BUILT_IN usage in tree-nested.c

[Richard Kenner]

    But Java does not need that one so it does not initializes it.  That is 
    all. Since Java, I think, un-nests its functions/classes itself before 
    getting to
    gimple does not need it at all.

Well, it's a confusing situation, at least.

gimplify.c references BUILT_IN_STACK_{SAVE,RESTORE},
BUILT_IN_MEMCPY, and BUILT_IN_PROFILE_{ENTER,EXIT}}; tree-mudflap.c
references BUILT_IN_STACK_ALLOC; tree-nested.c references
BUILT_IN_NONLOCAL_GOTO and BUILT_IN_{ADJUST,INIT}_TRAMPOLINE; and
tree-ssa-ccp.c references BUILT_IN_TRAP.

java/builtins.c defines *none* of those.

Where is it documented under what conditions front ends have to
initialize which builtins and how to do it?