fix(stdlib): delegate Array reverse/fill/copyWithin/sort to prototype

[dowdiness]

Summary

• Removes the four live fast-path arms for reverse, fill, copyWithin, and sort from get_array_method_with_interp, delegating each to Array.prototype via the normal prototype chain
• Cleans up the corresponding dead arms in get_array_method (these were unreachable since _with_interp handles them before the _ fallback fires)
• Deletes the now-dead direct helpers: array_direct_reverse, array_direct_fill, array_direct_copy_within, array_direct_sort_default, and the no-longer-referenced array_mutator_relative_index

Why safe: The removed _with_interp fast paths called the same indexed-property helpers (array_reverse_indexed_properties, array_fill_indexed_properties, etc.) that Array.prototype already uses — the bypass was redundant and skipped length-writable and Proxy trap semantics that route through indexed [[Set]]/[[Delete]].

Net change: −249 lines.

Test plan

• moon check — clean, 0 warnings
• moon test — 2249/2249 passed
• make test262-filter FILTER="reverse" — 64/64 (strict + non-strict)
• make test262-filter FILTER="fill" — 162/162
• make test262-filter FILTER="copyWithin" — 150/150
• make test262-filter FILTER="sort" — 130/144 (14 failures confirmed pre-existing on main)

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Updated array method behavior for reverse, fill, copyWithin, and sort to use the shared array prototype path.
  • Improved consistency for array operations when working with indexed elements.
  • Continued support for other array methods such as entries, keys, values, and splice.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6eb28163-e0cf-4475-92ce-3e6e237cf30a

📥 Commits

Reviewing files that changed from the base of the PR and between eced381 and 6875942.

📒 Files selected for processing (2)

• interpreter/stdlib/array_indexed_mutators.mbt
• interpreter/stdlib/builtins_array.mbt

💤 Files with no reviewable changes (1)

• interpreter/stdlib/array_indexed_mutators.mbt

---

📝 Walkthrough

Walkthrough

Array builtin dispatch now delegates reverse, fill, copyWithin, and sort through prototype handling in both method lookup paths, and the direct ArrayData helper implementations for those operations are removed.

Changes

Array method delegation updates

Layer / File(s)	Summary
Builtin dispatch delegates mutators interpreter/stdlib/builtins_array.mbt	get_array_method and get_array_method_with_interp switch reverse, fill, copyWithin, and sort to delegation-only handling.
Direct ArrayData helpers removed interpreter/stdlib/array_indexed_mutators.mbt	The relative-index helper and the direct reverse, fill, copyWithin, and default sort ArrayData helpers are deleted.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

(\/)
(••) I hopped along the array trail today,
/ >🍃 where reverse and fill now delegate away.
CopyWithin and sort took a gentler lane,
My little rabbit nose says: neat and plain!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: delegating Array reverse/fill/copyWithin/sort to the prototype.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/array-fastpath-remove-reverse-fill-copywith-sort

---

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[github-actions]

Benchmark Results

Run: https://github.com/dowdiness/js_engine/actions/runs/28223365312

startup/tiny_program is the PR #153 / issue #141 guardrail for built-in realm-stamping startup cost.

Stage summary

stage	benchmarks	total mean	slowest benchmark	slowest mean	noisy rows
startup	3	2.569 ms	startup/tiny_program	1.299 ms	0
frontend	7	0.931 ms	pipeline/parse_heavy	0.514 ms	2
execution	26	15293.423 ms	exec/fibonacci_30	13822.939 ms	2

Focused bytecode base-vs-head comparison

Base-vs-head deltas are reporting-only. Negative delta and PR/base < 1.00x mean the PR is faster; interpret high-CV or noisy rows cautiously.

