RFR: 8298935: fix independence bug in create_pack logic in SuperWord::find_adjacent_refs [v26]

[Vladimir Kozlov]

On Mon, 13 Mar 2023 10:13:40 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 40 commits:
>> 
>>  - Merge master after NULL -> nullptr conversion
>>  - Fixed wording from last commit
>>  - A little renaming and improved comments
>>  - resolve merge conflict after Roland's fix
>>  - TestDependencyOffsets.java: add vanilla run
>>  - TestDependencyOffsets.java: parallelize it + various AVX settings
>>  - TestOptionVectorizeIR.java: removed PopulateIndex IR rule - fails on x86 32bit - see Matcher::match_rule_supported
>>  - Merge branch 'master' into JDK-8298935
>>  - Reworked TestDependencyOffsets.java
>>  - remove negative IR rules for TestOptionVectorizeIR.java
>>  - ... and 30 more: https://git.openjdk.org/jdk/compare/5726d31e...731cc7b5
>
> https://github.com/openjdk/jdk/pull/12350#issuecomment-1461681129
> 
>> So is there a **4th Bug** lurking here?
> 
> @vnkozlov I now found an example that reveals this **Bug 4**. I want to fix it in a separate Bug [JDK-8304042](https://bugs.openjdk.org/browse/JDK-8304042).
> 
> This is the method:
> 
>     static void test(int[] dataI1, int[] dataI2, float[] dataF1, float[] dataF2) {
>         for (int i = 0; i < RANGE/2; i+=2) {
>             dataF1[i+0] = dataI1[i+0] + 0.33f;            // 1
>             dataI2[i+1] = (int)(11.0f * dataF2[i+1]);     // 2
> 
>             dataI2[i+0] = (int)(11.0f * dataF2[i+0]);     // 3
>             dataF1[i+1] = dataI1[i+1] + 0.33f;            // 4
>         }
>     }
> 
> 
> Note: `dataI1 == dataI2` and `dataF1 == dataF2`. I only had to use two references so that C2 does not know this, and does not optimize away load after store.
> 
> Lines 1 and 4 are `isomorphic` and `independent`. The same holds for line 2 and 3. We creates the packs `[1,4]` and `[2,3]`, and vectorize (with and without my patch). However, we have the following dependencies: `1->3` and `2->4`. This creates a cyclic dependency between the two packs.
> 
> As explained in the previous https://github.com/openjdk/jdk/pull/12350#issuecomment-1461681129, we have to verify that there are no cyclic dependencies between the packs, just before we schedule. The SuperWord paper states this in "3.7 Scheduling".

@eme64, I ran with `-XX:-TieredCompilation -Xbatch -XX:CICompilerCount=1 -XX:+TraceNewVectors` on AVX512 linux machine our vectors jtregs tests (including `jdk/incubator/vector) and everything is fine except one test:

compiler/loopopts/superword/TestPickLastMemoryState.java`

With these changes we almost don't generate vectors (may be 2 per `@run`). Without changes we got about 160 (50 per `@run`) new vectors. It has several `@run` commands for different vector sizes.

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PR: https://git.openjdk.org/jdk/pull/12350