RFR: 8282045: When loop strip mining fails, safepoints are removed from loop anyway

[Tobias Hartmann]

On Wed, 23 Feb 2022 09:32:58 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> I noticed that if loop strip mining fails because a safepoint is not
>> found right above the exit test (following partial peel for instance),
>> all safepoints are removed from the loop anyway. That's fixed by the
>> change in IdealLoopTree::counted_loop() where rather than test if loop
>> strip mining is enabled, the check now verifies that loop strip mining
>> was successful.
>> 
>> With that change,
>> compiler/c2/irTests/TestAutoVectorization2DArray.java fails. The loop
>> is not converted into a strip mined loop because there's no safepoint
>> above the exit test after partial peeling. The loop strip mining logic
>> is too strict when it comes to the safepoint location. Any safepoint
>> that dominates the exit and is in the loop as long as there's no side
>> effect between the safepoint and the exit can be used. The patch
>> implements that change as well. TestAutoVectorization2DArray.java
>> passes as a result.
>> 
>> The existing requirement to have no safepoint on the backedge is too
>> strict as well. If the loop has another safepoint that can be used for
>> strip mining, then the safepoint on the backedge can safely be
>> dropped. That's also implemented by the patch.
>
> src/hotspot/share/opto/loopnode.cpp line 1852:
> 
>> 1850:   }
>> 1851: 
>> 1852:   Node *sfpt2 = NULL;
> 
> Suggestion:
> 
>   Node* sfpt2 = NULL;

I would suggest to rename `sfpt2` to `sfpt` and `sfpt` below to `backedge_sfpt`.

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PR: https://git.openjdk.java.net/jdk/pull/7513