c++: Implement C++ DR 2406 - [[fallthrough]] attribute and iteration statements

c++: Implement C++ DR 2406 - [[fallthrough]] attribute and iteration statements

The following patch implements
CWG 2406 - [[fallthrough]] attribute and iteration statements
The genericization of some loops leaves nothing at all or just a label
after a body of a loop, so if the loop is later followed by
case or default label in a switch, the fallthrough statement isn't
diagnosed.

The following patch implements it by marking the IFN_FALLTHROUGH call
in such a case, such that during gimplification it can be pedantically
diagnosed even if it is followed by case or default label or some normal
labels followed by case/default labels.

While looking into this, I've discovered other problems.
expand_FALLTHROUGH_r is removing the IFN_FALLTHROUGH calls from the IL,
but wasn't telling that to walk_gimple_stmt/walk_gimple_seq_mod, so
the callers would then skip the next statement after it, and it would
return non-NULL if the removed stmt was last in the sequence.  This could
lead to wi->callback_result being set even if it didn't appear at the very
end of switch sequence.
The patch makes use of wi->removed_stmt such that the callers properly
know what happened, and use different way to handle the end of switch
sequence case.

That change discovered a bug in the gimple-walk handling of
wi->removed_stmt.  If that flag is set, the callback is telling the callers
that the current statement has been removed and so the innermost
walk_gimple_seq_mod shouldn't gsi_next.  The problem is that
wi->removed_stmt is only reset at the start of a walk_gimple_stmt, but that
can be too late for some cases.  If we have two nested gimple sequences,
say GIMPLE_BIND as the last stmt of some gimple seq, we remove the last
statement inside of that GIMPLE_BIND, set wi->removed_stmt there, don't
do gsi_next correctly because already gsi_remove moved us to the next stmt,
there is no next stmt, so we return back to the caller, but wi->removed_stmt
is still set and so we don't do gsi_next even in the outer sequence, despite
the GIMPLE_BIND (etc.) not being removed.  That means we walk the
GIMPLE_BIND with its whole sequence again.
The patch fixes that by resetting wi->removed_stmt after we've used that
flag in walk_gimple_seq_mod.  Nothing really uses that flag after the
outermost walk_gimple_seq_mod, it is just a private notification that
the stmt callback has removed a stmt.

2023-11-17  Jakub Jelinek  <jakub@redhat.com>

	PR c++/107571
gcc/
	* gimplify.cc (expand_FALLTHROUGH_r): Use wi->removed_stmt after
	gsi_remove, change the way of passing fallthrough stmt at the end
	of sequence to expand_FALLTHROUGH.  Diagnose IFN_FALLTHROUGH
	with GF_CALL_NOTHROW flag.
	(expand_FALLTHROUGH): Change loc into array of 2 location_t elts,
	don't test wi.callback_result, instead check whether first
	elt is not UNKNOWN_LOCATION and in that case pedwarn with the
	second location.
	* gimple-walk.cc (walk_gimple_seq_mod): Clear wi->removed_stmt
	after the flag has been used.
	* internal-fn.def (FALLTHROUGH): Mention in comment the special
	meaning of the TREE_NOTHROW/GF_CALL_NOTHROW flag on the calls.
gcc/c-family/
	* c-gimplify.cc (genericize_c_loop): For C++ mark IFN_FALLTHROUGH
	call at the end of loop body as TREE_NOTHROW.
gcc/testsuite/
	* g++.dg/DRs/dr2406.C: New test.