RFR: 8361702: C2: assert(is_dominator(compute_early_ctrl(limit, limit_ctrl), pre_end)) failed: node pinned on loop exit test? [v4]

[Emanuel Peter]

On Mon, 25 Aug 2025 14:20:03 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> A node in a pre loop only has uses out of the loop dominated by the
>> loop exit. `PhaseIdealLoop::try_sink_out_of_loop()` sets its control
>> to the loop exit projection. A range check in the main loop has this
>> node as input (through a chain of some other nodes). Range check
>> elimination needs to update the exit condition of the pre loop with an
>> expression that depends on the node pinned on its exit: that's
>> impossible and the assert fires. This is a variant of 8314024 (this
>> one was for a node with uses out of the pre loop on multiple paths). I
>> propose the same fix: leave the node with control in the pre loop in
>> this case.
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8361702
>  - Update src/hotspot/share/opto/loopopts.cpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/rangechecks/TestSunkRangeFromPreLoopRCE3.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/rangechecks/TestSunkRangeFromPreLoopRCE2.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update src/hotspot/share/opto/loopopts.cpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - Update test/hotspot/jtreg/compiler/rangechecks/TestSunkRangeFromPreLoopRCE2.java
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - tests
>  - fix

test/hotspot/jtreg/compiler/rangechecks/TestSunkRangeFromPreLoopRCE3.java line 28:

> 26:  * @bug 8361702
> 27:  * @summary C2: assert(is_dominator(compute_early_ctrl(limit, limit_ctrl), pre_end)) failed: node pinned on loop exit test?
> 28:  * @requires vm.flavor == "server"

Would this test fail without this requires? Or could we remove it, in the hopes of catching something else somewhere else?

test/hotspot/jtreg/compiler/rangechecks/TestSunkRangeFromPreLoopRCE3.java line 30:

> 28:  * @requires vm.flavor == "server"
> 29:  *
> 30:  * @run main/othervm -XX:-BackgroundCompilation -XX:LoopUnrollLimit=100 -XX:-UseLoopPredicate -XX:-UseProfiledLoopPredicate TestSunkRangeFromPreLoopRCE3

Could we have a run without any flags / fewer flags? Just in case it catches something else / related.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/26424#discussion_r2298393419
PR Review Comment: https://git.openjdk.org/jdk/pull/26424#discussion_r2298394619