On 10/21/2021 4:15 AM, Aldy Hernandez wrote:
On Wed, Oct 20, 2021 at 10:19 PM Jeff Law <jeffreya...@gmail.com> wrote:
So we want to keep some form of ssa-dom-thread-7. That' s the canonical
testcase for the case for the FSM optimization.
What we need to verify is that we thread jumps across the backedge of
the loop through the switch statement to a particular case (thus
bypassing the indirect jump for the switch statement). How to do that
in a way that's easier to manage? I have no clue. I guess a gimple-fe
based test might help.
Ah, I see.
After lots of pain, I was able to distill a tiny testcase that does
just that (tree-ssa/ssa-thread-backedge.c), and is easy to maintain.
I've added a "backedge" marker to the path dumping code for easy
matching in the test. An alternative would be to convert it to a
gimple FE test examining the exact thread sequence through the
backedge, but I think that's overkill since the test is so small.
Well, and the worry with a smaller testcase is reducing too far with the
result not really being representative of the issue. This actually
happened during the development of the FSM bits. I got a test from the
ARM guys, evaluated it and concluded it could be addressed with the
forward threader.... Then I did the implementation work. Once done
they said it didn't work and gave me a better testcase which had more
"join" blocks we would have had to copy to realize the important jump
threads. At which point Steve E's FSM threader was the only viable choice.
Phew, I think we're finally converging on a useful set of threading tests :).
OK for trunk?
Mostly, I just worry about losing the key test for the FSM optimization.
Jeff
