codegen: fuse compare-then-branch to drop boolean materialization

Every NEScript condition (`if x < N`, `while i < end`, etc.) lowers in two IR ops: `CmpX(d, a, b)` materializes a 0/1 boolean into temp `d`, and the block's terminator `Branch(d, t, f)` reads `d` and branches on it. The codegen faithfully emitted both halves — `LDA / CMP / branch-to-true / LDA #0 / JMP done / true: LDA #1 / done:`, then later `LDA d_slot / BNE branch_t / JMP branch_f` — about 14 cycles + 13 bytes per condition. The 6502's natural pattern is one `CMP` + one branch on the flags it just set: 8 cycles, no register-clobber, no temp slot. Detect the canonical pattern in `gen_block` (last op is an 8-bit `CmpX` whose dest temp is what the terminator branches on, with no other uses) and emit the fused form directly via a new `gen_cmp_branch` helper. The temp's allocation, store, load, and the terminator's branch fall away. Bookkeeping subtlety: the source temps `a`/`b` must be retired *after* the fused emit, not before — the original `gen_op` order is "emit body of op, then `retire_op_sources`". Decrementing their use counts before the CMP would free their slots while they were still live; `load_temp(a)` would then re-allocate `a` to whatever stale slot the free list popped next. Got hit by this on the first attempt — the SHA-256 example dutifully returned all-zero hashes until the order was fixed. Updated `ir_codegen_local_label_suffix_is_bank_namespaced`: the test was relying on `if x == 0` to emit `__ir_cmp_*` labels for its bank-namespacing check, which the fusion now collapses into direct branches. Switched the test source to a shift-by-variable pattern (`x = x << n`), which always emits `__ir_shift_loop_*` labels regardless of future cmp/branch optimizations. Cycle savings: ~6 cycles per condition. The SHA-256 rotate loops alone account for ~9K cycles per block. Across all examples the cycle drift shows up as audio-tick phase shifts in five timing-sensitive ROMs (`audio_demo`, `friendly_assets`, `noise_triangle_sfx`, `platformer`, `sfx_pitch_envelope`); the goldens for those are refreshed in this commit, plus `platformer.gif` (the only demo gif whose bytes actually moved). Verified: cargo test/clippy/fmt clean on rustc 1.95.0; emulator harness 34/34; reproducibility diff clean; SHA-256 of "NES" still computes to AE9145DB…4E0D. https://claude.ai/code/session_01FRmSBruVWCufm3LsUVMs8v
2026-07-08 08:55:38 +00:00 · 2026-04-16 17:00:07 +00:00 · 2026-04-16 17:00:07 +00:00 · 0600f5b872
commit 0600f5b872
parent 726faef538
38 changed files with 184 additions and 20 deletions
--- a/examples/arrays_and_functions.nes
+++ b/examples/arrays_and_functions.nes
--- a/examples/audio_demo.nes
+++ b/examples/audio_demo.nes
--- a/examples/bitwise_ops.nes
+++ b/examples/bitwise_ops.nes
--- a/examples/bouncing_ball.nes
+++ b/examples/bouncing_ball.nes
--- a/examples/coin_cavern.nes
+++ b/examples/coin_cavern.nes
--- a/examples/comparisons.nes
+++ b/examples/comparisons.nes
--- a/examples/friendly_assets.nes
+++ b/examples/friendly_assets.nes
--- a/examples/function_chain.nes
+++ b/examples/function_chain.nes
--- a/examples/logic_ops.nes
+++ b/examples/logic_ops.nes
--- a/examples/loop_break_continue.nes
+++ b/examples/loop_break_continue.nes
--- a/examples/match_demo.nes
+++ b/examples/match_demo.nes
--- a/examples/metasprite_demo.nes
+++ b/examples/metasprite_demo.nes
--- a/examples/mmc1_banked.nes
+++ b/examples/mmc1_banked.nes
--- a/examples/mmc3_per_state_split.nes
+++ b/examples/mmc3_per_state_split.nes
--- a/examples/noise_triangle_sfx.nes
+++ b/examples/noise_triangle_sfx.nes
--- a/examples/palette_and_background.nes
+++ b/examples/palette_and_background.nes
--- a/examples/platformer.nes
+++ b/examples/platformer.nes
--- a/examples/pong.nes
+++ b/examples/pong.nes
--- a/examples/sfx_pitch_envelope.nes
+++ b/examples/sfx_pitch_envelope.nes
--- a/examples/sha256.nes
+++ b/examples/sha256.nes
--- a/examples/state_machine.nes
+++ b/examples/state_machine.nes
--- a/examples/structs_enums_for.nes
+++ b/examples/structs_enums_for.nes
--- a/examples/two_player.nes
+++ b/examples/two_player.nes
--- a/examples/uxrom_banked.nes
+++ b/examples/uxrom_banked.nes
--- a/examples/uxrom_banked_to_banked.nes
+++ b/examples/uxrom_banked_to_banked.nes
--- a/examples/uxrom_user_banked.nes
+++ b/examples/uxrom_user_banked.nes
--- a/examples/war.nes
+++ b/examples/war.nes