`remove_dead_loads` now scans past opcodes that touch neither A nor
the flags an LDA sets, so a redundant LDA gets caught by its
successor's overwrite even when an index load or counter bump sits
between them. The extension covers LDX/LDY/INX/INY/DEX/DEY and the
flag ops (CLC/SEC/CLI/SEI/CLD/SED/CLV) alongside the INC/DEC/STX/STY
opcodes the pass already stepped past.
The highest-leverage case is the shape every single-tile `draw`
emits. After copy propagation and dead-store elimination do their
work, the stream reads:
LDA #<y> ; stray producer, value never consumed
LDY oam_cursor
LDA #<y> ; real load before STA
STA $0200,Y
The first LDA was surviving because the pass bailed on the LDY.
With the step-past, it drops. One LDA gone per draw, 2 bytes each.
Measured LDA-count reduction on committed examples:
platformer 242 → 221 (-21, -8.7 %)
war 785 → 754 (-31, -4.0 %)
pong 843 → 827 (-16, -1.9 %)
**Audio goldens.** The cycle savings shift the main-loop/NMI boundary
in audio-emitting programs, which re-times which frame each SFX
trigger lands in. Six audio hashes re-baseline as a result:
audio_demo, friendly_assets, noise_triangle_sfx, platformer, pong,
war. All 50 PNG goldens, the platformer/war/pong demo gifs, and
every non-audio program stay byte-identical. The re-baselined
output is still sample-accurate; what changed is the first-SFX
offset within the captured 132 084-sample window. This is the
audio-shift tradeoff documented in future-work.
Two new peephole unit tests lock in the behaviour:
- `dead_load_elim_steps_past_ldx_ldy` — the DrawSprite shape folds.
- `dead_load_elim_preserves_lda_when_used_by_shift` — a subsequent
ASL on A keeps the LDA alive across an intervening LDY.
Also updates future-work.md to reflect the shipped change and the
remaining register-allocator wins worth chasing next.
Closes §H. 2×2 metatiles and a parallel collision map are now a
first-class construct. `metatileset Name { metatiles: [{ id, tiles,
collide }, ...] }` declares a library of 2×2 tile bundles. `room Name
{ metatileset: M, layout: [...] }` lays them out on a 16×15 grid. The
compiler expands each room at compile time into:
- a 960-byte nametable (`__room_tiles_<name>`)
- a 64-byte attribute table (`__room_attrs_<name>`)
- a 240-byte collision bitmap (`__room_col_<name>`)
`paint_room Name` reuses the vblank-safe `load_background` update
machinery for the nametable blit and installs the collision bitmap
pointer into `ZP_ROOM_COL_LO`/`ZP_ROOM_COL_HI` (ZP $18/$19).
`collides_at(x, y)` JSRs into a small runtime helper that reads
`(room_col),Y` with `Y = (y & 0xF0) | (x >> 4)` and returns 0/1.
The helper links in only when the `__collides_at_used` marker is
emitted, so programs that declare a room but never query it pay
zero bytes for the subroutine.
`parse_byte_array` grows a `[value; count]` shortcut — 240-entry
`layout` arrays are unwieldy to spell out a byte at a time.
See `examples/metatiles_demo.ne` for the end-to-end flow: a probe
sprite bounces off walls via `collides_at` and lands on the left
side of the playfield at frame 180 — direct evidence that the
collision query works.
Also defers the register-allocator work from §"Code quality /
tooling" and documents the audio-goldens constraint in future-work
so the next agent sees it.
