release: cut v2.1.8 "Fathom" (performance — "Tempo")

.github/release-notes/v2.1.8.md

@@ -0,0 +1,26 @@
+# RustyNES v2.1.8 — "Fathom" (performance — "Tempo")
+
+The **performance** step of the v2.1.5 → v2.2.0 "deepen the existing project" run — profile-first optimization of the hot render + display paths, every change proven byte-identical (or shipped default-off behind a runtime knob). The shipped default build is **byte-identical**: **AccuracyCoin 141/141 (100.00%)**, nestest 0-diff, `visual_regression` / `pal_apu_tests` unchanged. Two PRs (#284, #286).
+
+## Performance
+
+### Specialized visible-scanline fast PPU dot path (A1) — #284
+
+Profiling (`perf` on a mixed corpus) put `Ppu::tick` at **46.5% of frame self-time** — the real hot spot. A1 adds a straight-line fast path for the common visible-scanline case (visible scanline, dots 1..256, rendering stably enabled, no sub-dot disturbance), running the identical helper sequence with the statically-dead event branches pruned; the instant anything sub-scanline happens (a `$2000/$2001/$2005/$2006/$2007` write, a `$2007` read during render, a sprite-0-hit dot, rendering toggling, an armed OAMADDR corruption from the v2.1.7 `Rp2c02G` model, etc.) it **drops to the exact 2-cycle-ALE loop**.
+
+- **Measured: ~12% faster** on the rendering-heavy `nes_run_frame_nestest` (4.54 → 3.98 ms/frame); neutral on the rendering-disabled path.
+- **Byte-identity is the contract, and it's proven:** a new `fast_dotloop_diff` differential test runs the corpus (nestest, flowing_palette, oam_stress, AccuracyCoin, MMC1/MMC3 Holy Mapperel, the scanline demo — and a dedicated `Rp2c02G` OAMADDR-corruption dimension) through **both** the fast and exact paths and asserts **bit-identical** framebuffer + palette-index buffer + audio + CPU cycles + full snapshot, every frame.
+- **Shipped default-OFF** (opt-in `Nes::set_fast_dotloop`) — as the roadmap's highest-risk item, it's held off the shipped default this cut so the build stays byte-identical, with promotion-to-default recommended after review + a clean-host Criterion confirmation.
+- Honest architectural finding: a *whole-scanline batch* is precluded by the v2.0.0 lockstep every-cycle-bus-access scheduler (the PPU is never invited to run a scanline uninterrupted), so this is a per-dot specialization, not a dot-batch.
+
+### Vectorized software blitter + wasm size/startup (A2/A4) — #286
+
+- **A2 — SIMD software blitter** (`crates/rustynes-frontend/src/gfx_blit.rs`): scalar / `blit_u32` / `wide::u32x8` (desktop) / `core::arch::wasm32` `v128` (`+simd128`) / scalar-fallback paths for the palette-index → RGBA conversion, all asserted **SIMD == scalar byte-identical** (`simd_equals_scalar_byte_identical`, sweeping all 512 LUT entries for both the composite-2C02 and RGB-2C05 LUTs). Honest measured result: the conversion is a **memory-bound LUT gather** (~19 GiB/s, the box's single-thread DRAM ceiling), so no SIMD path clears the >3% adoption bar — the dispatcher **keeps scalar-`u32` as the default** and the SIMD variants are validated + available but not forced. (The NTSC ladders are GPU WGSL shaders, so there's no hot CPU display transform there to vectorize.)
+- **A4 — wasm size/startup**: `wasm-opt -O4` (SIMD + bulk-memory preserved) shrinks the wasm-bindgen output 12.7 → 11.1 MiB raw; streaming instantiation documented. Real `trunk build --release`: **3.99 MiB gzip total — 1.01 MiB headroom under the 5 MiB budget.**
+
+## Notes
+
+- Version: workspace `2.1.7 → 2.1.8` (Cargo.lock refreshed). No dependency or mobile changes (the mobile apps re-release at v2.3.0).
+- **No default-build behavior change.** The fast dot path is default-off (and differential-tested bit-identical when on); the SIMD blitter is byte-identical to scalar and stays non-default. AccuracyCoin stays **141/141 (100.00%)**, nestest 0-diff, `visual_regression` byte-identical, save-state and the 60-ROM commercial oracle unchanged.
+- Docs updated in-change: `docs/performance.md` (the profiled fast-path design + measured before/after + the A2/A4 tables), `docs/ppu-2c02.md`, `docs/frontend.md`. New `fast_dotloop_diff` + `gfx_blit` tests.
+- Full host gate green: fmt · clippy (native all-targets + feature combos + all wasm32 targets incl. `+simd128`) · rustdoc (`-D warnings`) · `no_std` cross-compile (`thumbv7em`) · markdownlint (pinned v0.39.0) · the CPU/PPU + `--features test-roms` suites (incl. `fast_dotloop_diff` 3/3, `gfx_blit` 3/3) · a real `trunk build --release` under the gzip budget · version consistency.

CHANGELOG.md

@@ -14,6 +14,8 @@ cycle-accurate core later replaced.
 
