Introduce gdb::function_view

This commit adds a new function_view type.  This type holds a
non-owning reference to a callable.  It is meant to be used as
callback type of functions, instead of using the C-style pair of
function pointer and 'void *data' arguments.  function_view allows
passing references to stateful function objects / lambdas with
captures as callbacks efficiently, while function pointer + 'void *'
does not.

See the intro in the new function-view.h header for more.

Unit tests included, put into a new gdb/unittests/ subdir.

gdb/ChangeLog:
2017-02-23  Pedro Alves  <palves@redhat.com>

	* Makefile.in (SUBDIR_UNITTESTS_SRCS, SUBDIR_UNITTESTS_OBS): New.
	(%.o) <unittests/%.c>: New pattern.
	* configure.ac ($development): Add $(SUBDIR_UNITTESTS_OBS) to
	CONFIG_OBS, and $(SUBDIR_UNITTESTS_SRCS) to CONFIG_SRCS.
	* common/function-view.h: New file.
	* unittests/function-view-selftests.c: New file.
	* configure: Regenerate.

1 files changed, 355 insertions, 0 deletions

diff --git a/gdb/common/function-view.h b/gdb/common/function-view.h
new file mode 100644
index 0000000..66a691b
--- /dev/null
+++ b/gdb/common/function-view.h
@@ -0,0 +1,355 @@
+/* Copyright (C) 2017 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#ifndef COMMON_FUNCTION_VIEW_H
+#define COMMON_FUNCTION_VIEW_H
+
+/* function_view is a polymorphic type-erasing wrapper class that
+   encapsulates a non-owning reference to arbitrary callable objects.
+
+   A way to put it is that function_view is to std::function like
+   std::string_view is to std::string.  While std::function stores a
+   type-erased callable object internally, function_view holds a
+   type-erased reference to an external callable object.
+
+   This is meant to be used as callback type of a function that:
+
+     #1 - Takes a callback as parameter.
+
+     #2 - Wants to support arbitrary callable objects as callback type
+	  (e.g., stateful function objects, lambda closures, free
+	  functions).
+
+     #3 - Does not store the callback anywhere; instead the function
+	  just calls the callback directly or forwards it to some
+	  other function that calls it.
+
+     #4 - Can't be, or we don't want it to be, a template function
+	  with the callable type as template parameter.  For example,
+	  when the callback is a parameter of a virtual member
+	  function, or when putting the function template in a header
+	  would expose too much implementation detail.
+
+   Note that the C-style "function pointer" + "void *data" callback
+   parameter idiom fails requirement #2 above.  Please don't add new
+   uses of that idiom.  I.e., something like this wouldn't work;
+
+    typedef bool (iterate_over_foos_cb) (foo *f, void *user_data),
+    void iterate_over_foos (iterate_over_foos_cb *callback, void *user_data);
+
+    foo *find_foo_by_type (int type)
+    {
+      foo *found = nullptr;
+
+      iterate_over_foos ([&] (foo *f, void *data)
+	{
+	  if (foo->type == type)
+	    {
+	      found = foo;
+	      return true; // stop iterating
+	    }
+	  return false; // continue iterating
+	}, NULL);
+
+      return found;
+    }
+
+   The above wouldn't compile, because lambdas with captures can't be
+   implicitly converted to a function pointer (because a capture means
+   some context data must be passed to the lambda somehow).
+
+   C++11 gave us std::function as type-erased wrapper around arbitrary
+   callables, however, std::function is not an ideal fit for transient
+   callbacks such as the use case above.  For this use case, which is
+   quite pervasive, a function_view is a better choice, because while
+   function_view is light and does not require any heap allocation,
+   std::function is a heavy-weight object with value semantics that
+   generally requires a heap allocation on construction/assignment of
+   the target callable.  In addition, while it is possible to use
+   std::function in such a way that avoids most of the overhead by
+   making sure to only construct it with callables of types that fit
+   std::function's small object optimization, such as function
+   pointers and std::reference_wrapper callables, that is quite
+   inconvenient in practice, because restricting to free-function
