Add a very simple plugin to demonstrate the GCC plugin infrastructure. This GCC
plugin computes the cyclomatic complexity of each function.

The complexity M of a function's control flow graph is defined as:
 M = E - N + 2P
where
 E = the number of edges
 N = the number of nodes
 P = the number of connected components (exit nodes).

---
 Makefile                          |   6 +-
 arch/Kconfig                      |  16 +++++
 tools/gcc/Makefile                |   4 ++
 tools/gcc/cyc_complexity_plugin.c | 120 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 145 insertions(+), 1 deletion(-)
 create mode 100644 tools/gcc/cyc_complexity_plugin.c

diff --git a/Makefile b/Makefile
index 96ce015..9f76c26 100644
--- a/Makefile
+++ b/Makefile

@@ -628,7 +628,11 @@ else
 PLUGINCC := $(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-plugin.sh "$(HOSTCC)" "$(HOSTCXX)" "$(CC)")
 endif
 ifneq ($(PLUGINCC),)
-export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGINS_AFLAGS
+ifdef CONFIG_GCC_PLUGIN_CYC_COMPLEXITY
+GCC_PLUGIN_CYC_COMPLEXITY_CFLAGS := -fplugin=$(objtree)/tools/gcc/cyc_complexity_plugin.so -DGCC_PLUGIN_CYC_COMPLEXITY
+endif
+GCC_PLUGINS_CFLAGS := $(GCC_PLUGIN_CYC_COMPLEXITY_CFLAGS)
+export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGINS_AFLAGS GCC_PLUGIN_CYC_COMPLEXITY
 ifeq ($(KBUILD_EXTMOD),)
 gcc-plugins:
        $(Q)$(MAKE) $(build)=tools/gcc

diff --git a/arch/Kconfig b/arch/Kconfig
index a95e5b1..6b7d4de 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -365,6 +365,22 @@ config HAVE_GCC_PLUGINS
          An arch should select this symbol if it supports building with
          gcc plugins.

+if HAVE_GCC_PLUGINS
+
+config GCC_PLUGIN_CYC_COMPLEXITY
+       bool "Compute the cyclomatic complexity of a function"
+       help

+
+endif # HAVE_GCC_PLUGINS
 endmenu # "GCC plugins"

 config HAVE_CC_STACKPROTECTOR

diff --git a/tools/gcc/Makefile b/tools/gcc/Makefile
index b2d64af..31c72bf 100644
--- a/tools/gcc/Makefile
+++ b/tools/gcc/Makefile

@@ -12,4 +12,8 @@ endif

 export GCCPLUGINS_DIR HOSTLIBS

+$(HOSTLIBS)-$(CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) := cyc_complexity_plugin.so
+
 always := $($(HOSTLIBS)-y)
+
+cyc_complexity_plugin-objs := cyc_complexity_plugin.o

diff --git a/tools/gcc/cyc_complexity_plugin.c b/tools/gcc/cyc_complexity_plugin.c
new file mode 100644
index 0000000..c6f0d58
--- /dev/null
+++ b/tools/gcc/cyc_complexity_plugin.c

@@ -0,0 +1,120 @@
+/*
+ * Copyright 2011-2016 by Emese Revfy <re.emese@gmail.com>
+ * Licensed under the GPL v2, or (at your option) v3
+ *
+ * Homepage:
+ * https://github.com/ephox-gcc-plugins/cyclomatic_complexity
+ *
+ * http://en.wikipedia.org/wiki/Cyclomatic_complexity
+ * The complexity M is then defined as:
+ * M = E - N + 2P
+ * where
+ *
+ *  E = the number of edges of the graph
+ *  N = the number of nodes of the graph
+ *  P = the number of connected components (exit nodes).
+ *
+ * Usage (4.5 - 5):
+ * $ make clean; make run
+ */
+
+#include "gcc-common.h"
+
+int plugin_is_GPL_compatible;
+
+static struct plugin_info cyc_complexity_plugin_info = {
+       .version        = "20150523",
+       .help           = "Cyclomatic Complexity\n",
+};
+
+static unsigned int handle_function(void)
+{
+       int complexity;
+       expanded_location xloc;
+
+       // M = E - N + 2P
+       complexity = n_edges_for_fn(cfun) - n_basic_blocks_for_fn(cfun) + 2;
+
+       xloc = expand_location(DECL_SOURCE_LOCATION(current_function_decl));
+       fprintf(stderr, "Cyclomatic Complexity %d %s:%s\n", complexity, xloc.file, DECL_NAME_POINTER(current_function_decl));
+
+       return 0;
+}
+
+#if BUILDING_GCC_VERSION >= 4009
+namespace {
+static const struct pass_data cyc_complexity_pass_data = {
+#else
+static struct gimple_opt_pass cyc_complexity_pass = {
+       .pass = {
+#endif
+               .type                   = GIMPLE_PASS,
+               .name                   = "cyc_complexity",
+#if BUILDING_GCC_VERSION >= 4008
+               .optinfo_flags          = OPTGROUP_NONE,
+#endif
+#if BUILDING_GCC_VERSION >= 5000
+#elif BUILDING_GCC_VERSION >= 4009
+               .has_gate               = false,
+               .has_execute            = true,
+#else
+               .gate                   = NULL,
+               .execute                = handle_function,
+               .sub                    = NULL,
+               .next                   = NULL,
+               .static_pass_number     = 0,
+#endif
+               .tv_id                  = TV_NONE,
+               .properties_required    = 0,
+               .properties_provided    = 0,
+               .properties_destroyed   = 0,
+               .todo_flags_start       = 0,
+               .todo_flags_finish      = TODO_dump_func
+#if BUILDING_GCC_VERSION < 4009
+       }
+#endif
+};
+
+#if BUILDING_GCC_VERSION >= 4009
+class cyc_complexity_pass : public gimple_opt_pass {
+public:
+       cyc_complexity_pass() : gimple_opt_pass(cyc_complexity_pass_data, g) {}
+#if BUILDING_GCC_VERSION >= 5000
+       virtual unsigned int execute(function *) { return handle_function(); }
+#else
+       unsigned int execute() { return handle_function(); }
+#endif
+};
+}
+
+static struct opt_pass *make_cyc_complexity_pass(void)
+{
+       return new cyc_complexity_pass();
+}
+#else
+static struct opt_pass *make_cyc_complexity_pass(void)
+{
+       return &cyc_complexity_pass.pass;
+}
+#endif
+
+int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+       const char * const plugin_name = plugin_info->base_name;
+       struct register_pass_info cyc_complexity_pass_info;
+
+       cyc_complexity_pass_info.pass                           = make_cyc_complexity_pass();
+       cyc_complexity_pass_info.reference_pass_name            = "ssa";
+       cyc_complexity_pass_info.ref_pass_instance_number       = 1;
+       cyc_complexity_pass_info.pos_op                         = PASS_POS_INSERT_AFTER;
+
+       if (!plugin_default_version_check(version, &gcc_version)) {
+               error(G_("incompatible gcc/plugin versions"));
+               return 1;
+       }
+
+       register_callback(plugin_name, PLUGIN_INFO, NULL, &cyc_complexity_plugin_info);
+       register_callback(plugin_name, PLUGIN_PASS_MANAGER_SETUP, NULL, &cyc_complexity_pass_info);
+
+       return 0;
+}