules. Entities within a submodule that has already been built are visible when building later submodules in that module. This can lead to fragile modules that depend on the build order used for the submodules of the module, and should not be relied upon. This behavior is subject to change.

 ------
 
 The C and C++ preprocessor assumes that the input text is a single linear buffer, but with modules this is not the case. It is possible to import two modules that have conflicting definitions for a macro (or where one ``#define``\s a macro and the other ``#undef``\ines it). The rules for handling macro definitions in the presence of modules are as follows:
 
 * Each definition and undefinition of a macro is considered to be a distinct entity.
 * Such entities are *visible* if they are from the current submodule or translation unit, or if they were exported from a submodule that has been imported.
 * A ``#define X`` or ``#undef X`` directive *overrides* all definitions of ``X`` that are visible at the point of the directive.
 * A ``#define`` or ``#undef`` directive is *active* if it is visible and no visible directive overrides it.
 * A set of macro directives is *consistent* if it consists of only ``#undef`` directives, or if all ``#define`` directives in the set define the macro name to the same sequence of tokens (following the usual rules for macro redefinitions).
 * If a macro name is used and the set of active directives is not consistent, the program is ill-formed. Otherwise, the (unique) meaning of the macro name is used.
 
 For example, suppose:
 
 * ``<stdio.h>`` defines a macro ``getc`` (and exports its ``#define``)
 * ``<cstdio>`` imports the ``<stdio.h>`` module and undefines the macro (and exports its ``#undef``)
   
 The ``#undef`` overrides the ``#define``, and a source file that imports both modules *in any order* will not see ``getc`` defined as a macro.

clang/include/clang/Driver/Options.td

def fmodules_cache_path : Joined<["-"], "fmodules-cache-path=">, Group<i_Group>,
   Flags<[DriverOption, CC1Option]>, MetaVarName<"<directory>">,
   HelpText<"Specify the module cache path">;
 def fmodules_user_build_path : Separate<["-"], "fmodules-user-build-path">, Group<i_Group>,
   Flags<[DriverOption, CC1Option]>, MetaVarName<"<directory>">,
   HelpText<"Specify the module user build path">;
 def fprebuilt_module_path : Joined<["-"], "fprebuilt-module-path=">, Group<i_Group>,
   Flags<[DriverOption, CC1Option]>, MetaVarName<"<directory>">,
   HelpText<"Specify the prebuilt module path">;
+def fprebuilt_implicit_modules : Flag <["-"], "fprebuilt-implicit-modules">, Group<f_Group>,
+  Flags<[DriverOption, CC1Option]>,
+  HelpText<"Look up implicit modules in the prebuilt module path">;
 def fmodules_prune_interval : Joined<["-"], "fmodules-prune-interval=">, Group<i_Group>,
   Flags<[CC1Option]>, MetaVarName<"<seconds>">,
   HelpText<"Specify the interval (in seconds) between attempts to prune the module cache">;
 def fmodules_prune_after : Joined<["-"], "fmodules-prune-after=">, Group<i_Group>,
   Flags<[CC1Option]>, MetaVarName<"<seconds>">,
   HelpText<"Specify the interval (in seconds) after which a module file will be considered unused">;
 def fmodules_search_all : Flag <["-"], "fmodules-search-all">, Group<f_Group>,
   Flags<[DriverOption, CC1Option]>,

clang/include/clang/Lex/HeaderSearch.h

class HeaderSearch {
   /// overridden.
   ///
   /// For a \#include "x" or \#include \<x> directive, the last string in this
   /// list which is a prefix of 'x' determines whether the file is treated as
   /// a system header.
   std::vector<std::pair<std::string, bool>> SystemHeaderPrefixes;
 
   /// The path to the module cache.
   std::string ModuleCachePath;
 
   /// All of the preprocessor-specific data about files that are
   /// included, indexed by the FileEntry's UID.
   mutable std::vector<HeaderFileInfo> FileInfo;
 
