RFR: 8320725: C2: Add "is_associative" flag for floating-point add-reduction [v4]

[Emanuel Peter]

On Wed, 3 Apr 2024 11:12:24 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:
> 
>   Add comments, revert to requires_strict_order and other minor changes

src/hotspot/share/opto/vectorIntrinsics.cpp line 1742:

> 1740:   if (mask != nullptr && !use_predicate) {
> 1741:     Node* reduce_identity = gvn().transform(VectorNode::scalar2vector(init,
> 1742:           num_elem, Type::get_const_basic_type(elem_bt)));

Suggestion:

          num_elem, Type::get_const_basic_type(elem_bt)));

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18034#discussion_r1553343739