Specialize Constant for the arithmetic-write fast path

[joseph-isaacs]

Part 17 of 25 in the stacked PR series adding vortex-row.

This PR contains exactly one commit; review just that diff in isolation.

What this commit does

When a ConstantArray feeds the fixed-before-varlen arithmetic path, the encoded scalar bytes are the same for every row. Hoists them into 1-2 register-sized loads outside the loop and emits direct write_unaligned stores per row. Specialized for encoded lengths 2 (bool/i8), 5 (i32), 9 (i64), 17 (i128). Other lengths fall back to copy_nonoverlapping. The var_prefix case (Constant after a varlen column) takes the same shape but computes per-row positions arithmetically rather than via a running cursor.

constant_i64_vortex_without_kernel 2.47 → ~6 GB/s (PR3 commit 3.3 adds the same specialization to the Constant kernel itself).

Stack

#	PR	Title	Branch
1	#7986	vortex-row: crate scaffolding	claude/row-c01-crate-scaffolding
2	#7987	vortex-row: add SortField and RowEncodeOptions	claude/row-c02-sortfield-options
3	#7988	vortex-row: codec for fixed-width canonical types	claude/row-c03-codec-fixed-width
4	#7989	vortex-row: codec for varlen canonical types	claude/row-c04-codec-varlen
5	#7990	vortex-row: codec for nested canonical types	claude/row-c05-codec-nested
6	#7991	vortex-row: compute_sizes helper and RowSize ScalarFn	claude/row-c06-rowsize-scalarfn
7	#7992	vortex-row: RowEncode ScalarFn	claude/row-c07-rowencode-scalarfn
8	#7993	vortex-row: convert_columns + tests + bench scaffolding	claude/row-c08-convert-columns-tests-bench
9	#7994	Skip ListView validation in row encoder output	claude/row-c09-skip-listview-validation
10	#7995	Add validity fast-path helper for the four pattern-matching encoders	claude/row-c10-validity-fast-path
11	#7996	Skip zero-init of output buffer	claude/row-c11-skip-zero-init
12	#7997	Auto-vectorize pure-fixed offsets construction	claude/row-c12-vectorize-pure-fixed-offsets
13	#7998	Auto-vectorize mixed-path offsets construction	claude/row-c13-vectorize-mixed-offsets
14	#7999	Rewrite varlen 32-byte block encoder with copy_nonoverlapping	claude/row-c14-varlen-block-copy-nonoverlapping
15	#8000	Walk VarBinView rows directly in row encoder hot loop	claude/row-c15-walk-varbinview-directly
16	#8001	Add arithmetic-write fast path for fixed-before-varlen columns	claude/row-c16-arith-write-fast-path
17	#8002	Specialize Constant for the arithmetic-write fast path	claude/row-c17-specialize-constant-arith
18	#8003	RowSizeKernel and RowEncodeKernel dispatch helpers	claude/row-c18-kernel-dispatch-helpers
19	#8004	Inventory-based registry for downstream encoding kernels	claude/row-c19-inventory-registry
20	#8005	Constant row-encode kernel	claude/row-c20-constant-kernel
21	#8006	Dict row-encode kernel	claude/row-c21-dict-kernel
22	#8007	Patched row-encode kernel	claude/row-c22-patched-kernel
23	#8008	RunEnd row-encode kernel (vortex-runend)	claude/row-c23-runend-kernel
24	#8009	BitPacked row-encode kernel (vortex-fastlanes)	claude/row-c24-bitpacked-kernel
25	#7985	FoR and Delta row-encode kernels (vortex-fastlanes)	claude/row-pr3-kernels

Base of this PR: #8001 (claude/row-c16-arith-write-fast-path)
Next in stack: #8003 (claude/row-c18-kernel-dispatch-helpers)

Combined context

For the full design + rationale, see PR #7985 (top of stack).

[codspeed-hq]

Merging this PR will degrade performance by 12.39%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

❌ 1 regressed benchmark
✅ 1220 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	new_alp_prim_test_between[f32, 16384]	103.7 µs	118.3 µs	-12.39%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing claude/row-c17-specialize-constant-arith (7987423) with claude/row-c16-arith-write-fast-path (224fdab)}