Add "extern template" declarations now that we use explicit instantiation.

This is supported by gcc and clang, but guarded by a macro for MSVC 2008.

The extern template declaration is not necessary but generally good
form. It can avoid extra instantiations of the template methods
defined inline.

The EXTERN_TEMPLATE_INSTANTIATION macro could probably be generalized to
handle multiple template parameters if someone thinks it's worthwhile.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158840 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineLoopInfo.h

@@ -7,7 +7,7 @@
 //
 //===----------------------------------------------------------------------===//
 //
-// This file defines the MachineLoopInfo class that is used to identify natural 
+// This file defines the MachineLoopInfo class that is used to identify natural
 // loops and determine the loop depth of various nodes of the CFG.  Note that
 // natural loops may actually be several loops that share the same header node.
 //
@@ -35,6 +35,12 @@
 
 namespace llvm {
 
+// Implementation in LoopInfoImpl.h
+#ifdef __GNUC__
+class MachineLoop;
+__extension__ extern template class LoopBase<MachineBasicBlock, MachineLoop>;
+#endif
+
 class MachineLoop : public LoopBase<MachineBasicBlock, MachineLoop> {
 public:
   MachineLoop();
@@ -57,6 +63,12 @@ private:
     : LoopBase<MachineBasicBlock, MachineLoop>(MBB) {}
 };
 
+// Implementation in LoopInfoImpl.h
+#ifdef __GNUC__
+__extension__ extern template
+class LoopInfoBase<MachineBasicBlock, MachineLoop>;
+#endif
+
 class MachineLoopInfo : public MachineFunctionPass {
   LoopInfoBase<MachineBasicBlock, MachineLoop> LI;
   friend class LoopBase<MachineBasicBlock, MachineLoop>;