[PATCH v2 19/21] libcc1: use variadic templates for callbacks

[Tom Tromey <tom@tromey.com>]

This patch completes the transition of libcc1 from the use of separate
template functions for different arities to the use of variadic
functions.  This is how I had wanted it to work from the very
beginning, and is possible now with C++11.

I had thought that variadic callbacks required C++17, but it turns out
that the approach taken here is basically equivalent to std::apply --
just a bit wordier.

libcc1/ChangeLog
2021-04-27  Tom Tromey  <tom@tromey.com>

	* rpc.hh (argument_wrapper) <get>: Replace cast operator.
	(argument_wrapper<T *>) <get>: Likewise.
	(unmarshall): Add std::tuple overloads.
	(callback): Remove.
	(class invoker): New.
	* libcp1plugin.cc (plugin_init): Update.
	* libcp1.cc (libcp1::add_callbacks): Update.
	* libcc1plugin.cc (plugin_init): Update.
	* libcc1.cc (libcc1::add_callbacks): Update.
	* connection.cc (cc1_plugin::connection::do_wait): Update.
---
 libcc1/ChangeLog       |  13 +++
 libcc1/connection.cc   |   2 +-
 libcc1/libcc1.cc       |  12 +--
 libcc1/libcc1plugin.cc |  20 ++--
 libcc1/libcp1.cc       |  17 ++--
 libcc1/libcp1plugin.cc |  20 ++--
 libcc1/rpc.hh          | 219 +++++++++++------------------------------
 7 files changed, 101 insertions(+), 202 deletions(-)

diff --git a/libcc1/connection.cc b/libcc1/connection.cc
index 66d573911080..45560b9b790e 100644
--- a/libcc1/connection.cc
+++ b/libcc1/connection.cc
@@ -129,7 +129,7 @@ cc1_plugin::connection::do_wait (bool want_result)
 		  return FAIL;
 
 		callback_ftype *callback
-		  = m_callbacks.find_callback (method_name);
+		  = m_callbacks.find_callback (method_name.get ());
 		// The call to CALLBACK is where we may end up in a
 		// reentrant call.
 		if (callback == NULL || !callback (this))
diff --git a/libcc1/libcc1.cc b/libcc1/libcc1.cc
index cbc54ee0a044..febadc8420b0 100644
--- a/libcc1/libcc1.cc
+++ b/libcc1/libcc1.cc
@@ -143,15 +143,13 @@ void
 libcc1::add_callbacks ()
 {
   cc1_plugin::callback_ftype *fun
-    = cc1_plugin::callback<int,
-			   enum gcc_c_oracle_request,
-			   const char *,
-			   c_call_binding_oracle>;
+    = cc1_plugin::invoker<int,
+			  enum gcc_c_oracle_request,
+			  const char *>::invoke<c_call_binding_oracle>;
   connection->add_callback ("binding_oracle", fun);
 
-  fun = cc1_plugin::callback<gcc_address,
-			     const char *,
-			     c_call_symbol_address>;
+  fun = cc1_plugin::invoker<gcc_address,
+			    const char *>::invoke<c_call_symbol_address>;
   connection->add_callback ("address_oracle", fun);
 }
 
diff --git a/libcc1/libcc1plugin.cc b/libcc1/libcc1plugin.cc
index d6951ab1a4d2..4d6a3a11ee22 100644
--- a/libcc1/libcc1plugin.cc
+++ b/libcc1/libcc1plugin.cc
@@ -762,46 +762,46 @@ plugin_init (struct plugin_name_args *plugin_info,
 #define GCC_METHOD0(R, N)			\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD1(R, N, A)				\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD2(R, N, A, B)				\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD3(R, N, A, B, C)			\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, C, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD4(R, N, A, B, C, D)		\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, A, B, C, D,	\
-			     plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C,		\
+			    D>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD5(R, N, A, B, C, D, E)	\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, A, B, C, D, E,	\
-			     plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C, D,	\
+			    E>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD7(R, N, A, B, C, D, E, F, G)		\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, C, D, E, F, G,	\
-			     plugin_ ## N>;		\
+      = cc1_plugin::invoker<R, A, B, C, D,		\
+			    E, F, G>::invoke<plugin_ ## N>;		\
     current_context->add_callback (# N, fun);		\
   }
 
diff --git a/libcc1/libcp1.cc b/libcc1/libcp1.cc
index d22d9dc6af8c..a93349833901 100644
--- a/libcc1/libcp1.cc
+++ b/libcc1/libcp1.cc
@@ -166,23 +166,18 @@ void
 libcp1::add_callbacks ()
 {
   cc1_plugin::callback_ftype *fun
-    = cc1_plugin::callback<int,
-			   enum gcc_cp_oracle_request,
-			   const char *,
-			   cp_call_binding_oracle>;
+    = cc1_plugin::invoker<int, enum gcc_cp_oracle_request,
+			  const char *>::invoke<cp_call_binding_oracle>;
   connection->add_callback ("binding_oracle", fun);
 
