RFR: 8304042: C2 SuperWord: schedule must remove packs with cyclic dependencies [v3]

[Emanuel Peter]

> I discovered this bug during the bug fix of [JDK-8298935](https://bugs.openjdk.org/browse/JDK-8298935) [PR](https://git.openjdk.org/jdk/pull/12350).
> 
> Currently, the SuperWord algorithm only ensures that all `packs` are `isomorphic` and `independent` (additionally memops are `adjacent`).
> 
> This is **not sufficient**. We need to ensure that the `packs` do not introduce `cycles` into the graph. Example:
> 
> https://github.com/openjdk/jdk/blob/ad580d18dbbf074c8a3692e2836839505b574326/test/hotspot/jtreg/compiler/loopopts/superword/TestIndependentPacksWithCyclicDependency.java#L217-L231
> 
> This is also mentioned in the [SuperWord Paper](https://groups.csail.mit.edu/cag/slp/SLP-PLDI-2000.pdf) (2000, Samuel Larsen and Saman Amarasinghe, Exploiting Superword Level Parallelism with Multimedia Instruction Sets):
> 
> 
> 3.7 Scheduling
> Dependence analysis before packing ensures that statements within a group can be executed
> safely in parallel. However, it may be the case that executing two groups produces a dependence
> violation. An example of this is shown in Figure 6. Here, dependence edges are drawn between
> groups if a statement in one group is dependent on a statement in the other. As long as there
> are no cycles in this dependence graph, all groups can be scheduled such that no violations
> occur. However, a cycle indicates that the set of chosen groups is invalid and at least one group
> will need to be eliminated. Although experimental data has shown this case to be extremely rare,
> care must be taken to ensure correctness.
> 
> 
> **Solution**
> 
> Just before scheduling, I introduced `SuperWord::remove_cycles`. It creates a `PacksetGraph`, based on nodes in the `packs`, and scalar-nodes which are not in a pack. The edges are taken from `DepPreds`. We check if the graph can be scheduled without cycles (via topological sort).
> 
> **FYI**
> 
> I found a further bug, this time I think it happens during scheduling. See [JDK-8304720](https://bugs.openjdk.org/browse/JDK-8304720). Because of that, I had to disable a test case (`TestIndependentPacksWithCyclicDependency::test5`). I also had to require 64 bit, and either `avx2` or `asimd`. I hope we can lift that again once we fix the other bug. The issue is this: the cyclic dependency example can degenerate to non-cyclic ones, that need to reorder the non-vectorized memory operations.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  Typo fix by Tobias
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13078/files
  - new: https://git.openjdk.org/jdk/pull/13078/files/adc297e4..8e7814c4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13078&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13078&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13078.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13078/head:pull/13078

PR: https://git.openjdk.org/jdk/pull/13078