benchmark	stage	base mean	PR mean	delta	PR/base	base CV	PR CV	noisy
baseline/bytecode/closure_factory	execution	13.844 ms	14.046 ms	+1.5%	1.01x	9.7%	6.2%	no
pipeline/bytecode/evaluate	execution	9.390 ms	8.695 ms	-7.4%	0.93x	1.9%	2.5%	no
isolate/bytecode/call_frame	execution	7.701 ms	7.333 ms	-4.8%	0.95x	4.9%	0.5%	no
isolate/bytecode/runtime_helpers	execution	10.681 ms	10.940 ms	+2.4%	1.02x	0.5%	0.4%	no
isolate/bytecode/local_access	execution	36.959 ms	39.236 ms	+6.2%	1.06x	0.9%	2.9%	no
isolate/bytecode/env_access	execution	36.848 ms	38.756 ms	+5.2%	1.05x	1.7%	2.9%	no
isolate/bytecode/captured_access	execution	37.313 ms	36.331 ms	-2.6%	0.97x	1.3%	1.8%	no
isolate/bytecode/dispatch_stack	execution	23.160 ms	23.690 ms	+2.3%	1.02x	0.6%	1.6%	no

Base-vs-head comparison

benchmark	stage	base mean	PR mean	delta	PR/base	base CV	PR CV	noisy
startup/tiny_program	startup	1.217 ms	1.299 ms	+6.8%	1.07x	5.9%	3.5%	no
lexer/small	frontend	0.034 ms	0.035 ms	+1.5%	1.02x	24.2%	25.8%	base, PR
lexer/large	frontend	0.299 ms	0.300 ms	+0.3%	1.00x	0.5%	0.7%	no
exec/fibonacci_30	execution	13206.599 ms	13822.939 ms	+4.7%	1.05x	0.5%	0.4%	no
exec/property_chain	execution	17.600 ms	15.889 ms	-9.7%	0.90x	31.7%	10.1%	base
startup/phase/parse_tiny	frontend	0.002 ms	0.002 ms	-2.5%	0.97x	0.5%	0.6%	no
startup/phase/new_interpreter	startup	1.234 ms	1.270 ms	+3.0%	1.03x	10.3%	10.6%	no
startup/phase/execute_preparsed_tiny	execution	0.001 ms	0.001 ms	-8.7%	0.91x	0.7%	1.2%	no
startup/phase/event_loop_drain_empty	startup	0.000 ms	0.000 ms	+3.5%	1.03x	0.8%	1.0%	no
startup/phase/result_stringify_output	execution	0.000 ms	0.000 ms	+0.2%	1.00x	0.6%	1.6%	no
exec/array_map_filter	execution	21.181 ms	20.848 ms	-1.6%	0.98x	20.2%	17.0%	base, PR
exec/closure_factory	execution	29.697 ms	31.552 ms	+6.2%	1.06x	6.7%	8.5%	no
baseline/closure_legacy/closure_factory	execution	27.697 ms	28.258 ms	+2.0%	1.02x	8.0%	9.0%	no
baseline/bytecode/closure_factory	execution	13.844 ms	14.046 ms	+1.5%	1.01x	9.7%	6.2%	no
isolate/bytecode/dispatch_stack	execution	23.160 ms	23.690 ms	+2.3%	1.02x	0.6%	1.6%	no
isolate/bytecode/local_access	execution	36.959 ms	39.236 ms	+6.2%	1.06x	0.9%	2.9%	no
isolate/bytecode/env_access	execution	36.848 ms	38.756 ms	+5.2%	1.05x	1.7%	2.9%	no
isolate/bytecode/captured_access	execution	37.313 ms	36.331 ms	-2.6%	0.97x	1.3%	1.8%	no
isolate/bytecode/call_frame	execution	7.701 ms	7.333 ms	-4.8%	0.95x	4.9%	0.5%	no
isolate/bytecode/runtime_helpers	execution	10.681 ms	10.940 ms	+2.4%	1.02x	0.5%	0.4%	no
isolate/bytecode/property_get	execution	42.758 ms	44.649 ms	+4.4%	1.04x	2.8%	1.7%	no
isolate/bytecode/property_set	execution	40.255 ms	40.858 ms	+1.5%	1.01x	1.2%	0.8%	no
isolate/bytecode/method_call	execution	8.298 ms	8.374 ms	+0.9%	1.01x	0.9%	0.4%	no
isolate/bytecode/object_literal	execution	13.914 ms	12.789 ms	-8.1%	0.92x	1.1%	0.6%	no
isolate/bytecode/array_literal	execution	14.948 ms	14.645 ms	-2.0%	0.98x	0.9%	1.4%	no
exec/for_of	execution	6.338 ms	5.921 ms	-6.6%	0.93x	9.1%	6.2%	no
exec/arithmetic_loop	execution	1018.451 ms	1007.042 ms	-1.1%	0.99x	0.6%	0.6%	no
exec/object_construction	execution	7.943 ms	7.164 ms	-9.8%	0.90x	5.2%	6.6%	no
exec/string_ops	execution	2.134 ms	1.933 ms	-9.4%	0.91x	21.4%	20.5%	base, PR
pipeline/exec/lex	frontend	0.031 ms	0.031 ms	+0.7%	1.01x	0.9%	0.8%	no
pipeline/exec/parse	frontend	0.027 ms	0.027 ms	+0.0%	1.00x	2.9%	2.9%	no
pipeline/exec/evaluate	execution	28.077 ms	26.555 ms	-5.4%	0.95x	7.9%	11.5%	no
pipeline/closure_legacy/evaluate	execution	26.055 ms	24.981 ms	-4.1%	0.96x	4.0%	4.4%	no
pipeline/bytecode/compile	frontend	0.022 ms	0.022 ms	+1.4%	1.01x	37.6%	32.7%	base, PR
pipeline/bytecode/evaluate	execution	9.390 ms	8.695 ms	-7.4%	0.93x	1.9%	2.5%	no
pipeline/parse_heavy	frontend	0.517 ms	0.514 ms	-0.6%	0.99x	4.1%	5.4%	no

