RFR: 8304042: C2 SuperWord: schedule must remove packs with cyclic dependencies [v2]

[Tobias Hartmann]

On Wed, 29 Mar 2023 10:30:52 GMT, Emanuel Peter <epeter 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.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review feedback implemented

Nice analysis and test coverage! The fix looks good to me.

test/hotspot/jtreg/compiler/loopopts/superword/TestIndependentPacksWithCyclicDependency.java line 28:

> 26:  * @test
> 27:  * @bug 8304042
> 28:  * @summary Test some examples with indepenednet packs with cyclic dependency

Suggestion:

 * @summary Test some examples with independent packs with cyclic dependency

test/hotspot/jtreg/compiler/loopopts/superword/TestIndependentPacksWithCyclicDependency2.java line 28:

> 26:  * @test
> 27:  * @bug 8304042
> 28:  * @summary Test some examples with indepenednet packs with cyclic dependency

Suggestion:

 * @summary Test some examples with independent packs with cyclic dependency

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

Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13078#pullrequestreview-1368325513
PR Review Comment: https://git.openjdk.org/jdk/pull/13078#discussion_r1155506558
PR Review Comment: https://git.openjdk.org/jdk/pull/13078#discussion_r1155506717