[jdk16] RFR: 8260370: C2: LoopLimit node is not eliminated

[Vladimir Ivanov]

On Tue, 26 Jan 2021 17:26:30 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> As part of JDK-8255000 `C->major_progress` check was replaced with an explicit phase check (`post_loop_opts_phase()`).
>> But `LoopLimit` was not covered and still relied on `C->major_progress()`. It exposed a problem: sometimes IGVN sets `major_progress` (e.g., see `LoopNode::Ideal()`) and it blocks `LoopLimit` node elimination if the node is processed later (`LoopLimitNode::Ideal()` is misled that loop opts phase is not over).         
>> 
>> The fix migrates `LoopLimitNode::Ideal()` to  `post_loop_opts_phase()`. 
>> 
>> Also, post-loop opts IGVN pass conservatively clears `major_progress()` just in case.  
>>    
>> Testing:
>> - [x] failing test
>> - [x] hs-tier1 - hs-tier4
>
> Looks good.
> Do we have other places where we should replace major_progress()?

Thanks for the prompt review, Vladimir.

> Do we have other places where we should replace major_progress()?

Yes, there's at least one check left (in `SafePointNode::Identity`). I plan to address it separately.

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PR: https://git.openjdk.java.net/jdk16/pull/134