+   callables would imply no state/capture/closure, which we need in
+   most cases, and std::reference_wrapper implies remembering to use
+   std::ref/std::cref where the callable is constructed, with the
+   added inconvenience that std::ref/std::cref have deleted rvalue-ref
+   overloads, meaning you can't use unnamed/temporary lambdas with
+   them.
+
+   Note that because function_view is a non-owning view of a callable,
+   care must be taken to ensure that the callable outlives the
+   function_view that calls it.  This is not really a problem for the
+   use case function_view is intended for, such as passing a temporary
+   function object / lambda to a function that accepts a callback,
+   because in those cases, the temporary is guaranteed to be live
+   until the called function returns.
+
+   Calling a function_view with no associated target is undefined,
+   unlike with std::function, which throws std::bad_function_call.
+   This is by design, to avoid the otherwise necessary NULL check in
+   function_view::operator().
+
+   Since function_view objects are small (a pair of pointers), they
+   should generally be passed around by value.
+
+   Usage:
+
+   Given this function that accepts a callback:
+
+    void
+    iterate_over_foos (gdb::function_view<void (foo *)> callback)
+    {
+       for (auto &foo : foos)
+	 callback (&foo);
+    }
+
+   you can call it like this, passing a lambda as callback:
+
+    iterate_over_foos ([&] (foo *f)
+      {
+	process_one_foo (f);
+      });
+
+   or like this, passing a function object as callback:
+
+    struct function_object
+    {
+      void operator() (foo *f)
+      {
+	if (s->check ())
+	  process_one_foo (f);
+      }
+
+      // some state
+      state *s;
+    };
+
+    state mystate;
+    function_object matcher {&mystate};
+    iterate_over_foos (matcher);
+
+  or like this, passing a function pointer as callback:
+
+    iterate_over_foos (process_one_foo);
+
+  You can find unit tests covering the whole API in
+  unittests/function-view-selftests.c.  */
+
+namespace gdb {
+
+namespace traits {
+  /* A few trait helpers.  */
+  template<typename Predicate>
+  struct Not : public std::integral_constant<bool, !Predicate::value>
+  {};
+
+  template<typename...>
+  struct Or;
+
+  template<>
+  struct Or<> : public std::false_type
+  {};
+
+  template<typename B1>
+  struct Or<B1> : public B1
+  {};
+
+  template<typename B1, typename B2>
+  struct Or<B1, B2>
+    : public std::conditional<B1::value, B1, B2>::type
+  {};
+
+  template<typename B1,typename B2,typename B3, typename... Bn>
+  struct Or<B1, B2, B3, Bn...>
+    : public std::conditional<B1::value, B1, Or<B2, B3, Bn...>>::type
+  {};
+} /* namespace traits */
+
+namespace fv_detail {
+/* Bits shared by all function_view instantiations that do not depend
+   on the template parameters.  */
+
+/* Storage for the erased callable.  This is a union in order to be
+   able to save both a function object (data) pointer or a function
+   pointer without triggering undefined behavior.  */
+union erased_callable
+{
+  /* For function objects.  */
+  void *data;
+
+    /* For function pointers.  */
+  void (*fn) ();
+};
+
+} /* namespace fv_detail */
+
+/* Use partial specialization to get access to the callable's
+   signature. */
+template<class Signature>
+struct function_view;
+
+template<typename Res, typename... Args>
+class function_view<Res (Args...)>
+{
+  template<typename From, typename To>
+  using CompatibleReturnType
+    = traits::Or<std::is_void<To>,
+		 std::is_same<From, To>,
+		 std::is_convertible<From, To>>;
+
+  /* True if Func can be called with Args, and either the result is
+     Res, convertible to Res or Res is void.  */
+  template<typename Callable,
+	   typename Res2 = typename std::result_of<Callable &(Args...)>::type>
+  struct IsCompatibleCallable : CompatibleReturnType<Res2, Res>
+  {};
+
+  /* True if Callable is a function_view.  Used to avoid hijacking the
+     copy ctor.  */
+  template <typename Callable>
+  struct IsFunctionView
+    : std::is_same<function_view, typename std::decay<Callable>::type>
+  {};
+
+  /* Helper to make SFINAE logic easier to read.  */
