fuse flt/map over window into stride-1 in-place loop#325
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New 2-word opcode that materialises xs[idx..idx+n] into a destination list register, reusing the existing Vec in place when its strong count is 1. Backs the fused `flt fn (window n xs)` / `map fn (window n xs)` emitter coming in a follow-up commit. The reuse path mirrors the RC-peek pattern already used by OP_ADD_SS, OP_LISTAPPEND, and OP_MSET. When the destination doesn't hold a uniquely owned list, falls back to allocating fresh and drop_rc'ing the previous value. Also threads the new opcode through find_block_leaders (2-word skip) and chunk_is_all_numeric (non-numeric result).
Adds jit_window_view (mirrors the OP_WINDOW_VIEW VM dispatcher's reuse arm) and wires it into both the JIT and AOT compile paths. Each path declares the 5-arg helper, registers it in the relocation table, and emits an OP_WINDOW_VIEW arm that calls the helper with (cur, xs, idx, n, span). The helper takes the destination register's current value, attempts the in-place reuse fast path when strong count is 1, and otherwise allocates fresh. Returning the same NanVal bits on reuse keeps def_var as a no-op write so the SSA variable's identity is preserved across iterations. Also marks OP_WINDOW_VIEW as a non-numeric / non-bool writer in both Cranelift flag-flow analyses.
Pattern-matches `flt fn (window n xs)` and `map fn (window n xs)` at bytecode emit time and replaces the eager `L (L t)` materialisation with a tight stride-1 loop that reuses one scratch list as the per-call window. The reuse hinges on the new OP_WINDOW_VIEW arm's RC=1 fast path. Why this matters: bioinformatics rerun5 hit a 5.8x slowdown on `flt all-hydro (window 15 (chars seq))` over an 11.4M-residue input — the unfused form allocated ~170M small lists. With fusion, drop iterations and map iterations reuse a single Vec; only flt's keep iterations force a fresh allocation (because the accumulator clone_rc's the window into itself, bumping RC > 1). The non-fused `window` path is unchanged: when the result escapes to a binding or isn't consumed by an outer flt/map, the eager OP_WINDOW dispatcher fires as before. Tree-walker keeps the reference impl; the VM dispatcher and Cranelift helper handle the reuse.
Pin the fused emitter's output against tree, VM, and Cranelift engines:
- flt/map over window across happy paths
- all-pass case (every iter clone_rc's, forcing fresh allocation)
- empty source / n > len(xs) edge cases (limit reg <= 0, loop body
never runs)
- size-1 windows (smallest non-trivial case)
- non-bool predicate error path (matches the unfused error message)
- negative case: window result bound to a variable, fusion must not
fire and the eager OP_WINDOW path produces the expected list-of-
lists
Adds an examples/window-stream.ilo demonstrator that the examples_engines
harness runs across every engine.
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danieljohnmorris
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May 16, 2026
eight fixes since 0.11.4, all from rerun5 personas: bare-bang silent-nil regression (#324), Cranelift AArch64 panic catch_unwind fallback (#319), multi-line body span drift (#318), HOF tree-bridge error parity on Cranelift (#321), bool-ternary brace sugar (#323), single-line body diagnostic with brace-block bodies (#322), unknown-subcommand error in multi-fn files (#320), window perf cliff fused flt/map (#325).
This was referenced May 17, 2026
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Summary
Manifesto framing: idiomatic
flt all-hydro (window 15 (chars seq))was~6x slower than the hand-rolled imperative form on the bioinformatics
rerun5 corpus (11.4M residues). Root cause: the unfused emitter
materialised an
L (L t)ofn-k+1small inner lists, allocating ~170Mfresh Vecs total. AI agents reaching for the idiomatic shape paid a real
latency tax; the imperative escape hatch is more tokens to emit and
defeats the point of having higher-order list builtins.
This PR fuses
flt fn (window n xs)andmap fn (window n xs)at emittime into a stride-1 loop that walks
xsonce with a single reusablescratch list. The reuse path piggy-backs on the same RC=1 in-place
mutation pattern already used by
OP_ADD_SS,OP_LISTAPPEND, andOP_MSET.Result on the bioinformatics workload (local repro, full corpus): the
fused path matches the hand-rolled imperative form within noise. Agents
can now write the idiomatic shape and pay no penalty.
Repro
Before (unfused, on main):
After (fused, this PR):
What's in the diff
Four commits, one coherent change per commit:
opcode, VM dispatcher arm with RC=1 in-place reuse fast path,
find_block_leaders2-word skip,chunk_is_all_numericexclusion.jit_window_view(mirrors the VM dispatcher's reuse logic), wiredinto both JIT and AOT compile paths. Non-numeric / non-bool writer
flag updates.
compile_fused_window_hof, pattern-match in the(Builtin::Flt, 2)and
(Builtin::Map, 2)arms (only when the inner call is exactlywindow n xswith no unwrap mode). Tree-walker keeps the eager pathas the reference impl.
regression tests across tree / VM / Cranelift plus an
examples/window-stream.iloexercised by the examples_enginesharness.
Test plan
cargo test --release --features cranelift --test regression_window_fused(11/11)cargo test --release --features cranelift --test examples_engines(1/1)cargo test --release --features cranelift(full suite green)cargo fmt --checkcargo clippy --release --features cranelift --all-targets -- -D warningsFollow-ups
None blocking. A future PR could extend the same fusion to
fold fn init (window n xs)andflt+map fusionif the bioinformaticswork surfaces more patterns.