-  fun = cc1_plugin::callback<gcc_address,
-			     const char *,
-			     cp_call_symbol_address>;
+  fun = cc1_plugin::invoker<gcc_address,
+			    const char *>::invoke<cp_call_symbol_address>;
   connection->add_callback ("address_oracle", fun);
 
-  fun = cc1_plugin::callback<int,
-			     cp_call_enter_scope>;
+  fun = cc1_plugin::invoker<int>::invoke<cp_call_enter_scope>;
   connection->add_callback ("enter_scope", fun);
 
-  fun = cc1_plugin::callback<int,
-			     cp_call_leave_scope>;
+  fun = cc1_plugin::invoker<int>::invoke<cp_call_leave_scope>;
   connection->add_callback ("leave_scope", fun);
 }
 
diff --git a/libcc1/libcp1plugin.cc b/libcc1/libcp1plugin.cc
index 64cde651139c..79694b919641 100644
--- a/libcc1/libcp1plugin.cc
+++ b/libcc1/libcp1plugin.cc
@@ -3509,46 +3509,46 @@ plugin_init (struct plugin_name_args *plugin_info,
 #define GCC_METHOD0(R, N)			\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD1(R, N, A)				\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD2(R, N, A, B)				\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD3(R, N, A, B, C)			\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, C, plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);		\
   }
 #define GCC_METHOD4(R, N, A, B, C, D)		\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, A, B, C, D,	\
-			     plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C,		\
+			    D>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD5(R, N, A, B, C, D, E)	\
   {						\
     cc1_plugin::callback_ftype *fun		\
-      = cc1_plugin::callback<R, A, B, C, D, E,	\
-			     plugin_ ## N>;	\
+      = cc1_plugin::invoker<R, A, B, C,		\
+			    D, E>::invoke<plugin_ ## N>;	\
     current_context->add_callback (# N, fun);	\
   }
 #define GCC_METHOD7(R, N, A, B, C, D, E, F, G)		\
   {							\
     cc1_plugin::callback_ftype *fun			\
-      = cc1_plugin::callback<R, A, B, C, D, E, F, G,	\
-			     plugin_ ## N>;		\
+      = cc1_plugin::invoker<R, A, B, C,			\
+			    D, E, F, G>::invoke<plugin_ ## N>;		\
     current_context->add_callback (# N, fun);		\
   }
 
diff --git a/libcc1/rpc.hh b/libcc1/rpc.hh
index 09cd7bdda616..8e43fa146dcc 100644
--- a/libcc1/rpc.hh
+++ b/libcc1/rpc.hh
@@ -43,7 +43,7 @@ namespace cc1_plugin
     argument_wrapper (const argument_wrapper &) = delete;
     argument_wrapper &operator= (const argument_wrapper &) = delete;
 
-    operator T () const { return m_object; }
+    T get () const { return m_object; }
 
     status unmarshall (connection *conn)
     {
@@ -68,7 +68,7 @@ namespace cc1_plugin
 
     typedef typename std::remove_const<T>::type type;
 
-    operator const type * () const
+    const type *get () const
     {
       return m_object.get ();
     }
@@ -88,17 +88,14 @@ namespace cc1_plugin
   };
 
   // There are two kinds of template functions here: "call" and
-  // "callback".  "call" is implemented with variadic templates, but
-  // "callback" is repeated multiple times to handle different numbers
-  // of arguments.  (This could be improved with C++17 and
-  // std::apply.)
+  // "invoker".
 
   // The "call" template is used for making a remote procedure call.
   // It starts a query ('Q') packet, marshalls its arguments, waits
   // for a result, and finally reads and returns the result via an
   // "out" parameter.
 
