[flang][driver] Add support for Frontend Plugins

Introducing a plugin API and a simple HelloWorld Plugin example.
This patch adds the `-load` and `-plugin` flags to frontend driver and
the code around using custom frontend actions from within a plugin
shared library object.

It also adds to the Driver-help test to check the help option with the
updated driver flags.

Additionally, the patch creates a plugin-example test to check the
HelloWorld plugin example runs correctly. As part of this, a new CMake
flag (`FLANG_BUILD_EXAMPLES`) is added to allow the example to be built
and for the test to run.

This Plugin API has only been tested on Linux.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D106137

1 files changed, 31 insertions, 0 deletions

diff --git a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
index a17b6b5..677f8cd 100644
--- a/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
+++ b/flang/lib/FrontendTool/ExecuteCompilerInvocation.cpp
@@ -13,6 +13,7 @@
 
 #include "flang/Frontend/CompilerInstance.h"
 #include "flang/Frontend/FrontendActions.h"
+#include "flang/Frontend/FrontendPluginRegistry.h"
 #include "clang/Driver/Options.h"
 #include "llvm/Option/OptTable.h"
 #include "llvm/Option/Option.h"
@@ -62,6 +63,19 @@ static std::unique_ptr<FrontendAction> CreateFrontendBaseAction(
     return std::make_unique<GetSymbolsSourcesAction>();
   case InitOnly:
     return std::make_unique<InitOnlyAction>();
+  case PluginAction: {
+    for (const FrontendPluginRegistry::entry &plugin :
+        FrontendPluginRegistry::entries()) {
+      if (plugin.getName() == ci.frontendOpts().ActionName) {
+        std::unique_ptr<PluginParseTreeAction> p(plugin.instantiate());
+        return std::move(p);
+      }
+    }
+    unsigned diagID = ci.diagnostics().getCustomDiagID(
+        clang::DiagnosticsEngine::Error, "unable to find plugin '%0'");
+    ci.diagnostics().Report(diagID) << ci.frontendOpts().ActionName;
+    return nullptr;
+  }
   default:
     break;
     // TODO:
@@ -82,6 +96,7 @@ std::unique_ptr<FrontendAction> CreateFrontendAction(CompilerInstance &ci) {
 
   return act;
 }
+
 bool ExecuteCompilerInvocation(CompilerInstance *flang) {
   // Honor -help.
   if (flang->frontendOpts().showHelp) {
@@ -99,6 +114,22 @@ bool ExecuteCompilerInvocation(CompilerInstance *flang) {
     return true;
   }
 
+  // Load any requested plugins.
+  for (const std::string &Path : flang->frontendOpts().plugins) {
+    std::string Error;
+    if (llvm::sys::DynamicLibrary::LoadLibraryPermanently(
+            Path.c_str(), &Error)) {
+      unsigned diagID = flang->diagnostics().getCustomDiagID(
+          clang::DiagnosticsEngine::Error, "unable to load plugin '%0': '%1'");
+      flang->diagnostics().Report(diagID) << Path << Error;
+    }
+  }
+
+  // If there were errors in processing arguments, don't do anything else.
+  if (flang->diagnostics().hasErrorOccurred()) {
+    return false;
+  }
+
   // Create and execute the frontend action.
   std::unique_ptr<FrontendAction> act(CreateFrontendAction(*flang));
   if (!act)