RFR: 8320725: C2: Add "is_associative" flag for floating-point add-reduction [v3]

[Andrew Haley]

On Fri, 15 Mar 2024 11:03:08 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 incrementally with one additional commit since the last revision:
> 
>   Naming changes: replace strict/non-strict with more technical terms

src/hotspot/cpu/aarch64/aarch64_vector.ad line 2858:

> 2856: // reduction addF
> 2857: instruct reduce_add2F_neon(vRegF dst, vRegF fsrc, vReg vsrc) %{
> 2858:   predicate(Matcher::vector_length(n->in(2)) == 2 && n->as_Reduction()->is_associative());

This `vector_length(n->in(2)) == 2` is very obscure. I suspect that anyone coming across this code would not understand it.

What exactly is the reason that this pattern is only applied for the 16b case? You need to give a justification in a comment right here.

src/hotspot/share/opto/vectornode.hpp line 235:

> 233:   // Floating-point addition and multiplication are non-associative, so
> 234:   // AddReductionVF/D and MulReductionVF/D require strict-ordering
> 235:   // in auto-vectorization. Currently, Vector API allows

Don't say "currently".

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18034#discussion_r1528972669
PR Review Comment: https://git.openjdk.org/jdk/pull/18034#discussion_r1528974623