`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 the §A follow-up gap: ordering compares (`<`, `<=`, `>`, `>=`)
on signed integer types now use the canonical 6502 `CMP / SBC / BVC /
EOR #$80` overflow-correction idiom so the N flag reflects the true
sign of the difference, instead of the previous BCC/BCS-based path
that always treated `$FFxx` as greater than `$00yy`.
The same change also fixes narrow-to-wide widening: assigning a
runtime `i8` expression to an `i16` variable now sign-extends the
high byte via a new `IrOp::SignExtend` op instead of zero-extending
it, so `var w: i16 = some_i8_neg` round-trips negative values.
The lowerer tracks signedness on each IR temp (analogous to the
existing `wide_hi` map) and threads it onto the new `Signedness`
field of `CmpLt`/`CmpGt`/`CmpLtEq`/`CmpGtEq` and their 16-bit
variants. The optimizer's constant-folder uses the same flag to
fold compares correctly under either signedness. Casts to `u8`/`u16`
strip the signed flag so an explicit `as` opt-out stays unsigned.
`examples/signed_compare.ne` exercises both bit widths through the
emulator harness — the four pip sprites at the top of the screen
show three lit (signed-correct) and one dark (would only light if
the compare regressed to unsigned semantics).
