tailc: Deal with trivially useless EH cleanups [PR119491]

tailc: Deal with trivially useless EH cleanups [PR119491]

The following testcases FAIL, because EH cleanup is performed only before
IPA and then right before musttail pass.
At -O2 etc. (except for -O0/-Og) we handle musttail calls in the tailc
pass though, and we can fail at that point because the calls might appear
to throw internal exceptions which just don't do anything interesting
(perhaps have debug statements or clobber statements in them) before they
continue with resume of the exception (i.e. throw it externally).

As Richi said in the PR (and I agree) that moving passes is risky at this
point, the following patch instead teaches the tail{r,c} and musttail
passes to deal with such extra EDGE_EH edges.

It is fairly simple thing, if we see an EDGE_EH edge from the call we
just look up where it lands and if there are no
non-debug/non-clobber/non-label statements before resx which throws
externally, such edge can be ignored for tail call optimization or
tail recursion.  At other spots I just need to avoid using
single_succ/single_succ_edge because the bb might have another edge -
EDGE_EH.

To make this less risky, this is done solely for the musttail calls for now.

2025-04-02  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/119491
	* tree-tailcall.cc (single_non_eh_succ_edge): New function.
	(independent_of_stmt_p): Use single_non_eh_succ_edge (bb)->dest
	instead of single_succ (bb).
	(empty_eh_cleanup): New function.
	(find_tail_calls): Diagnose throwing of exceptions which do not
	propagate only if there are no EDGE_EH successor edges.  If there are
	and the call is musttail, use empty_eh_cleanup to find if the cleanup
	is not empty.  If not or the call is not musttail, use different
	diagnostics.  Set is_noreturn even if there are successor edges.  Use
	single_non_eh_succ_edge (abb) instead of single_succ_edge (abb).  Punt
	on internal noreturn calls.
	(decrease_profile): Don't assert 0 or 1 successor edges.
	(eliminate_tail_call): Use
	single_non_eh_succ_edge (gsi_bb (t->call_gsi)) instead of
	single_succ_edge (gsi_bb (t->call_gsi)).
	(tree_optimize_tail_calls_1): Also look into basic blocks with
	single succ edge which is EDGE_EH for noreturn musttail calls.

	* g++.dg/opt/musttail3.C: New test.
	* g++.dg/opt/musttail4.C: New test.
	* g++.dg/opt/musttail5.C: New test.