RFR: 8278296: Generalize long range check transformation [v3]

[John R Rose]

On Mon, 24 Jan 2022 15:01:24 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> 8259609 (C2: optimize long range checks in long counted loops) only
>> covered the case of a counted loop with a positive stride and a range
>> check with a positive scale. This change generalizes the long range
>> check transformation to all 4 combinations of stride and scale signs.
>> 
>> The stride > 0, scale > 0 case (covered 8259609) was tweaked so it now
>> uses Qmax computed as the inclusive limit of j*K+Q. That helps in
>> generalizing the formulas to other cases.
>> 
>> The addition of PhaseIdealLoop::is_scaled_iv_plus_extra_offset() was
>> required for the case of negative scale in an int loop. The range
>> check then has the shape:
>> 
>> (CmpUL (AddL (ConvI2L (SubI ConI (LshiftI (Phi
>> 
>> with ConI, the zero constant.
>> 
>> This change also addresses this comment from John:
>> 
>> https://github.com/openjdk/jdk/pull/6576#discussion_r765343664
>> 
>> as part of 8276116 (C2: optimize long range checks in int counted loops)
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
> 
>  - Merge branch 'master' into JDK-8278296
>  - more review
>  - review
>  - whitespaces
>  - test & fix

I went through loopTransform thoroughly and came up with a bunch of comments and suggested changes.  Here is the diff.

[loopTransform-review-1.diff.txt](https://github.com/openjdk/jdk/files/7930019/loopTransform-review-1.diff.txt)
.

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Changes requested by jrose (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6989