perf(gc, codegen): recover the instruction cost of the 56 B → 48 B header shrink (#8122)

.github/workflows/test.yml

@@ -982,6 +982,26 @@ jobs:
             done
           fi
 
+      # #8113 — the perry-ffi <-> perry-runtime ABI mirror, which had NEVER
+      # EXECUTED. `perry_ffi::types::layout_tests` is
+      # `#[cfg(all(test, feature = "runtime-link"))]`, `runtime-link` was
+      # enabled nowhere in `.github/`, and the scope loop above runs
+      # `cargo test -p perry-ffi` with DEFAULT features — so the module never
+      # even compiled. Deleting a mirrored field still went red (an `offset_of!`
+      # on a missing field stops compiling), but a SIZE or PADDING divergence
+      # between the two structs was invisible, which is precisely the failure
+      # mode of a header-layout change.
+      #
+      # Unconditional, not scope-gated: perry-ffi's optional dependency on
+      # perry-runtime means a runtime-only diff need not pull perry-ffi into
+      # scope, and this is the one check that says the published ABI mirror
+      # still matches the runtime it mirrors.
+      - name: perry-ffi ABI mirror matches the runtime (#8113)
+        env:
+          CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS: "-C linker-features=-lld"
+          CARGO_PROFILE_TEST_DEBUG: "0"
+        run: cargo test -p perry-ffi --features runtime-link --lib
+
   # ---------------------------------------------------------------------------
   # Scoped e2e: run the integration suites NAMED BY THE DIFF (#5960)
   #

TYPE_LOWERING.md

@@ -614,7 +614,7 @@ can bypass part of the generic NaN-boxing overhead:
 
 ### `ObjectHeader` Layout
 
-Every heap object has: `object_type` (u32), `class_id` (u32), `field_count` (u32), `keys_array` pointer. Inline property slots follow immediately in memory.
+Every heap object has: `class_id` (u32), `parent_class_id` (u32, which carries the runtime `ShapeId` once stamped), `keys_array` pointer, `meta` pointer — 24 bytes on LP64, 16 on ILP32. Inline property slots follow immediately in memory. (#8113 removed the `object_type` and `field_count` words: the receiver kind comes from `GcHeader.obj_type` plus the ShapeId descriptor's `object_kind`, and the live inline-slot bound from the same descriptor's `live_inline_slot_count`.)
 
 - **Shape caching**: Objects with the same key set share a `keys_array` pointer.
 - **`KEYS_INDEX`**: FNV-1a hash map built when `keys_array.length > 32` for O(1) lookup.