RFR: 8320725: C2: Add "requires_strict_order" flag for floating-point add-reduction [v6]

[Bhavana Kilambi]

> 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:

  Address some more review comments

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18034/files
  - new: https://git.openjdk.org/jdk/pull/18034/files/71a86deb..f38dae21

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18034&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18034&range=04-05

  Stats: 11 lines in 3 files changed: 5 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/18034.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18034/head:pull/18034

PR: https://git.openjdk.org/jdk/pull/18034