RFR: 8282722: Regard mapping array in enum switches as stable for constant folding

[Jatin Bhateja]

On Mon, 7 Mar 2022 07:13:20 GMT, Joshua Zhu <jzhu at openjdk.org> wrote:

> I came across a performance issue when using scatter store VectorAPI for Integer and Long simultaneously in the same application. The poor performance was caused by vector intrinsic inlining failure because of non-determined IntSpecies for a constant VectorShape of IndexMap in this scenario.
> 
> For ScatterStore operation of LongVector.SPECIES_512/IntVector.SPECIES_512, VectorShape.S_256_BIT/S_512_BIT is the actual length of indexMap vector respectively.
> 
> IntSpecies species(VectorShape s)
> 
> returns the corresponding IntSpecies by Switch on Enum type "VectorShape". [1] 
> 
> With this change introduced, elements in the SwitchMap array (initialized in clinit) can be constant-folded so that determined IntSpecies can be acquired for a constant VectorShape.
> 
> jtreg test passed without new failure.
> Please help review this change and let me know if any comments.
> 
> [1] https://github.com/openjdk/jdk/blob/894ffb098c80bfeb4209038c017d01dbf53fac0f/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/IntVector.java#L4043

Fix is checking an integer array field name starting with "$SwitchMap"  to be marked as stable if it is decorated with ACC_STATIC | ACC_FINAL attributes.  Wandering why is not being honored during constant folding currently.  

https://github.com/openjdk/jdk/blob/master/src/hotspot/share/ci/ciField.hpp#L129


  // Is this field a constant?
  //
  // Clarification: A field is considered constant if:
  //   1. The field is both static and final
  //   2. The field is not one of the special static/final
  //      non-constant fields.  These are java.lang.System.in
  //      and java.lang.System.out.  Abomination.
  //
  // A field is also considered constant if
  // - it is marked @Stable and is non-null (or non-zero, if a primitive) or
  // - it is trusted or
  // - it is the target field of a CallSite object.
  //
  // See ciField::initialize_from() for more details.
  //
  // A user should also check the field value (constant_value().is_valid()), since
  // constant fields of non-initialized classes don't have values yet.
  bool is_constant() const { return _is_constant; }

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PR: https://git.openjdk.java.net/jdk/pull/7721