RFR: 8300257: C2: vectorization fails on some simple Memory Segment loops [v4]

[Roland Westrelin]

> 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 eight commits:

 - review
 -  Merge branch 'master' into JDK-8300257
 - Update test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMultiInvar.java
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
 - Update test/hotspot/jtreg/compiler/c2/irTests/TestVectorizationMultiInvar.java
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
 - Update src/hotspot/share/opto/superword.hpp
   
   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>
 - NULL -> nullptr
 - Merge branch 'master' into JDK-8300257
 - fix & test

-------------

Changes: https://git.openjdk.org/jdk/pull/12942/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12942&range=03
  Stats: 273 lines in 3 files changed: 211 ins; 23 del; 39 mod
  Patch: https://git.openjdk.org/jdk/pull/12942.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12942/head:pull/12942

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