RFR: 8336726: C2: assert(!do_asserts || projs->fallthrough_ioproj != nullptr) failed: must be found

[Christian Hagedorn]

On Thu, 10 Oct 2024 13:29:11 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> Post-parse call devirtualization asserts when calling `CallNode::extract_projections` on a virtual call that does not have the `fallthrough_ioproj` anymore. The projection was removed because the call is followed by an endless loop that does not have any IO uses.
> 
> Similar to incremental inlining, we should not assert that all call projections are still there for post-parse call devirtualization because parts of the graph might have been removed already:
> https://github.com/openjdk/jdk/blob/580eb62dc097efeb51c76b095c1404106859b673/src/hotspot/share/opto/callnode.cpp#L963-L965
> 
> Thanks,
> Tobias

That looks reasonable to me. As we have discussed offline, it's probably not worth/too complex to verify that we always end in an infinite loop afterwards.

test/hotspot/jtreg/compiler/c2/TestCallDevirtualizationWithInfiniteLoop.java line 53:

> 51:     public static void test(boolean flag) {
> 52:         // Avoid executing endless loop
> 53:         if (flag) return;

You should add braces here:
Suggestion:

        if (flag) {
            return;
        }

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21450#pullrequestreview-2362827732
PR Review Comment: https://git.openjdk.org/jdk/pull/21450#discussion_r1796968384