perf: fastpath for Poseidon2 arm64 accelerator IV≠0

[Tabaie]

Unlocks optimizations where constant-length hashes may omit the initial incorporation of an initialization vector without compromising collision resistance.

Case                         Median time
------------------------------------------
Zero-IV baseline (HEAD)      174.8 µs
Zero-IV current              172.2 µs
Non-zero-IV accelerated      175.9 µs
Non-zero-IV generic          1.49 ms

Non-zero-IV speedup: ~8.5×

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Note

Medium Risk
Changes Poseidon2 compression output for non-nil state on arm64 if the new state load layout or register fix (nbSteps vs state pointer) is wrong; correctness should be validated against the generic path.

Overview
Enables the ARM64 NEON column-compression kernel when Compressx16ColumnsWithState supplies a non-zero initial state, instead of always falling back to pure Go on arm64 whenever state != nil.

The Go dispatcher now passes an optional state pointer into permutation16x16xN_columns_arm64 (same pattern as the existing AVX-512 columns kernel). The assembly loads per-batch initial capacity coordinates from column-major state[pos*16+lane] when the pointer is non-nil, and still zero-initializes when it is nil (zero-IV / Compressx16Columns behavior).

Stub signatures, codegen templates, and the ARM64 generator were updated so regenerated .s files match; a benchmark compares accelerated vs generic Compressx16ColumnsWithState at colSize=512.

^{Reviewed by Cursor Bugbot for commit 8a928eb. Bugbot is set up for automated code reviews on this repo. Configure here.}

[Copilot]

Pull request overview

This PR enables the ARM64 NEON Poseidon2 “columns” accelerator to handle Compressx16ColumnsWithState with a non-nil initial state (IV ≠ 0), avoiding a fallback to the pure-Go implementation and significantly improving performance for non-zero-IV workloads.

Changes:

• Extend the ARM64 columns-kernel ABI to accept an optional state *fr.Element pointer and plumb it through the Go dispatcher and templates.
• Update the ARM64 assembly generator and regenerated .s output to conditionally load the initial capacity state when provided (or zero-initialize when nil).
• Add a benchmark to compare accelerated vs generic Compressx16ColumnsWithState at colSize=512.

Reviewed changes

Copilot reviewed 7 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/generator/field/template/poseidon2/poseidon2.purego.go.tmpl	Updates pure-Go stub signature for the ARM64 columns kernel to include state.
internal/generator/field/template/poseidon2/poseidon2.go.tmpl	Removes the arm64+state fast-path disable and passes an optional state pointer into the ARM64 kernel.
internal/generator/field/template/poseidon2/poseidon2.arm64.go.tmpl	Updates ARM64 assembly declaration signature to include state.
internal/generator/field/template/poseidon2/poseidon2.amd64.go.tmpl	Updates amd64-side stub signature for the ARM64 columns kernel to include state.
internal/generator/field/asm/arm64/element_vec_F31_poseidon2.go	Updates ARM64 codegen to accept/load an optional state pointer and adjust arg sizing.
field/koalabear/poseidon2/poseidon2.go	Enables ARM64 accelerated path for non-nil state by passing statePtr into the ARM64 kernel.
field/koalabear/poseidon2/poseidon2_purego.go	Updates pure-Go stub signature for permutation16x16xN_columns_arm64 to include state.
field/koalabear/poseidon2/poseidon2_bench_test.go	Adds a benchmark for accelerated vs generic Compressx16ColumnsWithState at colSize=512.
field/koalabear/poseidon2/poseidon2_arm64.s	Regenerated ARM64 assembly: new arg frame size and conditional state load logic.
field/koalabear/poseidon2/poseidon2_arm64.go	Updates ARM64 function declaration to include state.
field/koalabear/poseidon2/poseidon2_amd64.go	Updates amd64 stub signature for the ARM64 columns kernel to include state.

Files not reviewed (4)

• field/koalabear/poseidon2/poseidon2.go: Generated file
• field/koalabear/poseidon2/poseidon2_amd64.go: Generated file
• field/koalabear/poseidon2/poseidon2_arm64.go: Generated file
• field/koalabear/poseidon2/poseidon2_purego.go: Generated file

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