perf(gc): declare array element layout once at allocation instead of per push

Cargo.toml

@@ -315,7 +315,7 @@ codegen-units = 16
 codegen-units = 16
 
 [workspace.package]
-version = "0.5.1288"
+version = "0.5.1289"
 edition = "2021"
 license = "MIT"
 repository = "https://github.com/PerryTS/perry"

changelog.d/7501-layout-declared-at-alloc.md

@@ -0,0 +1,64 @@
+**GC element layout is now declared once at allocation for proven pointer arrays,
+instead of maintained per push (#7469 mutator-cost campaign).**
+
+An `out = []` + `out.push({ … })` loop paid two runtime calls on every iteration
+of its inline store: `js_gc_note_slot_layout`, which reaches into the
+`LAYOUT_SLOT_MASKS` thread-local hashmap to set one bit of a per-array pointer
+bitmap, and `js_array_note_numeric_write`, which clears a raw-f64 flag that is
+already clear. The mask entry the first push creates also makes
+`LAYOUT_SLOT_MASKS` non-empty for the rest of the process, so the `is_empty()`
+fast-out in `layout_forget_object` — probed on **every** object allocation —
+starts hashing instead.
+
+When codegen can see the whole life of an array local (a single `[]` binding,
+every store a push of a fresh allocation, no rebind, no keyed store, no closure
+capture, no in-place mutator, and at least one such push in the same region), it
+now emits one `js_array_declare_all_pointer_elements` at the allocation site and
+drops both per-push calls. New collector: `perry-codegen/src/collectors/
+all_pointer_arrays.rs`.
+
+**Soundness does not rest on that static proof.** Every elided store is fronted
+by a test of the live GC header, folded into the frozen/sealed integrity test the
+inline push already performed — the same `_reserved` load, the same one `and` +
+one `icmp`, no added instruction: `(_reserved & 0xF487) == 0xA000`. That demands
+the four integrity bits clear (as before), **both** raw-f64 layout bits clear
+(which is what makes eliding the numeric-write note sound — there is nothing left
+for it to clear), and `GC_LAYOUT_SIDE_MASK | GC_LAYOUT_ALL_POINTERS` set (which is
+what makes eliding the layout note sound — in that state the collector visits
+every slot in `0..length`, so the slot being written is scanned whether or not a
+mask bit was recorded). A push that fails the test falls through to
+`js_array_push_f64`, which notes the slot exactly as it always did.
+
+Testing the live header rather than trusting the declaration is load-bearing,
+because the runtime revokes it: `js_array_is_numeric_f64_layout` on a still-empty
+declared array verifies vacuously and re-publishes it as RawF64 + `POINTER_FREE`,
+and `rebuild_array_layout` (sort/splice) installs a precise mask. Both are now
+regression tests.
+
+`layout_note_slot` gained one refinement: a pointer **append** at an array's
+append position (`slot_index == length` — every append protocol in the tree
+records the slot before bumping `length`) preserves an all-pointer declaration
+instead of downgrading it. Without that the first growth through
+`js_array_push_f64` demoted the array and every later push fell off the declared
+path permanently.
+
+`js_string_addref_if_heap_string` is now gated independently of the layout note
+at array pushes (`store_needs_string_addref`). The declaration predicate admits
+`Expr::New` — HIR rewrites closed-shape object literals to
+`New { class_name: "__AnonShape_<hash>" }` before codegen runs, so the loop this
+targets *is* the `new` form — and a `new` can be re-pointed by a constructor
+return override, so it must never gate the string demote.
+
+Witness coverage in `perry-runtime/src/gc/tests/copying/all_pointer_elements_7469.rs`:
+an array filled through the elided sequence survives a copying minor with every
+element relocated and every slot rewritten, and a permanent sabotage arm asserts
+the *undeclared* array enumerates zero child slots — so a green positive test
+means the declaration was load-bearing, not that nothing was tried. End to end, a
+compiled smoke over these shapes is byte-identical to `node` both normally and
+under `PERRY_GC_MOVING_LOOP_POLLS=1` + `PERRY_GC_ZEAL=1
+PERRY_GC_PROTECT_FROMSPACE=1 PERRY_GC_VERIFY_EVACUATION=1`, with `PERRY_GC_DIAG=1`
+confirming thousands of protected from-space retirements actually ran.
+
+Known non-firing shapes: both halves of a deforested producer/consumer pair (the
+producer's `const out = []` becomes a `__deforest_out` parameter; the consumer's
+literal has no push in its own region).