RFR: 8281322: C2: always construct strip mined loop initially (even if strip mining is disabled)

[Roland Westrelin]

On Fri, 11 Feb 2022 10:05:44 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

>> Some of the long range check transformations take advantage of the
>> safepoint captured by loop strip mining to extract jvm state (in order
>> to add back empty predicates to the inner loop of a loop nest). As a
>> consequence, irTests/TestLongRangeChecks.java fails with strip mining
>> off and users might experience performance anomalies where changing
>> GCs affect purely computational code.
>> 
>> The strip mined loop nest creation is a 2 step process:
>> 
>> 1- when a CountedLoop is created, an OuterStripMinedLoop is also added
>>   but it's not fully constructed
>> 
>> 2- at macro expansion time, the OuterStripMinedLoop is turned into an
>>   actual loop by adding Phis and a proper exit condition
>> 
>> I propose always doing 1- whether loop strip mining is enabled or
>> not. This causes the safepoint to always be captured. Loop strip ming
>> is not expected to get in the way of loop transformations so this
>> change in itself should be performance neutral. Then at 2-, if loop
>> strip mining is not enabled, the OuterStripMinedLoop can be removed
>> and the safepoint moved back into the loop in case
>> LoopStripMiningIter=1 or simply removed too.
>
> For the record: Christian's testing found issues (he reported them to Roland already).
> 
> I think it would be good to change the bug title to something more descriptive.

@TobiHartmann @chhagedorn thanks for the reviews

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PR: https://git.openjdk.java.net/jdk/pull/7364