   /// Keeps track of each lookup performed by LookupFile.
   struct LookupFileCacheInfo {
     /// Starting index in SearchDirs that the cached search was performed from.
public:
   /// \param FileMapOnly If true, then only look in the explicit module name
   //  to file name map and skip the directory search.
   ///
   /// \returns The name of the module file that corresponds to this module,
   /// or an empty string if this module does not correspond to any module file.
   std::string getPrebuiltModuleFileName(StringRef ModuleName,
                                         bool FileMapOnly = false);
 
+  /// Retrieve the name of the prebuilt module file that should be used
+  /// to load the given module.
+  ///
+  /// \param Module The module whose module file name will be returned.
+  ///
+  /// \returns The name of the module file that corresponds to this module,
+  /// or an empty string if this module does not correspond to any module file.
+  std::string getPrebuiltImplicitModuleFileName(Module *Module);
+
   /// Retrieve the name of the (to-be-)cached module file that should
   /// be used to load a module with the given name.
   ///
   /// \param ModuleName The module whose module file name will be returned.
   ///
   /// \param ModuleMapPath A path that when combined with \c ModuleName
   /// uniquely identifies this module. See Module::ModuleMap.
   ///
private:
   ///
   /// \param AllowExtraModuleMapSearch Whether we allow to search modulemaps
   /// in subdirectories.
   ///
   /// \returns The module named ModuleName.
   Module *lookupModule(StringRef ModuleName, StringRef SearchName,
                        bool AllowExtraModuleMapSearch = false);
 
+  /// Retrieve the name of the (to-be-)cached module file that should
+  /// be used to load a module with the given name.
+  ///
+  /// \param ModuleName The module whose module file name will be returned.
+  ///
+  /// \param ModuleMapPath A path that when combined with \c ModuleName
+  /// uniquely identifies this module. See Module::ModuleMap.
+  ///
+  /// \param CachePath A path to the module cache.
+  ///
+  /// \returns The name of the module file that corresponds to this module,
+  /// or an empty string if this module does not correspond to any module file.
+  std::string getCachedModuleFileNameImpl(StringRef ModuleName,
+                                          StringRef ModuleMapPath,
+                                          StringRef CachePath);
+
   /// Retrieve a module with the given name, which may be part of the
   /// given framework.
   ///
   /// \param Name The name of the module to retrieve.
   ///
   /// \param Dir The framework directory (e.g., ModuleName.framework).
   ///
   /// \param IsSystem Whether the framework directory is part of the system

clang/include/clang/Lex/HeaderSearchOptions.h

public:
   /// working directory (or the home directory of the module map file that
   /// contained the 'extern module' directive importing this module map file
   /// if any) rather than the directory containing the module map file.
   //
   /// The home directory is where we look for files named in the module map
   /// file.
   unsigned ModuleMapFileHomeIsCwd : 1;
 
+  /// Also search for prebuilt implicit modules in the prebuilt module cache
+  /// path.
+  unsigned EnablePrebuiltImplicitModules : 1;
+
   /// The interval (in seconds) between pruning operations.
   ///
   /// This operation is expensive, because it requires Clang to walk through
   /// the directory structure of the module cache, stat()'ing and removing
   /// files.
   ///
   /// The default value is large, e.g., the operation runs once a week.
   unsigned ModuleCachePruneInterval = 7 * 24 * 60 * 60;
public:
   ///
   /// This includes things like the full header search path, and enabled
   /// diagnostics.
   unsigned ModulesStrictContextHash : 1;
 
