RFR: 8281322: irTests/TestLongRangeChecks.java fails with strip mining off

[Tobias Hartmann]

On Mon, 7 Feb 2022 08:53:41 GMT, Roland Westrelin <roland 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.

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