vm + cranelift: native map HOF dispatch (PR 2 of 4)

[danieljohnmorris]

Summary

PR 2 of the HOF dispatch chain. PR 1 (#274) wired FnRef NaN-tagging so a function reference could survive a NanVal round-trip on VM and Cranelift. This PR puts that plumbing to work for the first HOF.

map fn xs now runs natively on every engine. The compiler emits a real bytecode loop using the OP_CALL_DYN opcode that PR 1 reserved, and Cranelift lowers OP_CALL_DYN through a new jit_call_dyn helper that re-enters the VM for user-fn callbacks and routes builtin callbacks through the tree-bridge.

Token-cost framing: the manifesto bet is that each engine should agree with the others on whatever ilo prints, so an agent never has to second-guess which engine its -- run-vm example ran under. engine-skip: vm lines and "tree-only" hedges in HOF examples were a tax on that promise. This PR cashes in the first slice of that for map.

Repro before / after

sq x:n>n;*x x
main xs:L n>L n;map sq xs

Before:

$ ilo file.ilo --run-vm main [1,2,3,4,5]
Compile error: undefined function: map   # the HOF arm errored out

$ ilo file.ilo --run-cranelift main [1,2,3,4,5]
Compile error: ...                       # same

After:

$ ilo file.ilo --run-tree main [1,2,3,4,5]
[1, 4, 9, 16, 25]
$ ilo file.ilo --run-vm main [1,2,3,4,5]
[1, 4, 9, 16, 25]
$ ilo file.ilo --run-cranelift main [1,2,3,4,5]
[1, 4, 9, 16, 25]

What's in the diff (per commit)

• vm: thread-local active program + jit_call_dyn helper for OP_CALL_DYN — adds the runtime plumbing Cranelift needs. New ACTIVE_PROGRAM TLS slot published by with_active_registry (cleared by the same drop guard as the existing FUNC_NAMES / REGISTRY TLS). New jit_call_dyn(callee_bits, argc, regs_ptr) -> u64 extern "C" helper: Builtin FnRef → tree-bridge (same path as jit_call_builtin_tree), User FnRef → fresh VM::new(active_program).call(idx, args). The TLS-plus-helper combo is the minimum surface that lets Cranelift dispatch a FnRef whose target id is only known at runtime.

• cranelift: lower OP_CALL_DYN through the jit_call_dyn helper — replaces the PR-1 placeholders in both Cranelift backends (compile_cranelift returned an error, jit_cranelift bailed JIT compilation with None). Mirrors the OP_CALL_BUILTIN_TREE pattern: spill arg slots into a sized stack slot and pass (callee_bits, argc, regs_ptr) to the helper. Both backends register the helper FuncId.

• vm: native map HOF loop using OP_CALL_DYN — (Builtin::Map, 2) arm in the bytecode compiler. Emits OP_LISTNEW for the accumulator, allocates contiguous result + arg scratch regs (so OP_CALL_DYN's R[A+1..=A+argc] layout is correct), drives the iteration with OP_FOREACHPREP / OP_FOREACHNEXT (the same pair @i xs uses), and grows the accumulator with OP_LISTAPPEND on the in-place RC=1 fast path. is_tree_bridge_eligible doc-comment loses map from the not-yet list. Also drops #[ignore] from vm_map_squares and vm_map_wrong_list_arg.

• test + example: cross-engine coverage for native map HOF — six cross-engine tests in tests/regression_hof_map.rs covering user-fn / builtin / empty / single-element / chained / tail-position shapes. examples/map-fnref.ilo doubles as an agent-facing learning sample and runs through tests/examples_engines.rs on every engine.

• example: lift vm/cranelift skip from window.ilo — sums>L n;map sum (window 3 [1,2,3,4,5]) now runs on every engine and no longer needs the skip annotation.

Test plan

• cargo build --release --features cranelift clean
• cargo test --release --features cranelift — all 4803 tests pass, 0 failures across 113 suites (includes the new regression_hof_map and the cross-engine examples_engines harness which exercises the updated window.ilo)
• cargo clippy --release --features cranelift --all-targets -- -D warnings clean
• cargo fmt --check clean
• Manual cross-engine smoke: map sq [1,2,3,4,5] and map abs [-3,0,4,-7] return identical output on --run-tree, --run-vm, --run-cranelift

Follow-ups (PR 3, not in this diff)

• flt, fld, grp, flatmap, uniqby, partition, 2-arg srt, dynamic-fmt wr — same native-loop pattern, each with its own emitter arm
• The 23+ tests still gated with #[ignore] // Dynamic dispatch (OP_CALL_DYN emission) arrives in PR 2. for the other HOFs
• 3-arg closure-bind map fn ctx xs (verifier accepts it; the emitter currently only handles 2-arg)
• Value::Text-as-callable dispatch (the vm_map_with_text_fn_name shape) — needs OP_CALL_DYN to accept a Text fallback alongside FnRef

[codecov]

Codecov Report

❌ Patch coverage is 77.50000% with 27 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/vm/compile_cranelift.rs	5.26%	18 Missing ⚠️
src/vm/mod.rs	90.12%	8 Missing ⚠️
src/vm/jit_cranelift.rs	95.00%	1 Missing ⚠️

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