Make llvmc work again.

Chris recently broke llvmc with his Makefile changes (r75379). That patch made
the global change .o -> .a, which caused built-in llvmc plugins to stop working
since plugin initialization in llvmc is based on static variables not referenced
from the main executable. This patch implements auto-generated forced references
to the plugin libraries.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74000 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CompilerDriver/ForceLinkage.h

@@ -0,0 +1,82 @@
+//===--- ForceLinkage.h - The LLVM Compiler Driver --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open
+// Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//  A bit of preprocessor magic to force references to static libraries. Needed
+//  because plugin initialization is done via static variables.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_H
+#define LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_H
+
+#include "llvm/CompilerDriver/ForceLinkageMacros.h"
+
+namespace llvmc {
+
+// Declare all ForceLinkage$(PluginName) functions.
+
+#ifdef LLVMC_BUILTIN_PLUGIN_1
+      LLVMC_FORCE_LINKAGE_DECL(LLVMC_BUILTIN_PLUGIN_1);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_2
+      LLVMC_FORCE_LINKAGE_DECL(LLVMC_BUILTIN_PLUGIN_2);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_3
+      LLVMC_FORCE_LINKAGE_DECL(LLVMC_BUILTIN_PLUGIN_3);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_4
+      LLVMC_FORCE_LINKAGE_DECL(LLVMC_BUILTIN_PLUGIN_4);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_5
+      LLVMC_FORCE_LINKAGE_DECL(LLVMC_BUILTIN_PLUGIN_5);
+#endif
+
+namespace force_linkage {
+
+  struct LinkageForcer {
+
+    LinkageForcer() {
+
+// Call all ForceLinkage$(PluginName) functions.
+#ifdef LLVMC_BUILTIN_PLUGIN_1
+      LLVMC_FORCE_LINKAGE_CALL(LLVMC_BUILTIN_PLUGIN_1);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_2
+      LLVMC_FORCE_LINKAGE_CALL(LLVMC_BUILTIN_PLUGIN_2);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_3
+      LLVMC_FORCE_LINKAGE_CALL(LLVMC_BUILTIN_PLUGIN_3);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_4
+      LLVMC_FORCE_LINKAGE_CALL(LLVMC_BUILTIN_PLUGIN_4);
+#endif
+
+#ifdef LLVMC_BUILTIN_PLUGIN_5
+      LLVMC_FORCE_LINKAGE_CALL(LLVMC_BUILTIN_PLUGIN_5);
+#endif
+
+    }
+  };
+} // End namespace force_linkage.
+
+// The only externally used bit.
+void ForceLinkage() {
+  force_linkage::LinkageForcer dummy;
+}
+
+} // End namespace llvmc.
+
+#endif // LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_H

include/llvm/CompilerDriver/ForceLinkageMacros.h

@@ -0,0 +1,29 @@
+//===--- ForceLinkageMacros.h - The LLVM Compiler Driver --------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open
+// Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+//  Preprocessor magic that forces references to static libraries - common
+//  macros used by both driver and plugins.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_MACROS_H
+#define LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_MACROS_H
+
+#define LLVMC_FORCE_LINKAGE_PREFIX(PluginName) ForceLinkage ## PluginName
+
+#define LLVMC_FORCE_LINKAGE_FUN(PluginName) \
+  LLVMC_FORCE_LINKAGE_PREFIX(PluginName)
+
+#define LLVMC_FORCE_LINKAGE_DECL(PluginName) \
+  void LLVMC_FORCE_LINKAGE_FUN(PluginName) ()
+
+#define LLVMC_FORCE_LINKAGE_CALL(PluginName) \
+  LLVMC_FORCE_LINKAGE_FUN(PluginName) ()
+
+#endif // LLVM_INCLUDE_COMPILER_DRIVER_FORCE_LINKAGE_MACROS_H

include/llvm/CompilerDriver/Main.inc

@@ -19,6 +19,7 @@
 
 #include "llvm/CompilerDriver/CompilationGraph.h"
 #include "llvm/CompilerDriver/Error.h"
+#include "llvm/CompilerDriver/ForceLinkage.h"
 #include "llvm/CompilerDriver/Plugin.h"
 
 #include "llvm/System/Path.h"
@@ -85,6 +86,8 @@ namespace {
 
 int main(int argc, char** argv) {
   try {
+    ForceLinkage();
+
     LanguageMap langMap;
     CompilationGraph graph;