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Re: [PATCH 1/3] Introduce gdb::function_view
On 2017-02-22 09:50, Pedro Alves wrote:
This commit adds new function_view type. This type holds a a
non-owning reference to a callable. It is meant to be used as
callback type of functions, instead of using C-style pair of function
pointer and 'void *data' arguments. function_view allows passing
references to stateful function objects / lambdas w/ 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. I added a new gdb/unittests/ subdir this time,
instead of putting the tests under gdb/. If this is agreed to be a
good idea, some of the current selftests that exercise gdb/common/
things but live in gdb/ could move here (e.g., gdb/utils-selftests.c).
gdb/ChangeLog:
yyyy-mm-dd Pedro Alves <palves@redhat.com>
* Makefile.in (SUBDIR_UNITTESTS_SRCS, SUBDIR_UNITTESTS_OBS): New.
(SFILES): Add $(SUBDIR_UNITTEST_SRCS).
(COMMON_OBS): Add $(SUBDIR_UNITTEST_OBS).
(%.o) <unittests/%.c>: New pattern.
(INIT_FILES): Add $(SUBDIR_UNITTESTS_SRCS).
* common/function-view.h: New file.
* unittests/function-view-selftests.c: New file.
---
gdb/Makefile.in | 24 ++-
gdb/common/function-view.h | 320
++++++++++++++++++++++++++++++++
gdb/unittests/function-view-selftests.c | 183 ++++++++++++++++++
3 files changed, 524 insertions(+), 3 deletions(-)
create mode 100644 gdb/common/function-view.h
create mode 100644 gdb/unittests/function-view-selftests.c
diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 43253d3..a4cac36 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -523,6 +523,12 @@ SUBDIR_PYTHON_DEPS =
SUBDIR_PYTHON_LDFLAGS =
SUBDIR_PYTHON_CFLAGS =
+SUBDIR_UNITTESTS_SRCS = \
+ unittests/function-view-selftests.c
+
+SUBDIR_UNITTESTS_OBS = \
+ function-view-selftests.o
+
# Opcodes currently live in one of two places. Either they are in the
# opcode library, typically ../opcodes, or they are in a header file
# in INCLUDE_DIR.
@@ -1216,7 +1222,8 @@ SFILES = \
common/xml-utils.c \
mi/mi-common.c \
target/waitstatus.c \
- $(SUBDIR_GCC_COMPILE_SRCS)
+ $(SUBDIR_GCC_COMPILE_SRCS) \
+ $(SUBDIR_UNITTEST_SRCS)
LINTFILES = $(SFILES) $(YYFILES) $(CONFIG_SRCS) init.c
@@ -1800,7 +1807,8 @@ COMMON_OBS = $(DEPFILES) $(CONFIG_OBS) $(YYOBJ) \
xml-syscall.o \
xml-tdesc.o \
xml-utils.o \
- $(SUBDIR_GCC_COMPILE_OBS)
+ $(SUBDIR_GCC_COMPILE_OBS) \
+ $(SUBDIR_UNITTESTS_OBS)
TSOBS = inflow.o
@@ -1909,6 +1917,10 @@ all: gdb$(EXEEXT) $(CONFIG_ALL)
$(COMPILE) $<
$(POSTCOMPILE)
+%.o: ${srcdir}/unittests/%.c
+ $(COMPILE) $<
+ $(POSTCOMPILE)
+
# Specify an explicit rule for gdb/common/agent.c, to avoid a clash
with the
# object file generate by gdb/agent.c.
common-agent.o: $(srcdir)/common/agent.c
@@ -2124,7 +2136,13 @@ test-cp-name-parser$(EXEEXT):
test-cp-name-parser.o $(LIBIBERTY)
# duplicates. Files in the gdb/ directory can end up appearing in
# COMMON_OBS (as a .o file) and CONFIG_SRCS (as a .c file).
-INIT_FILES = $(COMMON_OBS) $(TSOBS) $(CONFIG_SRCS)
$(SUBDIR_GCC_COMPILE_SRCS)
+INIT_FILES = \
+ $(COMMON_OBS) \
+ $(TSOBS) \
+ $(CONFIG_SRCS) \
+ $(SUBDIR_GCC_COMPILE_SRCS) \
+ $(SUBDIR_UNITTESTS_SRCS)
+
init.c: $(INIT_FILES)
@echo Making init.c
@rm -f init.c-tmp init.l-tmp
diff --git a/gdb/common/function-view.h b/gdb/common/function-view.h
new file mode 100644
index 0000000..cd455f8
--- /dev/null
+++ b/gdb/common/function-view.h
@@ -0,0 +1,320 @@
+/* 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 - Does not store the callback anywhere; instead if just calls
if -> it
+ it or forwards it to some other function that calls it.
+
+ #3 - When we don't want, or can't make said function 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.
+
+ For this use case, which is quite pervasive, a function_view is a
+ better choice for callback type than std::function. It is a better
+ choice because std::function is a heavy-weight object with value
+ semantics that generally requires a heap allocation on
+ construction/assignment of the target callable, while function_view
+ is light and does not require any heap allocation. 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, however,
+ that is quite inconvenient in practice, because restricting to
+ free-function callables would imply no state/capture (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 those function 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 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:
It's not necessary, but it would be nice to have the equivalent example
of how it would've been done before (with a function pointer) so that
people can relate the following example to something they already know.
+ void
+ iterate_over_foos (gdb::function_view<void (foo *)> callback)
+ {
+ for (auto & : foos)
I think you're missing a "foo" here.
+ 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;
+ };
+
+ 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
+{
Is it intended to have this { on a separate line, unlike the other
namespace declarations?
+ /* 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 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 the result, and the
+ result is convertible to Res, unless 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
I am not going to try to understand any of this... but as long as it
works I'm happy.
Simon