builtin functions and how to handle them in the CFG

[Ben Elliston <bje at au1 dot ibm dot com>]

Target-specific builtins (that almost always map to a single
corresponding machine instruction) are presently registered with
add_builtin_function().  This function takes an `attrs' parameter that
can attached additional attributes to the builtin.

Some builtins can indeed be marked pure or const, depending on what the
corresponding machine instructions do.  However, some do modify global
memory and can be marked as neither.

When generating coverage information for calls, the flow graph code adds
a fake edge to the exit block of the enclosing function, as the compiler
is unaware that the builtin function cannot alter control flow by
throwing an exception or calling longjmp.

What is needed is an ECF_ flag that indicates that the function *will*
return and will not alter control flow in any way.  It looks like
ECF_NOTHROW is not quite right, nor ECF_RETURNS_TWICE.  What is really
needed is something like "ECF_RETURNS_ONCE", and for need_fake_edge_p to
handle this flag appropriately.

Does anyone have any advice?

Thanks,
Ben