RFR: 8263058: Optimize vector shift with zero shift count [v2]

[Eric Liu]

> Like scalar shift, vector shift could do nothing when shift count is
> zero.
> 
> This patch implements the 'Identity' method for all kinds of vector
> shift nodes to optimize out 'ShiftVCntNode 0', which is typically a
> redundant 'mov' in final generated code like below:
> 
>         add     x17, x12, x14
>         ldr     q16, [x17, #16]
>         mov     v16.16b, v16.16b
>         add     x14, x13, x14
>         str     q16, [x14, #16]
> 
> With this patch, the code above could be optimized as below:
> 
>         add     x17, x12, x14
>         ldr     q16, [x17, #16]
>         add     x14, x13, x14
>         str     q16, [x14, #16]
> 
> [TESTS]
> compiler/vectorapi/TestVectorShiftImm.java, jdk/incubator/vector,
> hotspot::tier1 passed without new failure.

Eric Liu has updated the pull request incrementally with one additional commit since the last revision:

  fix build failure
  
  Change-Id: Id7d6f4bbb5d8dde8d1a6c5c9cd5052d64fd978a9

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2911/files
  - new: https://git.openjdk.java.net/jdk/pull/2911/files/2a978dac..2bb8a513

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2911&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2911&range=00-01

  Stats: 11 lines in 1 file changed: 6 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2911.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2911/head:pull/2911

PR: https://git.openjdk.java.net/jdk/pull/2911