+  template<typename Condition>
+  using Requires = typename std::enable_if<Condition::value, void>::type;
+
+ public:
+
+  /* NULL by default.  */
+  constexpr function_view () noexcept
+    : m_erased_callable {},
+      m_invoker {}
+  {}
+
+  /* Default copy/assignment is fine.  */
+  function_view (const function_view &) = default;
+  function_view &operator= (const function_view &) = default;
+
+  /* This is the main entry point.  Use SFINAE to avoid hijacking the
+     copy constructor and to ensure that the target type is
+     compatible.  */
+  template
+    <typename Callable,
+     typename = Requires<traits::Not<IsFunctionView<Callable>>>,
+     typename = Requires<IsCompatibleCallable<Callable>>>
+  function_view (Callable &&callable) noexcept
+  {
+    bind (callable);
+  }
+
+  /* Construct a NULL function_view.  */
+  constexpr function_view (std::nullptr_t) noexcept
+    : m_erased_callable {},
+      m_invoker {}
+  {}
+
+  /* Clear a function_view.  */
+  function_view &operator= (std::nullptr_t) noexcept
+  {
+    m_invoker = nullptr;
+    return *this;
+  }
+
+  /* Return true if the wrapper has a target, false otherwise.  Note
+     we check M_INVOKER instead of M_ERASED_CALLABLE because we don't
+     know which member of the union is active right now.  */
+  constexpr explicit operator bool () const noexcept
+  { return m_invoker != nullptr; }
+
+  /* Call the callable.  */
+  Res operator () (Args... args) const
+  { return m_invoker (m_erased_callable, std::forward<Args> (args)...); }
+
+ private:
+
+  /* Bind this function_view to a compatible function object
+     reference.  */
+  template <typename Callable>
+  void bind (Callable &callable) noexcept
+  {
+    m_erased_callable.data = (void *) std::addressof (callable);
+    m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
+      noexcept (noexcept (callable (std::forward<Args> (args)...))) -> Res
+      {
+	auto &restored_callable = *static_cast<Callable *> (ecall.data);
+	/* The explicit cast to Res avoids a compile error when Res is
+	   void and the callable returns non-void.  */
+	return (Res) restored_callable (std::forward<Args> (args)...);
+      };
+  }
+
+  /* Bind this function_view to a compatible function pointer.
+
+     Making this a separate function allows avoiding one indirection,
+     by storing the function pointer directly in the storage, instead
+     of a pointer to pointer.  erased_callable is then a union in
+     order to avoid storing a function pointer as a data pointer here,
+     which would be undefined.  */
+  template<class Res2, typename... Args2>
+  void bind (Res2 (*fn) (Args2...)) noexcept
+  {
+    m_erased_callable.fn = reinterpret_cast<void (*) ()> (fn);
+    m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
+      noexcept (noexcept (fn (std::forward<Args> (args)...))) -> Res
+      {
+	auto restored_fn = reinterpret_cast<Res2 (*) (Args2...)> (ecall.fn);
+	/* The explicit cast to Res avoids a compile error when Res is
+	   void and the callable returns non-void.  */
+	return (Res) restored_fn (std::forward<Args> (args)...);
+      };
+  }
+
+  /* Storage for the erased callable.  */
+  fv_detail::erased_callable m_erased_callable;
+
+  /* The invoker.  This is set to a capture-less lambda by one of the
+     'bind' overloads.  The lambda restores the right type of the
+     callable (which is passed as first argument), and forwards the
+     args.  */
+  Res (*m_invoker) (fv_detail::erased_callable, Args...);
+};
+
+/* Allow comparison with NULL.  Defer the work to the in-class
+   operator bool implementation.  */
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator== (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
+{ return !static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator== (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
+{ return !static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator!= (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
+{ return static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator!= (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
+{ return static_cast<bool> (f); }
+
+} /* namespace gdb */
+
+#endif