RFR: 8300257: C2: vectorization fails on some simple Memory Segment loops [v2]

[Tobias Hartmann]

On Tue, 14 Mar 2023 10:58:11 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> In the test case `testByteLong1` (that's extracted from a memory
>> segment micro benchmark), the address of the store is initially:
>> 
>> 
>> (AddP#204 base#195 base#195 (AddL#164 (ConvI2L#158 (CastII#157 (LshiftI#107 iv#101))) invar#163))
>> 
>> 
>> (#numbers are node numbers to help the discussion).
>> 
>> `iv#101` is the `Phi` of a counted loop. `invar#163` is the
>> `baseOffset` load.
>> 
>> To eliminate the range check, the loop is transformed into a loop nest
>> and as a consequence the address above becomes:
>> 
>> 
>> (AddP#204 base#195 base#195 (AddL#164 (ConvI2L#158 (CastII#157 (LShiftI#107 (AddI#326 invar#308 iv#321)))) invar#163))
>> 
>> 
>> `invar#308` is some expression from a `Phi` of the outer loop.
>> 
>> That `AddP` is transformed multiple times to push the invariants out of loop:
>> 
>> 
>> (AddP#568 base#195 (AddP#556 base#195 base#195 invar#163) (ConvI2L#158 (CastII#157 (AddI#566 (LShiftI#565 iv#321) invar#577))))
>> 
>> 
>> then:
>> 
>> 
>> (AddP#568 base#195 (AddP#847 (AddP#556 base#195 base#195 invar#163) (AddL#838 (ConvI2L#793 (LShiftL#760 iv#767)) (ConvI2L#818 (CastII#779 invar#577)))))
>> 
>> 
>> and finally:
>> 
>> 
>> (AddP#568 base#195 (AddP#949 base#195 (AddP#855 base#195 (AddP#556 base#195 base#195 invar#163) (ConvI2L#818 (CastII#809 invar#577))) (ConvI2L#938 (LShiftI#896 iv#908))))
>> 
>> 
>> `AddP#855` is out of the inner loop. 
>> 
>> This doesn't vectorize because:
>> 
>> - there are 2 invariants in the address expression but superword only
>>   support one (tracked by `_invar` in `SWPointer`)
>> 
>> - there are more levels of `AddP` (4) than superword supports (3)
>> 
>> To fix that, I propose to no longer track the address elements in
>> `_invar`, `_negate_invar` and `_invar_scale` but instead to have a
>> single `_invar` which is an expression built by superword as it
>> follows chains of `addP` nodes. I kept the previous `_invar`,
>> `_negate_invar` and `_invar_scale` as debugging and use them to check
>> that what vectorized with the previous scheme still does.
>> 
>> I also propose lifting the restriction on 3 levels of `AddP` entirely.
>
> Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - NULL -> nullptr
>  - Merge branch 'master' into JDK-8300257
>  - fix & test

Performance and correctness testing looks good.

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PR: https://git.openjdk.org/jdk/pull/12942