peephole: step past non-A ops in remove_dead_loads
`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.
2026-04-19 01:43:58 +00:00
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