   HeaderSearchOptions(StringRef _Sysroot = "/")
       : Sysroot(_Sysroot), ModuleFormat("raw"), DisableModuleHash(false),
         ImplicitModuleMaps(false), ModuleMapFileHomeIsCwd(false),
-        UseBuiltinIncludes(true), UseStandardSystemIncludes(true),
-        UseStandardCXXIncludes(true), UseLibcxx(false), Verbose(false),
+        EnablePrebuiltImplicitModules(false), UseBuiltinIncludes(true),
+        UseStandardSystemIncludes(true), UseStandardCXXIncludes(true),
+        UseLibcxx(false), Verbose(false),
         ModulesValidateOncePerBuildSession(false),
         ModulesValidateSystemHeaders(false),
         ValidateASTInputFilesContent(false), UseDebugInfo(false),
         ModulesValidateDiagnosticOptions(true), ModulesHashContent(false),
         ModulesStrictContextHash(false) {}
 
   /// AddPath - Add the \p Path path to the specified \p Group list.
   void AddPath(StringRef Path, frontend::IncludeDirGroup Group,

clang/lib/Driver/ToolChains/Clang.cpp

static void RenderModulesOptions(Compilation &C, const Driver &D,
 
   if (HaveModules) {
     // -fprebuilt-module-path specifies where to load the prebuilt module files.
     for (const Arg *A : Args.filtered(options::OPT_fprebuilt_module_path)) {
       CmdArgs.push_back(Args.MakeArgString(
           std::string("-fprebuilt-module-path=") + A->getValue()));
       A->claim();
     }
+    if (Args.hasFlag(options::OPT_fprebuilt_implicit_modules, false))
+      CmdArgs.push_back("-fprebuilt-implicit-modules");
     if (Args.hasFlag(options::OPT_fmodules_validate_input_files_content,
                      options::OPT_fno_modules_validate_input_files_content,
                      false))
       CmdArgs.push_back("-fvalidate-ast-input-files-content");
   }
 
   // -fmodule-name specifies the module that is currently being built (or
   // used for header checking by -fmodule-maps).

clang/lib/Frontend/CompilerInstance.cpp

if (BuiltModuleIt != BuiltModules.end()) {
     return MS_ModuleBuildPragma;
   }
 
   // Try to load the module from the prebuilt module path.
   const HeaderSearchOptions &HSOpts = HS.getHeaderSearchOpts();
   if (!HSOpts.PrebuiltModuleFiles.empty() ||
       !HSOpts.PrebuiltModulePaths.empty()) {
     ModuleFilename = HS.getPrebuiltModuleFileName(ModuleName);
+    if (HSOpts.EnablePrebuiltImplicitModules && ModuleFilename.empty())
+      ModuleFilename = HS.getPrebuiltImplicitModuleFileName(M);
     if (!ModuleFilename.empty())
       return MS_PrebuiltModulePath;
   }
 
   // Try to load the module from the module cache.
   if (M) {
     ModuleFilename = HS.getCachedModuleFileName(M);
     return MS_ModuleCache;

clang/lib/Frontend/CompilerInvocation.cpp

static void ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args,
   for (const auto *A : Args.filtered(OPT_fprebuilt_module_path))
     Opts.AddPrebuiltModulePath(A->getValue());
   Opts.DisableModuleHash = Args.hasArg(OPT_fdisable_module_hash);
   Opts.ModulesHashContent = Args.hasArg(OPT_fmodules_hash_content);
   Opts.ModulesValidateDiagnosticOptions =
       !Args.hasArg(OPT_fmodules_disable_diagnostic_validation);
   Opts.ImplicitModuleMaps = Args.hasArg(OPT_fimplicit_module_maps);
   Opts.ModuleMapFileHomeIsCwd = Args.hasArg(OPT_fmodule_map_file_home_is_cwd);
+  Opts.EnablePrebuiltImplicitModules =
+      Args.hasArg(OPT_fprebuilt_implicit_modules);
   Opts.ModuleCachePruneInterval =
       getLastArgIntValue(Args, OPT_fmodules_prune_interval, 7 * 24 * 60 * 60);
   Opts.ModuleCachePruneAfter =
       getLastArgIntValue(Args, OPT_fmodules_prune_after, 31 * 24 * 60 * 60);
   Opts.ModulesValidateOncePerBuildSession =
       Args.hasArg(OPT_fmodules_validate_once_per_build_session);
   Opts.BuildSessionTimestamp =
       getLastArgUInt64Value(Args, OPT_fbuild_session_timestamp, 0);