Mean-time chart (log scale)

benchmark	stage	mean	chart
startup/tiny_program	startup	1.299 ms	##
lexer/small	frontend	0.035 ms ⚠	#
lexer/large	frontend	0.300 ms	#
exec/fibonacci_30	execution	13822.939 ms	##############################
exec/property_chain	execution	15.889 ms	########
startup/phase/parse_tiny	frontend	0.002 ms	#
startup/phase/new_interpreter	startup	1.270 ms	##
startup/phase/execute_preparsed_tiny	execution	0.001 ms	#
startup/phase/event_loop_drain_empty	startup	0.000 ms	#
startup/phase/result_stringify_output	execution	0.000 ms	#
exec/array_map_filter	execution	20.848 ms ⚠	#########
exec/closure_factory	execution	31.552 ms	##########
baseline/closure_legacy/closure_factory	execution	28.258 ms	##########
baseline/bytecode/closure_factory	execution	14.046 ms	########
isolate/bytecode/dispatch_stack	execution	23.690 ms	##########
isolate/bytecode/local_access	execution	39.236 ms	###########
isolate/bytecode/env_access	execution	38.756 ms	###########
isolate/bytecode/captured_access	execution	36.331 ms	###########
isolate/bytecode/call_frame	execution	7.333 ms	######
isolate/bytecode/runtime_helpers	execution	10.940 ms	#######
isolate/bytecode/property_get	execution	44.649 ms	############
isolate/bytecode/property_set	execution	40.858 ms	###########
isolate/bytecode/method_call	execution	8.374 ms	#######
isolate/bytecode/object_literal	execution	12.789 ms	########
isolate/bytecode/array_literal	execution	14.645 ms	########
exec/for_of	execution	5.921 ms	######
exec/arithmetic_loop	execution	1007.042 ms	#####################
exec/object_construction	execution	7.164 ms	######
exec/string_ops	execution	1.933 ms ⚠	###
pipeline/exec/lex	frontend	0.031 ms	#
pipeline/exec/parse	frontend	0.027 ms	#
pipeline/exec/evaluate	execution	26.555 ms	##########
pipeline/closure_legacy/evaluate	execution	24.981 ms	##########
pipeline/bytecode/compile	frontend	0.022 ms ⚠	#
pipeline/bytecode/evaluate	execution	8.695 ms	#######
pipeline/parse_heavy	frontend	0.514 ms	#

Closure-conversion comparison

• unavailable