RFR: 8345485: C2 MergeLoads: merge adjacent array/native memory loads into larger load [v6]

[kuaiwei]

> In this patch, I extent the merge stores optimization to merge adjacents loads. Tier1 tests are passed in my machine.
> 
> The benchmark result of MergeLoadBench.java
> AMD EPYC 9T24 96-Core Processor:
> 
> |name   | -MergeLoads   | +MergeLoads   |delta|
> |---|---|---|---|
> |MergeLoadBench.getCharB        |4352.150       |4407.435       | 55.29 |
> |MergeLoadBench.getCharBU       |4075.320       |4084.663       | 9.34 |
> |MergeLoadBench.getCharBV       |3221.302       |3221.528       | 0.23 |
> |MergeLoadBench.getCharC        |2235.433       |2238.796       | 3.36 |
> |MergeLoadBench.getCharL        |4363.244       |4372.281       | 9.04 |
> |MergeLoadBench.getCharLU       |4072.550       |4075.744       | 3.19 |
> |MergeLoadBench.getCharLV       |2227.825       |2231.612       | 3.79 |
> |MergeLoadBench.getIntB |11199.935      |6869.030       | -4330.90 |
> |MergeLoadBench.getIntBU        |6853.862       |2763.923       | -4089.94 |
> |MergeLoadBench.getIntBV        |306.953        |309.911        | 2.96 |
> |MergeLoadBench.getIntL |10426.843      |6523.716       | -3903.13 |
> |MergeLoadBench.getIntLU        |6740.847       |2602.701       | -4138.15 |
> |MergeLoadBench.getIntLV        |2233.151       |2231.745       | -1.41 |
> |MergeLoadBench.getIntRB        |11335.756      |8980.619       | -2355.14 |
> |MergeLoadBench.getIntRBU       |7439.873       |3190.208       | -4249.66 |
> |MergeLoadBench.getIntRL        |16323.040      |7786.842       | -8536.20 |
> |MergeLoadBench.getIntRLU       |7457.745       |3364.140       | -4093.61 |
> |MergeLoadBench.getIntRU        |2512.621       |2511.668       | -0.95 |
> |MergeLoadBench.getIntU |2501.064       |2500.629       | -0.43 |
> |MergeLoadBench.getLongB        |21175.442      |21103.660      | -71.78 |
> |MergeLoadBench.getLongBU       |14042.046      |2512.784       | -11529.26 |
> |MergeLoadBench.getLongBV       |606.448        |606.171        | -0.28 |
> |MergeLoadBench.getLongL        |23142.178      |23217.785      | 75.61 |
> |MergeLoadBench.getLongLU       |14112.972      |2237.659       | -11875.31 |
> |MergeLoadBench.getLongLV       |2230.416       |2231.224       | 0.81 |
> |MergeLoadBench.getLongRB       |21152.558      |21140.583      | -11.98 |
> |MergeLoadBench.getLongRBU      |14031.178      |2520.317       | -11510.86 |
> |MergeLoadBench.getLongRL       |23248.506      |23136.410      | -112.10 |
> |MergeLoadBench.getLongRLU      |14125.032      |2240.481       | -11884.55 |
> |MergeLoadBench.getLongRU       |3071.881       |3066.606       | -5.27 |
> |Merg...

kuaiwei has updated the pull request incrementally with two additional commits since the last revision:

 - Enable StressIGVN and riscv platform
 - Change tests as review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24023/files
  - new: https://git.openjdk.org/jdk/pull/24023/files/1eba9308..ed5590a9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24023&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24023&range=04-05

  Stats: 728 lines in 3 files changed: 434 ins; 118 del; 176 mod
  Patch: https://git.openjdk.org/jdk/pull/24023.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24023/head:pull/24023

PR: https://git.openjdk.org/jdk/pull/24023