This is a super minimalistic GCC plugin that makes avr-g++ put tables of
vitual functions into the program memory saving RAM. It is meant to be
built as a plugin for the existing avr-g++ and modify its default behavior
when loaded. It is loaded into the compiler during compilation step. It
has no effect at linking step. Libraries that define classes which vtables
are intended to be placed into flash must be compiled with this plugin.

Building.

To build this plugin you need avr-g++ with plugins directory installed
and the host gcc. You may also need to install additional headers not
provided by the gcc, like gmp.h

When building for linux run

  make

in the current directory.

When building for windows-hosted compiler using mingw toolchain run

  make HOST_GCC=x86_64-w64-mingw32 GCCPLUGINS_DIR=<full-path-to-win-avr-gcc-plugin-dir>

in the current directory. Make sure the HOST_GCC matches gcc that was
used to build the target compiler, otherwise plugin build will fail at
linking step b/o missing symbols.

Using.

Invoke avr-g++ for compilation with additional parameter

  -fplugin=<full-path-to>/avr-flash-vtbl.so

More info.

https://habrahabr.ru/company/amperka/blog/264041

More technical info.

This plugin merely adds __flash attribute to the C++ vtable and vtable
pointer types. The rest is taken care of by the existing address space
machinery of the gcc and flash address space support of the AVR backend.