RFR: 8320725: AArch64: C2: Add "requires_strict_order" flag for floating-point add and mul reduction [v12]

[Emanuel Peter]

On Fri, 7 Jun 2024 13:30:35 GMT, Bhavana Kilambi <bkilambi at openjdk.org> wrote:

>> Floating-point addition is non-associative, that is adding floating-point elements in arbitrary order may get different value. Specially, Vector API does not define the order of reduction intentionally, which allows platforms to generate more efficient codes [1]. So that needs a node to represent non strictly-ordered add-reduction for floating-point type in C2.
>> 
>> To avoid introducing new nodes, this patch adds a bool field in `AddReductionVF/D` to distinguish whether they require strict order. It also removes `UnorderedReductionNode` and adds a virtual function `bool requires_strict_order()` in `ReductionNode`. Besides `AddReductionVF/D`, other reduction nodes' `requires_strict_order()` have a fixed value.
>> 
>> With this patch, Vector API would always generate non strictly-ordered `AddReductionVF/D' on SVE machines with vector length <= 16B as it is more beneficial to generate non-strictly ordered instructions on such machines compared to strictly ordered ones.
>> 
>> [AArch64]
>> On Neon, non strictly-ordered `AddReductionVF/D` cannot be generated. Auto-vectorization has already banned these nodes in JDK-8275275 [2].
>> 
>> This patch adds matching rules for non strictly-ordered `AddReductionVF/D`.
>> 
>> No effects on other platforms.
>> 
>> [Performance]
>> FloatMaxVector.ADDLanes [3] measures the performance of add reduction for floating-point type. With this patch, it improves ~3x on my SVE machine (128-bit).
>> 
>> ADDLanes
>> 
>> Benchmark                 Before     After      Unit
>> FloatMaxVector.ADDLanes   1789.513   5264.226   ops/ms
>> 
>> 
>> Final code is as below:
>> 
>> Before:
>> `        fadda        z17.s, p7/m, z17.s, z16.s
>> `
>> After:
>> 
>>         faddp        v17.4s, v21.4s, v21.4s
>>         faddp        s18, v17.2s
>>         fadd         s18, s18, s19
>> 
>> 
>> 
>> 
>> [Test]
>> Full jtreg passed on AArch64 and x86.
>> 
>> [1] https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/FloatVector.java#L2529
>> [2] https://bugs.openjdk.org/browse/JDK-8275275
>> [3] https://github.com/openjdk/panama-vector/blob/vectorIntrinsics/test/micro/org/openjdk/bench/jdk/incubator/vector/operation/FloatMaxVector.java#L316
>
> Bhavana Kilambi has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:
> 
>  - Merge with master
>  - Make changes in IR rules for JTREG tests
>  - Modify JTREG IR rules and some style/format changes
>  - Add dump_spec and JTREG IR tests for Add/Mul Reduction Nodes
>  - Merge master
>  - Adjust format for the backend rules changed in previous commit
>  - Address some more review comments
>  - Revert to previous indentation
>  - Add comments, revert to requires_strict_order and other minor changes
>  - Naming changes: replace strict/non-strict with more technical terms
>  - ... and 2 more: https://git.openjdk.org/jdk/compare/d923efaf...35e6258d

Testing is good, thanks for bearing with all the comments and suggestions. Thanks for the work!

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

Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18034#pullrequestreview-2107169313