RFR: 8374349: [VectorAPI]: AArch64: Prefer merging mode SVE CPY instruction

[Eric Fang]

On Thu, 22 Jan 2026 12:31:19 GMT, Eric Fang <erfang at openjdk.org> wrote:

> When optimizing some VectorMask related APIs , we found an optimization opportunity related to the `cpy (immediate, zeroing)` instruction [1]. Implementing the functionality of this instruction using `cpy (immediate, merging)` instruction [2] leads to better performance.
> 
> Currently the `cpy (imm, zeroing)` instruction is used in code generated by `VectorStoreMaskNode` and `VectorReinterpretNode`. Doing this optimization benefits all vector APIs that generate these two IRs potentially, such as `VectorMask.intoArray()` and `VectorMask.toLong()`.
> 
> Microbenchmarks show this change brings performance uplift ranging from **11%** to **33%**, depending on the specific operation and data types.
> 
> The specific changes in this PR:
> 1. Achieve the functionality of the `cpy (imm, zeroing)` instruction with the `movi + cpy (imm, merging)` instructions in assembler:
> 
> cpy  z17.d, p1/z, #1 =>
> 
> movi v17.2d, #0       // this instruction is zero cost
> cpy  z17.d, p1/m, #1
> 
> 
> 2. Add a new option `PreferSVEMergingModeCPY` to indicate whether to apply this optimization or not.
> - This option belongs to the Arch product category.
> - The default value is true on Neoverse-V1/V2 where the improvement has been confirmed, false on others.
> - When its value is true, the change is applied.
> 
> 3. Add a jtreg test to verify the behavior of this option.
> 
> This PR was tested on aarch64 and x86 machines with different configurations, and all tests passed.
> 
> JMH benchmarks:
> 
> On a Nvidia Grace (Neoverse-V2) machine with 128-bit SVE2:
> 
> Benchmark	        	Unit	size	Before		Error	After		Error	Uplift
> byteIndexInRange		ops/ms	7.00	471816.15	1125.96	473237.77	1593.92	1.00
> byteIndexInRange		ops/ms	256.00	149654.21	416.57	149259.95	116.59	1.00
> byteIndexInRange		ops/ms	259.00	177850.31	991.13	179785.19	1110.07	1.01
> byteIndexInRange		ops/ms	512.00	133393.26	167.26	133484.61	281.83	1.00
> doubleIndexInRange		ops/ms	7.00	302176.39	12848.8	299813.02	37.76	0.99
> doubleIndexInRange		ops/ms	256.00	47831.93	56.70	46708.70	56.11	0.98
> doubleIndexInRange		ops/ms	259.00	11550.02	27.95	15333.50	10.40	1.33
> doubleIndexInRange		ops/ms	512.00	23687.76	61.65	23996.08	69.52	1.01
> floatIndexInRange		ops/ms	7.00	412195.79	124.71	411770.23	78.73	1.00
> floatIndexInRange		ops/ms	256.00	84479.98	70.69	84237.31	70.15	1.00
> floatIndexInRange		ops/ms	259.00	22585.65	80.07	28296.21	7.98	1.25
> floatIndexInRange		ops/ms	512.00	46902.99	51.60	46686.68	66.01	1.00
> intIndexInRange			ops/ms	7.00	413411.70	50.59	420684.66	253.55	1.02
> intIndexInRange			ops/...

Hi @fg1417, this PR optimizes the use of the `sve cpy (imm, zeroing)` instruction. I have confirmed this optimization on Neoverse-V1/V2, but I don't have any more AArch64 testing environments, so I'm not sure if this optimization can be applied to more environments. Do you have more testing environments ? If so, could you help me test and review this PR? Thanks!

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PR Comment: https://git.openjdk.org/jdk/pull/29359#issuecomment-3784204017