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, arm64 and
powerpc 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.