clang/lib/Lex/HeaderSearch.cpp

for (const std::string &Dir : HSOpts->PrebuiltModulePaths) {
     llvm::sys::fs::make_absolute(Result);
     llvm::sys::path::append(Result, ModuleName + ".pcm");
     if (getFileMgr().getFile(Result.str()))
       return std::string(Result);
   }
   return {};
 }
 
+std::string HeaderSearch::getPrebuiltImplicitModuleFileName(Module *Module) {
+  const FileEntry *ModuleMap =
+      getModuleMap().getModuleMapFileForUniquing(Module);
+  StringRef ModuleName = Module->Name;
+  StringRef ModuleMapPath = ModuleMap->getName();
+  StringRef ModuleCacheHash = HSOpts->DisableModuleHash
+                                  ? ""
+                                  : llvm::sys::path::stem(getModuleCachePath());
+  for (const std::string &Dir : HSOpts->PrebuiltModulePaths) {
+    SmallString<256> CachePath(Dir);
+    llvm::sys::fs::make_absolute(CachePath);
+    llvm::sys::path::append(CachePath, ModuleCacheHash);
+    std::string FileName =
+        getCachedModuleFileNameImpl(ModuleName, ModuleMapPath, CachePath);
+    if (!FileName.empty() && getFileMgr().getFile(FileName))
+      return FileName;
+  }
+  return {};
+}
+
 std::string HeaderSearch::getCachedModuleFileName(StringRef ModuleName,
                                                   StringRef ModuleMapPath) {
+  return getCachedModuleFileNameImpl(ModuleName, ModuleMapPath,
+                                     getModuleCachePath());
+}
+
+std::string HeaderSearch::getCachedModuleFileNameImpl(StringRef ModuleName,
+                                                      StringRef ModuleMapPath,
+                                                      StringRef CachePath) {
   // If we don't have a module cache path or aren't supposed to use one, we
   // can't do anything.
-  if (getModuleCachePath().empty())
+  if (CachePath.empty())
     return {};
 
-  SmallString<256> Result(getModuleCachePath());
+  SmallString<256> Result(CachePath);
   llvm::sys::fs::make_absolute(Result);
 
   if (HSOpts->DisableModuleHash) {
     llvm::sys::path::append(Result, ModuleName + ".pcm");
   } else {
     // Construct the name <ModuleName>-<hash of ModuleMapPath>.pcm which should
     // ideally be globally unique to this particular module. Name collisions
     // in the hash are safe (because any translation unit can only import one

clang/lib/Serialization/ASTReader.cpp

bool ASTReader::ParseHeaderSearchOptions(const RecordData &Record,
   }
 
   HSOpts.ResourceDir = ReadString(Record, Idx);
   HSOpts.ModuleCachePath = ReadString(Record, Idx);
   HSOpts.ModuleUserBuildPath = ReadString(Record, Idx);
   HSOpts.DisableModuleHash = Record[Idx++];
   HSOpts.ImplicitModuleMaps = Record[Idx++];
   HSOpts.ModuleMapFileHomeIsCwd = Record[Idx++];
+  HSOpts.EnablePrebuiltImplicitModules = Record[Idx++];
   HSOpts.UseBuiltinIncludes = Record[Idx++];
   HSOpts.UseStandardSystemIncludes = Record[Idx++];
   HSOpts.UseStandardCXXIncludes = Record[Idx++];
   HSOpts.UseLibcxx = Record[Idx++];
   std::string SpecificModuleCachePath = ReadString(Record, Idx);
 
