sprite-per-scanline: add cycle_sprites runtime flicker + debug telemetry
W0109 (shipped last commit) catches the 8-sprites-per-scanline
hardware limit at compile time for static layouts, but the
dynamic case — enemy formations, projectile clusters, animated
NPCs where coordinates come from variables — was still silent.
This change adds two layers of defense on top of W0109:
Layer 2: `cycle_sprites` runtime flicker intrinsic
New keyword statement that rotates the OAM DMA start offset
one slot per call. When called once per `on frame`, the PPU's
sprite evaluation picks up a different subset of the 12+
overlapping sprites each frame, so the permanent-dropout
failure mode becomes visible flicker — the classic NES
technique used by Gradius, Battletoads, and every shmup.
Implementation:
- Lexer keyword `KwCycleSprites` and parser production.
- AST `Statement::CycleSprites(Span)`.
- `IrOp::CycleSprites` lowered by the IR pass.
- Codegen emits `LDA $07EF / CLC / ADC #4 / STA $07EF` with
natural u8 wrap, plus a one-shot `__sprite_cycle_used`
marker label the first time it fires.
- Linker detects the marker and switches `gen_nmi` to the
cycling variant, which reads the rotating offset from
`$07EF` into OAM_ADDR before the DMA instead of writing
a literal 0. Programs that don't call `cycle_sprites`
skip the marker and get byte-identical ROM output.
Layer 3: debug-mode sprite overflow telemetry
Mirrors the frame-overrun pair (`debug.frame_overrun_count` /
`debug.frame_overran`). In debug builds the NMI handler reads
`$2002` at the top of vblank, masks bit 5 (the PPU's sprite
overflow flag), and if set bumps a cumulative counter at
`$07FD` plus a sticky bit at `$07FC`. The sticky bit clears
on every `wait_frame`.
New debug builtins:
- `debug.sprite_overflow_count()` → u8 peek of $07FD
- `debug.sprite_overflow()` → u8 peek of $07FC (sticky bit)
The hardware flag has well-known quirks but is correct for
the overwhelming majority of cases and costs ~15 cycles per
frame to sample. Release builds emit no overflow-check code
at all, so the four bytes at `$07EF` / `$07FC`-`$07FD` stay
free for user allocation.
Related changes:
- `gen_nmi` now takes an `NmiOptions` struct. Four bool
parameters tripped clippy's `fn_params_excessive_bools`.
- CLI `build` now renders analyzer warnings on a successful
build. Previously warnings were silently dropped unless
the user also ran `nescript check`, which made W0109
effectively invisible to CI and local dev alike. Existing
pre-existing W0103 / W0106 warnings on `coin_cavern`,
`mmc3_per_state_split`, `sprites_and_palettes` surface
too — not regressions, just now visible.
New example: `examples/sprite_flicker_demo.ne`
Draws 12 sprites into a 4-pixel band, W0109 fires at compile
time with nine labels pointing at the offenders, and a
`cycle_sprites` call at the end of `on frame` turns the
hardware dropout into flicker. The committed emulator golden
captures one frame of the cycling pattern (deterministic).
Tests:
- `runtime::tests::nmi_debug_mode_samples_sprite_overflow`
- `runtime::tests::nmi_sprite_cycle_variant_reads_rotating_offset`
- `ir_codegen::*::debug_sprite_overflow_count_loads_07fd`
- `ir_codegen::*::debug_sprite_overflow_flag_loads_07fc`
- `ir_codegen::*::wait_frame_clears_sprite_overflow_sticky_in_debug_mode`
- `ir_codegen::*::wait_frame_release_does_not_touch_sprite_overflow_sticky`
- `ir_codegen::*::cycle_sprites_emits_marker_and_add4`
- `ir_codegen::*::cycle_sprites_marker_dedup_across_multiple_calls`
- `ir_codegen::*::program_without_cycle_sprites_emits_no_marker`
- `analyzer::*::accepts_debug_sprite_overflow_builtins`
- `analyzer::*::rejects_unknown_debug_method_lists_all_four_known_names`
- `analyzer::*::accepts_cycle_sprites_statement`
Docs: `examples/war/COMPILER_BUGS.md` §4 now describes all three
layers (W0109, `cycle_sprites`, debug telemetry) with reasoning
for when each applies. `README.md` and `examples/README.md` add
the new example to their tables.
All 32 emulator goldens still match — the cycling is opt-in
and programs that don't call `cycle_sprites` or enable debug
mode are byte-identical to the pre-change output.
https://claude.ai/code/session_0143dTgh3UeRrtfHgQwzcv5z
2026-04-15 22:07:19 +00:00
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