 ## [Unreleased]
 
+## [2.1.8] - 2026-07-12 - "Fathom" (performance — default-off specialized fast PPU dot-loop (differential-tested byte-identical, +12% rendering-heavy) + SIMD software blitter + wasm size/startup pass; "Tempo")
+
 ### Performance
 
 - **Specialized visible-scanline fast dot path (v2.1.8 "Performance" A1;

Cargo.toml

@@ -21,7 +21,7 @@ members = [
 ]
 
 [workspace.package]
-version = "2.1.7"
+version = "2.1.8"
 edition = "2024"
 rust-version = "1.96"
 license = "MIT OR Apache-2.0"

README.md

@@ -9,7 +9,7 @@
 </p>
 
 <p align="center">
-  <a href="https://github.com/doublegate/RustyNES/actions"><img src="https://github.com/doublegate/RustyNES/workflows/CI/badge.svg" alt="Build Status"></a> <a href="#license"><img src="https://img.shields.io/badge/license-MIT%20OR%20Apache--2.0-blue.svg" alt="License: MIT OR Apache-2.0"></a> <a href="https://github.com/doublegate/RustyNES/releases"><img src="https://img.shields.io/badge/version-v2.1.7-blue.svg" alt="Version"></a> <a href="rust-toolchain.toml"><img src="https://img.shields.io/badge/rust-1.96-orange.svg" alt="Rust: 1.96"></a><br>
+  <a href="https://github.com/doublegate/RustyNES/actions"><img src="https://github.com/doublegate/RustyNES/workflows/CI/badge.svg" alt="Build Status"></a> <a href="#license"><img src="https://img.shields.io/badge/license-MIT%20OR%20Apache--2.0-blue.svg" alt="License: MIT OR Apache-2.0"></a> <a href="https://github.com/doublegate/RustyNES/releases"><img src="https://img.shields.io/badge/version-v2.1.8-blue.svg" alt="Version"></a> <a href="rust-toolchain.toml"><img src="https://img.shields.io/badge/rust-1.96-orange.svg" alt="Rust: 1.96"></a><br>
   <a href="#compatibility-and-accuracy"><img src="https://img.shields.io/badge/AccuracyCoin-100%25%20(141%2F141)-brightgreen.svg" alt="AccuracyCoin"></a> <a href="#compatibility-and-accuracy"><img src="https://img.shields.io/badge/nestest-0--diff-brightgreen.svg" alt="nestest"></a> <a href="https://doublegate.github.io/RustyNES/"><img src="https://img.shields.io/badge/play-in%20browser-success.svg" alt="Try in browser"></a><br>
   <a href="#platform-support"><img src="https://img.shields.io/badge/platform-Windows%20%7C%20Linux%20%7C%20macOS%20%7C%20Web%20%7C%20Android%20%7C%20iOS-lightgrey.svg" alt="Platform"></a>
 </p>
@@ -775,16 +775,17 @@ and the Material-for-MkDocs documentation handbook at
 
 ## Current Release
 
-RustyNES's current release is **v2.1.7 "Fathom" ("Stepping")**, the hardware-revisions
-step of the v2.1.5 → v2.2.0 "deepen the project" run — it adds an opt-in, default-off
-**selectable PPU die-revision** model (2C02 rev-E+ OAMADDR/`$2003` corruption à la *Huge
-Insect*, power-up palette variants) and a **power-on RAM model** (all-zero default, opt-in
-seeded/patterned — for games that read uninitialized RAM like *Final Fantasy*), plus an
-opt-in **2A03 die-revision** (`Rp2A03G`/`Rp2A03H`) config that gates the "unexpected read"
-DMA behavior. The last is the honest **DMA frontier** work: direct instrumentation proved
-the die-revision DMA gate is a **no-op on every committed oracle ROM** (and no reference
-emulator branches DMA on die stepping), so the residual is documented in **ADR 0033**
-rather than faked. Every toggle defaults off, so the shipped build stays **byte-identical**:
+RustyNES's current release is **v2.1.8 "Fathom" ("Tempo")**, the performance step of the
+v2.1.5 → v2.2.0 "deepen the project" run — it adds a **specialized visible-scanline fast
+PPU dot path** (a straight-line renderer for clean dots that drops to the exact 2-cycle-ALE
+loop the instant any sub-scanline disturbance occurs; **~12% faster** on rendering-heavy
+frames), a set of **SIMD-validated software palette→RGBA blitter paths** (scalar / `wide` /
+wasm `+simd128`, all proven byte-identical — but the conversion is a **memory-bound LUT
+gather**, so no SIMD path clears the >3% bar and **scalar stays the default**; the vector
+paths are validated and available, not a shipped speedup), and a **wasm size/startup pass**
+(`wasm-opt -O4` → 3.99 MiB gzip, comfortably under budget). The fast dot path is
+**default-off** and **differential-tested bit-identical** to the exact path across the
+corpus (framebuffer + audio + cycles + snapshot) — so the shipped build stays byte-identical:
 **AccuracyCoin stays 141/141 (100.00%)**, nestest 0-diff, `visual_regression` /
 `pal_apu_tests` unchanged.
 
@@ -912,7 +913,7 @@ If you use RustyNES in academic research, please cite:
   author  = {RustyNES Contributors},
   title   = {RustyNES: A Cycle-Accurate NES Emulator in Rust},
   year    = {2026},
-  version = {2.1.7},
+  version = {2.1.8},
   url     = {https://github.com/doublegate/RustyNES},
   note    = {Cycle-accurate NES emulator on a master-clock-precise scheduler;
              AccuracyCoin 100\% (141/141), nestest 0-diff; 172 mapper families,