RFR: 8300865: C2: product reduction in ProdRed_Double is not vectorized [v4]

[Sandhya Viswanathan]

> This PR fixes the problem with double reduction on x86_64. 
> 
> In the test compiler.loopopts.superword.ProdRed_Double, the product reduction loop in prodReductionImplement() was not getting vectorized when run as follows:
> jtreg -XX:CompileCommand=PrintAssembly,compiler.loopopts.superword.ProdRed_Double::prodReductionImplement compiler/loopopts/superword/ProdRed_Double.java
> The print assembly generated in the pid-xxx.log output in JTwork/scratch directory was not showing any vector_reduction_double node.
> 
> This was happening as the ReductionNode::implemented was passed a vector size of one element. For the vector reduction implemented we need to check with at least vector size of two elements.
> 
> With this PR the vector_reduction_double node is generated.
> 
> Please review.
> 
> Best Regards,
> Sandhya

Sandhya Viswanathan has updated the pull request incrementally with one additional commit since the last revision:

  Add jmh test case

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14065/files
  - new: https://git.openjdk.org/jdk/pull/14065/files/ba3b5dfa..1c29051c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14065&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14065&range=02-03

  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/14065.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14065/head:pull/14065

PR: https://git.openjdk.org/jdk/pull/14065