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

[Emanuel Peter]

On Wed, 29 Mar 2023 04:26:48 GMT, Fei Gao <fgao at openjdk.org> wrote:

>> 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.
>
> src/hotspot/share/opto/superword.cpp line 2449:
> 
>> 2447:       Node* n = _block.at(i);
>> 2448:       int pid = get_pid_or_zero(n);
>> 2449:       if (pid == 0 || pid <= max_pid_packset) {
> 
> Is `pid == 0 ||` repetitive here? We have `max_pid_packset >= 0`, right?

Good catch.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13078#discussion_r1151657752