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performance.md: remove — all six wins shipped on this branch
Anchored on commit33640f8; the milestone is closed: #1 33640f8..0b5470b codegen: skip leaf prologue spill #4 0b5470b..726faef sha256: specialize rotr_wk per amount #2 726faef..0600f5b codegen: fuse compare-then-branch #3df71c2bpeephole: drop dead LDA #imm #6f2623cbsha256/computing: track byte offsets directly #54afd196ir: inline-asm {param} substitution after splice6696d79codegen+ir: code-review followups (UTF-8, leaf, tests) The how-it-works writeups live in each commit's message. The explanatory inline comments in `IrCodeGen::new` (leaf detection), `gen_block` (cmp/branch fusion), `peephole.rs::remove_dead_loads` (JMP-following), `try_inline_call_stmt` (inline-asm const-arg constraint), and the new `function_is_leaf_detects_jsr_emitting_ops` + `inline_fun_with_asm_param_*` tests are the ongoing reference. https://claude.ai/code/session_01FRmSBruVWCufm3LsUVMs8v
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performance.md
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performance.md
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# Performance work
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Six performance wins surfaced by the SHA-256 example's inner-loop
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analysis. Working through them on this branch; this file is a
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scratchpad for the milestone and gets deleted in the final commit
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once everything is shipped.
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The numbers are per 64-byte SHA-256 block on the example, where
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the baseline is ~550K cycles ≈ 18 NTSC frames per block.
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## #1 — Skip parameter spill in leaf functions
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**Status**: TODO
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**Where**: `src/codegen/ir_codegen.rs`
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**Estimate**: ~66K cycles/block, ~2.2 frames
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Every function currently opens with `LDA $04 / STA <local>` to
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spill the param transport slots into a per-function RAM slot,
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defending against nested calls that would re-clobber `$04-$07`.
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Functions that never `JSR` from inside their body are leaves —
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the spill is dead work for them.
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Fix: in `IrCodeGen::new`, scan each function for an `IrOp::Call`.
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Functions with none get their parameters mapped directly to the
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transport slots `$04-$07`, and `gen_function` skips emitting the
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spill prologue.
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## #2 — Direct-branch comparisons (drop bool materialization)
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**Status**: TODO
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**Where**: `src/codegen/ir_codegen.rs` (`Lt`/`Gt`/`Eq`/etc. lowering)
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**Estimate**: ~9K cycles/block, ~0.3 frames
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`if x < N` currently lowers to "compute the result as 0 or 1 in A,
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then `BEQ` on it":
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```
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LDA x ; 3
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CMP #N ; 2
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BCC cmp_t ; 2/3
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LDA #0 ; 2
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JMP cmp_e ; 3
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cmp_t: LDA #1 ; 2
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cmp_e: BEQ end ; 3
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```
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The canonical 6502 idiom is one `CMP` + one branch (8 cycles vs.
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~16). The IR already gives us the false target label — we just
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need to teach `gen_op` for the comparison ops to branch directly
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when their result feeds straight into a conditional.
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## #3 — Drop dead `LDA #imm` before `INC`/`DEC`
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**Status**: TODO
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**Where**: `src/codegen/peephole.rs`
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**Estimate**: ~5K cycles/block, ~0.2 frames
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`i += 1` currently emits:
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```
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LDA #1 ; 2 — A is overwritten by the next op
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INC ZeroPage(i) ; 5
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```
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The `LDA #1` is dead. A peephole rule "drop `LDA #imm` if A is
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re-written or never read before the next `LDA`/branch/RTS" should
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catch this. The same rule fires elsewhere (any `+= const` /
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`-= const` that strength-reduced to INC/DEC).
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## #4 — Specialize `rotr_wk` per amount (.ne refactor)
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**Status**: TODO
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**Where**: `examples/sha256/sha_core.ne`
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**Estimate**: ~45K cycles/block, ~1.5 frames
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`rotr_wk(dst, n)` is a generic loop wrapper. Every SHA-256
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rotation amount is a compile-time constant (2, 6, 7, 11, 13, 17,
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18, 19, 22, 25), so the loop body is wasted work — the compiler
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can't see through the runtime `n`.
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Fix: declare one `rotr_wk_<N>(dst)` per used amount, each calling
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the appropriate sequence of `byte_rotr_wk` and `rotr1_wk`. The
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sigma helpers swap `rotr_wk(SIG, 6)` → `rotr_wk_6(SIG)`. The
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loop wrapper stays available for any future caller that needs a
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runtime amount.
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## #5 — Inline-asm `{param}` substitution after `inline fun` splice
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**Status**: DEFERRED
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**Where**: `src/ir/lowering.rs::try_inline_call_stmt` +
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`src/codegen/ir_codegen.rs::substitute_asm_vars`
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**Estimate**: ~45K cycles/block (potential), ~1.5 frames
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Marking primitives `inline fun` would eliminate JSR + RTS + the
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rest of the call apparatus — but the inline-asm `{param}`
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substitution today resolves names against the analyzer's per-
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function allocation table, which doesn't see the inline frame.
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A spliced `cp_wk(32, 28)` ends up emitting `LDX {dst}` against
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the *caller's* scope where `dst` doesn't exist.
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Properly fixing this is non-trivial: substitution needs to be
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addressing-mode-aware (immediate `#$20` vs. zero-page `$27` vs.
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absolute) and depends on whether the inline arg is a constant or
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a runtime value. Documenting the design here so a future pass can
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take it.
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The simpler half of the win: at inline expansion time, build a
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per-frame map from param-name → arg-temp. Pass that map down to
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`gen_op`'s asm handler, which substitutes `{name}` with the
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arg-temp's allocated slot instead of the param's address. For
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constant args, fold further to immediates. This is ~150 lines of
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Rust; deferring to a follow-up.
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## #6 — Const-fold `r << 2` style index math
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**Status**: TODO
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**Where**: `src/optimizer/mod.rs`
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**Estimate**: <1K cycles/block, negligible
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`round_one(r << 2)` recomputes the shift on every iteration of the
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phased compression driver. The optimizer already folds shifts when
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both operands are constant; extending it to fold "shift a constant
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by a constant" inside the IR would catch this case. Trivial in
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cycles but worth the cleanliness.
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---
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## After-each-change checklist
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For every codegen change, verify:
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- [ ] `cargo fmt --check`
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- [ ] `cargo clippy --all-targets -- -D warnings` on rustc 1.95.0
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- [ ] `cargo test --all-targets` passes
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- [ ] Rebuild every committed `examples/*.nes`
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- [ ] Run the emulator harness — if any golden drifts, eyeball the
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diff and update with `UPDATE_GOLDENS=1`
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- [ ] Regenerate `docs/{platformer,war,pong}.gif` if any of those
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examples' captured frames changed
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For each `.ne` change:
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- [ ] Just the example-side checks (rebuild ROM, harness, gif).
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