The purpose of GENERIC is simply to provide a
language-independent way of representing an entire function in
trees. To this end, it was necessary to add a few new tree codes
to the back end, but most everything was already there. If you
can express it with the codes in gcc/tree.def, it's
GENERIC.
Early on, there was a great deal of debate about how to think
about statements in a tree IL. In GENERIC, a statement is
defined as any expression whose value, if any, is ignored. A
statement will always have TREE_SIDE_EFFECTS set (or it
will be discarded), but a non-statement expression may also have
side effects. A CALL_EXPR, for instance.
It would be possible for some local optimizations to work on the
GENERIC form of a function; indeed, the adapted tree inliner
works fine on GENERIC, but the current compiler performs inlining
after lowering to GIMPLE (a restricted form described in the next
section). Indeed, currently the frontends perform this lowering
before handing off to tree_rest_of_compilation, but this
seems inelegant.
If necessary, a front end can use some language-dependent tree codes in its GENERIC representation, so long as it provides a hook for converting them to GIMPLE and doesn't expect them to work with any (hypothetical) optimizers that run before the conversion to GIMPLE. The intermediate representation used while parsing C and C++ looks very little like GENERIC, but the C and C++ gimplifier hooks are perfectly happy to take it as input and spit out GIMPLE.