GCC plugin infrastructure
  =========================
  
  
  1. Introduction
  ===============
  
  GCC plugins are loadable modules that provide extra features to the
  compiler [1]. They are useful for runtime instrumentation and static analysis.
  We can analyse, change and add further code during compilation via
  callbacks [2], GIMPLE [3], IPA [4] and RTL passes [5].
  
  The GCC plugin infrastructure of the kernel supports all gcc versions from
  4.5 to 6.0, building out-of-tree modules, cross-compilation and building in a
  separate directory.
  Plugin source files have to be compilable by both a C and a C++ compiler as well
  because gcc versions 4.5 and 4.6 are compiled by a C compiler,
  gcc-4.7 can be compiled by a C or a C++ compiler,
  and versions 4.8+ can only be compiled by a C++ compiler.
  
  Currently the GCC plugin infrastructure supports only the x86, arm and arm64
  architectures.
  
  This infrastructure was ported from grsecurity [6] and PaX [7].
  
  --
  [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html
  [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API
  [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html
  [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html
  [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html
  [6] https://grsecurity.net/
  [7] https://pax.grsecurity.net/
  
  
  2. Files
  ========
  
  $(src)/scripts/gcc-plugins
  	This is the directory of the GCC plugins.
  
  $(src)/scripts/gcc-plugins/gcc-common.h
  	This is a compatibility header for GCC plugins.
  	It should be always included instead of individual gcc headers.
  
  $(src)/scripts/gcc-plugin.sh
  	This script checks the availability of the included headers in
  	gcc-common.h and chooses the proper host compiler to build the plugins
  	(gcc-4.7 can be built by either gcc or g++).
  
  $(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h
  $(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h
  $(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h
  $(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h
  	These headers automatically generate the registration structures for
  	GIMPLE, SIMPLE_IPA, IPA and RTL passes. They support all gcc versions
  	from 4.5 to 6.0.
  	They should be preferred to creating the structures by hand.
  
  
  3. Usage
  ========
  
  You must install the gcc plugin headers for your gcc version,
  e.g., on Ubuntu for gcc-4.9:
  
  	apt-get install gcc-4.9-plugin-dev
  
  Enable a GCC plugin based feature in the kernel config:
  
  	CONFIG_GCC_PLUGIN_CYC_COMPLEXITY = y
  
  To compile only the plugin(s):
  
  	make gcc-plugins
  
  or just run the kernel make and compile the whole kernel with
  the cyclomatic complexity GCC plugin.
  
  
  4. How to add a new GCC plugin
  ==============================
  
  The GCC plugins are in $(src)/scripts/gcc-plugins/. You can use a file or a directory
  here. It must be added to $(src)/scripts/gcc-plugins/Makefile,
  $(src)/scripts/Makefile.gcc-plugins and $(src)/arch/Kconfig.
  See the cyc_complexity_plugin.c (CONFIG_GCC_PLUGIN_CYC_COMPLEXITY) GCC plugin.