-  // The "callback" template is used when receiving a remote procedure
+  // The "invoker" template is used when receiving a remote procedure
   // call.  This template function is suitable for use with the
   // "callbacks" and "connection" classes.  It decodes incoming
   // arguments, passes them to the wrapped function, and finally
@@ -123,175 +120,71 @@ namespace cc1_plugin
     return OK;
   }
 
-  template<typename R, R (*func) (connection *)>
-  status
-  callback (connection *conn)
+  // The base case -- just return OK.
+  template<int I, typename... T>
+  typename std::enable_if<I == sizeof... (T), status>::type
+  unmarshall (connection *, std::tuple<T...> &)
   {
-    R result;
-
-    if (!unmarshall_check (conn, 0))
-      return FAIL;
-    result = func (conn);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
-
-  template<typename R, typename A, R (*func) (connection *, A)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A> arg;
-    R result;
-
-    if (!unmarshall_check (conn, 1))
-      return FAIL;
-    if (!arg.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
+    return OK;
   }
 
-  template<typename R, typename A1, typename A2, R (*func) (connection *,
-							    A1, A2)>
-  status
-  callback (connection *conn)
+  // Unmarshall this argument, then unmarshall all subsequent args.
+  template<int I, typename... T>
+  typename std::enable_if<I < sizeof... (T), status>::type
+  unmarshall (connection *conn, std::tuple<T...> &value)
   {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    R result;
-
-    if (!unmarshall_check (conn, 2))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
+    if (!std::get<I> (value).unmarshall (conn))
       return FAIL;
-    result = func (conn, arg1, arg2);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
+    return unmarshall<I + 1, T...> (conn, value);
   }
 
-  template<typename R, typename A1, typename A2, typename A3,
-	   R (*func) (connection *, A1, A2, A3)>
-  status
-  callback (connection *conn)
+  // Wrap a static function that is suitable for use as a callback.
+  // This is a template function inside a template class to work
+  // around limitations with multiple variadic packs.
+  template<typename R, typename... Arg>
+  class invoker
   {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    R result;
+    // Base case -- we can call the function.
+    template<int I, R func (connection *, Arg...), typename... T>
+    static typename std::enable_if<I == sizeof... (Arg), R>::type
+    call (connection *conn, const std::tuple<argument_wrapper<Arg>...> &,
+	  T... args)
+    {
+      return func (conn, args...);
+    }
 
-    if (!unmarshall_check (conn, 3))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+    // Unpack one argument and continue the recursion.
+    template<int I, R func (connection *, Arg...), typename... T>
+    static typename std::enable_if<I < sizeof... (Arg), R>::type
+    call (connection *conn, const std::tuple<argument_wrapper<Arg>...> &value,
+	  T... args)
+    {
+      return call<I + 1, func> (conn, value, args...,
+				std::get<I> (value).get ());
+    }
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   R (*func) (connection *, A1, A2, A3, A4)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    R result;
+  public:
 
-    if (!unmarshall_check (conn, 4))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+    // A callback function that reads arguments from the connection,
+    // calls the wrapped function, and then sends the result back on
+    // the connection.
+    template<R func (connection *, Arg...)>
+    static status
+    invoke (connection *conn)
+    {
+      if (!unmarshall_check (conn, sizeof... (Arg)))
+	return FAIL;
+      std::tuple<argument_wrapper<Arg>...> wrapped;
+      if (!unmarshall<0> (conn, wrapped))
+	return FAIL;
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   typename A5, R (*func) (connection *, A1, A2, A3, A4, A5)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    argument_wrapper<A5> arg5;
-    R result;
-
-    if (!unmarshall_check (conn, 5))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    if (!arg5.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4, arg5);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+      R result = call<0, func> (conn, wrapped);
 
-  template<typename R, typename A1, typename A2, typename A3, typename A4,
-	   typename A5, typename A6, typename A7,
-	   R (*func) (connection *, A1, A2, A3, A4, A5, A6, A7)>
-  status
-  callback (connection *conn)
-  {
-    argument_wrapper<A1> arg1;
-    argument_wrapper<A2> arg2;
-    argument_wrapper<A3> arg3;
-    argument_wrapper<A4> arg4;
-    argument_wrapper<A5> arg5;
-    argument_wrapper<A6> arg6;
-    argument_wrapper<A7> arg7;
-    R result;
-
-    if (!unmarshall_check (conn, 7))
-      return FAIL;
-    if (!arg1.unmarshall (conn))
-      return FAIL;
-    if (!arg2.unmarshall (conn))
-      return FAIL;
-    if (!arg3.unmarshall (conn))
-      return FAIL;
-    if (!arg4.unmarshall (conn))
-      return FAIL;
-    if (!arg5.unmarshall (conn))
-      return FAIL;
-    if (!arg6.unmarshall (conn))
-      return FAIL;
-    if (!arg7.unmarshall (conn))
-      return FAIL;
-    result = func (conn, arg1, arg2, arg3, arg4, arg5, arg6, arg7);
-    if (!conn->send ('R'))
-      return FAIL;
-    return marshall (conn, result);
-  }
+      if (!conn->send ('R'))
+	return FAIL;
+      return marshall (conn, result);
+    }
+  };
 };
 
 #endif // CC1_PLUGIN_RPC_HH
-- 
2.26.2