Front ends may wish to keep some state associated with various GENERIC
trees while parsing. To support this, trees provide a set of flags
that may be used by the front end. They are accessed using
TREE_LANG_FLAG_n where ‘n’ is currently 0 through 6.
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.