RFR: 8336830: C2: assert(get_loop(lca)->_nest < n_loop->_nest || lca->in(0)->is_NeverBranch()) failed: must not be moved into inner loop

[Emanuel Peter]

On Thu, 25 Jul 2024 15:16:39 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> A store is sunk from a counted loop into an enclosing infinite
> loop. The assert fires because:
> 
> 
> get_loop(lca)->_nest < n_loop->_nest
> 
> 
> is false. That happens because the outer loop was found to be infinite
> in the current loop opts pass. When that happens, it's not properly
> attached to the loop tree. The second part of the assert was added to
> cover a similar case:
> 
> 
> lca->in(0)->is_NeverBranch()
> 
> 
> but it doesn't work in this case bcause lca is not a projection of the
> `NeverBranch`. It's the exit projection of the counted loop. The fix I
> propose changes that part of the assert to test that lca is, indeed,
> in an infinite loop in a way that's robust.
> 
> I also removed some code that I believe to be useless following
> 8335709.

LGTM, thanks for fixing this!

src/hotspot/share/opto/loopopts.cpp line 1275:

> 1273:   for (;;) {
> 1274:     Node* dom = idom(useblock);
> 1275:     if (loop->is_member(get_loop(dom))) {

This would not be backported, right? Do you think we should do it in a separate RFE? Or is it necessary for the fix?

-------------

Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20334#pullrequestreview-2199675821
PR Review Comment: https://git.openjdk.org/jdk/pull/20334#discussion_r1691680407