clang-cl: Support loading plugins on Windows

[• ehsan]

Summary

This builds on the support being added to LLVM to import and export
registries from DLLs. This will allow us to pick up the registry
entries added in the DLL's copy of FrontendPluginRegistry.

This will allow us to use plugins on Windows using:
$ clang-cl -Xclang -load -Xclang plugin.dll \

-Xclang -add-plugin -Xclang foo

clang-cl: Enable plugins on Windows

[• ehsan]

Updating based on the LLVM side changes.

[• ehsan]

Sorry for the noise, arconist fail...

[rnk]

lgtm

[Eugene.Zelenko]

Was committed in r260265, but reverted in r260536.

[v.g.vassilev]

@chapuni, you have reverted this patch and the commit message says "investigating". Did you come to a conclusion? Can we reland it?

[chapuni]

@v.g.vassilev, I couldn't explain what it caused. Fell free to recommit.

[thakis]

LLVM_EXPORT_REGISTRY no longer exists, so this can't be relanded.

[v.g.vassilev]

Indeed. IIUC, here is no way clang-cl to load plugins if compiled BUILD_SHARED_LIB=0. It is not clear to me if that's a design limitation or something else. I'd love to be able to load plugins on a 'standard' clang-cl instance.

[thakis]

Clang's plugin model is currently fundamentally incompatible with how imports and exports work on Windows, which I'm guessing is probably why LLVM_EXPORT_REGISTRY was removed as well. There's no easy way to get plugins to work on Windows.

docs/ClangPlugins.rst

bool ParseArgs(const CompilerInstance &CI,
     }
     return true;
   }
 
 Registering a plugin
 ====================
 
 A plugin is loaded from a dynamic library at runtime by the compiler. To
-register a plugin in a library, use ``FrontendPluginRegistry::Add<>``:
+register a plugin in a library, use ``FrontendPluginRegistry::Add<>``.
+On Windows, you also need to export your plugin registry using
+``LLVM_EXPORT_REGISTRY``.  Here is an example:
 
 .. code-block:: c++
 
   static FrontendPluginRegistry::Add<MyPlugin> X("my-plugin-name", "my plugin description");
+  LLVM_EXPORT_REGISTRY(FrontendPluginRegistry)
 
 Putting it all together
 =======================
 
 Let's look at an example plugin that prints top-level function names.  This
 example is checked into the clang repository; please take a look at
 the `latest version of PrintFunctionNames.cpp
 <http://llvm.org/viewvc/llvm-project/cfe/trunk/examples/PrintFunctionNames/PrintFunctionNames.cpp?view=markup>`_.

examples/PrintFunctionNames/PrintFunctionNames.cpp

protected:
   }
 
 };
 
 }
 
 static FrontendPluginRegistry::Add<PrintFunctionNamesAction>
 X("print-fns", "print function names");
+LLVM_EXPORT_REGISTRY(FrontendPluginRegistry)

lib/FrontendTool/ExecuteCompilerInvocation.cpp

if (Clang->getFrontendOpts().ShowVersion) {
     return true;
   }
 
   // Load any requested plugins.
   for (unsigned i = 0,
          e = Clang->getFrontendOpts().Plugins.size(); i != e; ++i) {
     const std::string &Path = Clang->getFrontendOpts().Plugins[i];
     std::string Error;
-    if (llvm::sys::DynamicLibrary::LoadLibraryPermanently(Path.c_str(), &Error))
+    llvm::sys::DynamicLibrary DL(
+        llvm::sys::DynamicLibrary::getPermanentLibrary(Path.c_str(), &Error));
+    if (DL.isValid()) {
+      // On Windows, we need to import the plugin front-end action
+      // dynamically.
+      LLVM_IMPORT_REGISTRY(FrontendPluginRegistry, DL);
+    } else {
       Clang->getDiagnostics().Report(diag::err_fe_unable_to_load_plugin)
         << Path << Error;
     }
+  }
 
   // Honor -mllvm.
   //
   // FIXME: Remove this, one day.
   // This should happen AFTER plugins have been loaded!
   if (!Clang->getFrontendOpts().LLVMArgs.empty()) {
     unsigned NumArgs = Clang->getFrontendOpts().LLVMArgs.size();
     auto Args = llvm::make_unique<const char*[]>(NumArgs + 2);

test/Frontend/plugins.c

 // RUN: %clang_cc1 -load %llvmshlibdir/PrintFunctionNames%pluginext -plugin print-fns %s 2>&1 | FileCheck %s
+// RUN: %clang_cl -Xclang -load -Xclang %llvmshlibdir/PrintFunctionNames%pluginext -Xclang -plugin -Xclang print-fns %s 2>&1 | FileCheck %s
 // REQUIRES: plugins, examples
 
 // CHECK: top-level-decl: "x"
 void x();

test/lit.cfg

def inferClang(PATH):
     return clang
 
 config.clang = inferClang(config.environment['PATH']).replace('\\', '/')
 if not lit_config.quiet:
     lit_config.note('using clang: %r' % config.clang)
 
 # Plugins (loadable modules)
 # TODO: This should be supplied by Makefile or autoconf.
-if sys.platform in ['win32', 'cygwin']:
+if sys.platform in ['cygwin']:
     has_plugins = (config.enable_shared == 1)
 else:
     has_plugins = True
 
 if has_plugins and config.llvm_plugin_ext:
     config.available_features.add('plugins')
 
 config.substitutions.append( ('%llvmshlibdir', config.llvm_shlib_dir) )