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+# Compiler plugins
+
+## Overview
+
+This directory contains code for compiler plugins. These are used to perform
+additional actions during compilation (such as additional warnings) and
+also to perform mass code refactoring.
+
+Currently only the Clang compiler is supported <http://wiki.documentfoundation.org/Development/Clang>.
+
+## Usage
+
+Compiler plugins are enabled automatically by `--enable-dbgutil` if Clang headers
+are found or explicitly using `--enable-compiler-plugins`.
+
+## Functionality
+
+There are two kinds of plugin actions:
+
+- compile checks - these are run during normal compilation
+- rewriters - these must be run manually and modify source files
+
+Each source has a comment saying whether it's compile check or a rewriter
+and description of functionality.
+
+### Compile Checks
+
+Used during normal compilation to perform additional checks.
+All warnings and errors are marked '[loplugin]' in the message.
+
+### Rewriters
+
+Rewriters analyse and possibly modify given source files.
+Usage: `make COMPILER_PLUGIN_TOOL=<rewriter_name>`
+Additional optional make arguments:
+
+- it is possible to also pass `FORCE_COMPILE=all` to make to trigger rebuild of all source files,
+    even those that are up to date. FORCE_COMPILE takes a list of gbuild targets specifying
+    where to run the rewriter ('all' means everything, '-' prepended means to not enable, '/' appended means
+    everything in the directory; there is no ordering, more specific overrides
+    more general, and disabling takes precedence).
+    Example: FORCE_COMPILE="all -sw/ -Library_sc"
+
+- `UPDATE_FILES=<scope>` - limits which modified files will be actually written back with the changes
+    - `mainfile` - only the main `.cxx` file will be modified (default)
+    - `all` - all source files involved will be modified (possibly even header files from other LO modules),
+        3rd party header files are however never modified
+    - `<module>` - only files in the given LO module (toplevel directory) will be modified (including headers)
+
+Modifications will be written directly to the source files.
+
+Some rewriter plugins are dual-mode and can also be used in a non-rewriting mode
+in which they emit warnings for problematic code that they would otherwise
+automatically rewrite.  When any rewriter is enabled explicitly via `make
+COMPILER_PLUGIN_TOOL=<rewriter_name>` it works in rewriting mode (and all other
+plugins are disabled), but when no rewriter is explicitly enabled (i.e., just
+`make`), all dual-mode rewriters are enabled in non-rewriting mode (along with
+all non-rewriter plugins; and all non--dual-mode plugins are disabled).  The
+typical process to use such a dual-mode rewriter X in rewriting mode is
+
+    make COMPILER_PLUGIN_WARNINGS_ONLY=X \
+    && make COMPILER_PLUGIN_TOOL=X FORCE_COMPILE=all UPDATE_FILES=all
+
+which first generates a full build without failing due to warnings from plugin
+X in non-rewriting mode (in case of `--enable-werror`) and then repeats the build
+in rewriting mode (during which no object files are generate).
+
+
+## Code Documentation / HowTos
+
+<https://wiki.documentfoundation.org/Clang_plugins>