[vector] RFR (L): Vector box elimination improvements

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/panama/vector.box_elimination/webrev.03/

Different improvements in vector box elimination to make it more robust:

   * Moved VectorBox/VectorUnbox to compile.cpp from macro;

   * Lazy vector box materialization (before EA, but after VB/VU 
elimination);

   * Introduced VectorBoxAllocate to capture JVM state for allocation 
materialization purposes;

   * Vector box rematerialization at safepoints (when used from debug info);

   * Aggressive vector reboxing;

   * Introduced VectorIntrinsics.maybeRebox() to help aggressive 
reboxing by capturing JVM state and keeping it during optimizations on 
VB/VU nodes;

Optimization phase for vector operations looks as follows:
  (1) Expand VectorUnboxNodes
  (2) Scalarize VectorBoxNodes
  (3) Inline VI.maybeRebox() calls

  (4) RemoveUseless + IGVN

  (5) Expand VectorBoxNodes/VectorBoxAllocateNodes
  (6) Eliminate unused VectorBoxAllocateNodes

  (7) RemoveUseless + IGVN


TRANSFORMATIONS

The following transformations (some details [1]) are the core of the 
optimization:

  (A) VB/VU elimination
        VectorUnbox (VectorBox box value)  =>  value
        LoadVector  (VectorBox box value)  =>  value

  (B) Merge-through-phi
        Phi (VectorBox o1 v1) (VectorBox o2 v2)
        =>
        VectorBox (Phi o1 o2) (Phi v1 v2)

  (C) Reboxing
        VectorBox (Phi o1 o2) (Phi v1 v2)
        =>
        VectorBox new_box (Phi v1 v2)

  (D) Expansion
        VectorBox   box value  => box
        VectorUnbox mem box    => LoadVector box

  (E) Rematerialization
        SafePoint … (VectorBox box value) … 

        =>
        
SafePoint … (SafePointScalarObject box_klass #value) … value


KNOWN ISSUES (will be fixed later):

   (1) Elimination of unused vector boxes can produce loops w/o safepoints.

   Can be fixed by replacing VBA nodes with a safepoint.

   (2) No distinction between indentity-preserving and identity-changing 
transformations: both aggressive reboxing and rematerialization can lead 
to observable changes in identity of vector boxes at runtime.

   The plan is to add a flag and explicitly enable identity-changing 
transformations. It's easy to detect when box identity is observable and 
avoid reboxing & rematerialization if the flag is turned off.


FUTURE IMPROVEMENTS / CLEANUPS

   - No need for VB/VU/VBA to stay macro nodes.

   - Convert all stores during box allocation (see 
Compile::expand_vbox_alloc_node) into initialization stores.

   - Aggressive unboxing (using VectorUnbox nodes) to help 
merge-through-phi transformation.

Thanks!

Best regards,
Vladimir Ivanov

[1] 
http://mail.openjdk.java.net/pipermail/panama-dev/2017-December/000840.html