1=========================
  2GCC plugin infrastructure
  3=========================
  4
  5
  6Introduction
  7============
  8
  9GCC plugins are loadable modules that provide extra features to the
 10compiler [1]_. They are useful for runtime instrumentation and static analysis.
 11We can analyse, change and add further code during compilation via
 12callbacks [2]_, GIMPLE [3]_, IPA [4]_ and RTL passes [5]_.
 13
 14The GCC plugin infrastructure of the kernel supports building out-of-tree
 15modules, cross-compilation and building in a separate directory.
 16Plugin source files have to be compilable by a C++ compiler.
 17
 18Currently the GCC plugin infrastructure supports only some architectures.
 19Grep "select HAVE_GCC_PLUGINS" to find out which architectures support
 20GCC plugins.
 21
 22This infrastructure was ported from grsecurity [6]_ and PaX [7]_.
 23
 24--
 25
 26.. [1] https://gcc.gnu.org/onlinedocs/gccint/Plugins.html
 27.. [2] https://gcc.gnu.org/onlinedocs/gccint/Plugin-API.html#Plugin-API
 28.. [3] https://gcc.gnu.org/onlinedocs/gccint/GIMPLE.html
 29.. [4] https://gcc.gnu.org/onlinedocs/gccint/IPA.html
 30.. [5] https://gcc.gnu.org/onlinedocs/gccint/RTL.html
 31.. [6] https://grsecurity.net/
 32.. [7] https://pax.grsecurity.net/
 33
 34
 35Purpose
 36=======
 37
 38GCC plugins are designed to provide a place to experiment with potential
 39compiler features that are neither in GCC nor Clang upstream. Once
 40their utility is proven, the goal is to upstream the feature into GCC
 41(and Clang), and then to finally remove them from the kernel once the
 42feature is available in all supported versions of GCC.
 43
 44Specifically, new plugins should implement only features that have no
 45upstream compiler support (in either GCC or Clang).
 46
 47When a feature exists in Clang but not GCC, effort should be made to
 48bring the feature to upstream GCC (rather than just as a kernel-specific
 49GCC plugin), so the entire ecosystem can benefit from it.
 50
 51Similarly, even if a feature provided by a GCC plugin does *not* exist
 52in Clang, but the feature is proven to be useful, effort should be spent
 53to upstream the feature to GCC (and Clang).
 54
 55After a feature is available in upstream GCC, the plugin will be made
 56unbuildable for the corresponding GCC version (and later). Once all
 57kernel-supported versions of GCC provide the feature, the plugin will
 58be removed from the kernel.
 59
 60
 61Files
 62=====
 63
 64**$(src)/scripts/gcc-plugins**
 65
 66	This is the directory of the GCC plugins.
 67
 68**$(src)/scripts/gcc-plugins/gcc-common.h**
 69
 70	This is a compatibility header for GCC plugins.
 71	It should be always included instead of individual gcc headers.
 72
 73**$(src)/scripts/gcc-plugins/gcc-generate-gimple-pass.h,
 74$(src)/scripts/gcc-plugins/gcc-generate-ipa-pass.h,
 75$(src)/scripts/gcc-plugins/gcc-generate-simple_ipa-pass.h,
 76$(src)/scripts/gcc-plugins/gcc-generate-rtl-pass.h**
 77
 78	These headers automatically generate the registration structures for
 79	GIMPLE, SIMPLE_IPA, IPA and RTL passes.
 80	They should be preferred to creating the structures by hand.
 81
 82
 83Usage
 84=====
 85
 86You must install the gcc plugin headers for your gcc version,
 87e.g., on Ubuntu for gcc-10::
 88
 89	apt-get install gcc-10-plugin-dev
 90
 91Or on Fedora::
 92
 93	dnf install gcc-plugin-devel libmpc-devel
 94
 95Or on Fedora when using cross-compilers that include plugins::
 96
 97	dnf install libmpc-devel
 98
 99Enable the GCC plugin infrastructure and some plugin(s) you want to use
100in the kernel config::
101
102	CONFIG_GCC_PLUGINS=y
103	CONFIG_GCC_PLUGIN_LATENT_ENTROPY=y
104	...
105
106Run gcc (native or cross-compiler) to ensure plugin headers are detected::
107
108	gcc -print-file-name=plugin
109	CROSS_COMPILE=arm-linux-gnu- ${CROSS_COMPILE}gcc -print-file-name=plugin
110
111The word "plugin" means they are not detected::
112
113	plugin
114
115A full path means they are detected::
116
117       /usr/lib/gcc/x86_64-redhat-linux/12/plugin
118
119To compile the minimum tool set including the plugin(s)::
120
121	make scripts
122
123or just run the kernel make and compile the whole kernel with
124the cyclomatic complexity GCC plugin.
125
126
1274. How to add a new GCC plugin
128==============================
129
130The GCC plugins are in scripts/gcc-plugins/. You need to put plugin source files
131right under scripts/gcc-plugins/. Creating subdirectories is not supported.
132It must be added to scripts/gcc-plugins/Makefile, scripts/Makefile.gcc-plugins
133and a relevant Kconfig file.