   return Listener.ReadHeaderSearchOptions(HSOpts, SpecificModuleCachePath,
                                           Complain);

clang/lib/Serialization/ASTWriter.cpp

#include "clang/Basic/Sanitizers.def"
   }
 
   AddString(HSOpts.ResourceDir, Record);
   AddString(HSOpts.ModuleCachePath, Record);
   AddString(HSOpts.ModuleUserBuildPath, Record);
   Record.push_back(HSOpts.DisableModuleHash);
   Record.push_back(HSOpts.ImplicitModuleMaps);
   Record.push_back(HSOpts.ModuleMapFileHomeIsCwd);
+  Record.push_back(HSOpts.EnablePrebuiltImplicitModules);
   Record.push_back(HSOpts.UseBuiltinIncludes);
   Record.push_back(HSOpts.UseStandardSystemIncludes);
   Record.push_back(HSOpts.UseStandardCXXIncludes);
   Record.push_back(HSOpts.UseLibcxx);
   // Write out the specific module cache path that contains the module files.
   AddString(PP.getHeaderSearchInfo().getModuleCachePath(), Record);
   Stream.EmitRecord(HEADER_SEARCH_OPTIONS, Record);
 

clang/test/Modules/Inputs/prebuilt-implicit-module/a.h

+const int a = 1;

clang/test/Modules/Inputs/prebuilt-implicit-module/module.modulemap

+module module_a { header "a.h" }

clang/test/Modules/prebuilt-implicit-modules.m

+// RUN: rm -rf %t
+// RUN: %clang_cc1 -x objective-c -fmodules %S/Inputs/prebuilt-implicit-module/module.modulemap -emit-module -fmodule-name=module_a -fmodules-cache-path=%t
+// RUN: find %t -name "module_a*.pcm" | grep module_a
+//
+// Check we use a prebuilt module when available, and do not build an implicit module.
+// RUN: rm -rf %t1
+// RUN: mkdir -p %t1
+// RUN: %clang_cc1 -x objective-c %s -I%S/Inputs/prebuilt-implicit-module -fmodules -fmodule-map-file=%S/Inputs/prebuilt-implicit-module/module.modulemap -fprebuilt-implicit-modules -fprebuilt-module-path=%t -fmodules-cache-path=%t1
+// RUN: find %t1 -name "module_a*.pcm" | not grep module_e
+//
+// Check that we correctly fall back to implicit modules if the prebuilt implicit module is not found.
+// RUN: %clang_cc1 -x objective-c %s -I%S/Inputs/prebuilt-implicit-module -fmodules -fmodule-map-file=%S/Inputs/prebuilt-implicit-module/module.modulemap -fprebuilt-implicit-modules -fprebuilt-module-path=%t -fmodules-cache-path=%t1 -fno-signed-char
+// RUN: find %t1 -name "module_a*.pcm" | grep module_a
+
+// Check that non-implicit prebuilt modules are always preferred to prebuilt implicit modules.
+// RUN: rm -rf %t2
+// RUN: mkdir -p %t2
+// RUN: %clang_cc1 -x objective-c -fmodules %S/Inputs/prebuilt-implicit-module/module.modulemap -emit-module -fmodule-name=module_a -fmodules-cache-path=%t
+// RUN: %clang_cc1 -x objective-c -fmodules %S/Inputs/prebuilt-implicit-module/module.modulemap -emit-module -fmodule-name=module_a -o %t/module_a.pcm -fno-signed-char
+// RUN: not %clang_cc1 -x objective-c %s -I%S/Inputs/prebuilt-implicit-module -fmodules -fmodule-map-file=%S/Inputs/prebuilt-implicit-module/module.modulemap -fprebuilt-implicit-modules -fprebuilt-module-path=%t -fmodules-cache-path=%t2
+// RUN: find %t2 -name "module_a*.pcm" | not grep module_a
+
+// expected-no-diagnostics
+@import module_a;
+int test() {
+  return a;
+}