test(bare-metal): rusty_zstd round-trips on an ESP32-S3

.gitignore

@@ -22,3 +22,7 @@
 # `rustc scripts/cpuprobe.rs` drops its binary wherever it is run from.
 /cpuprobe.exe
 /cpuprobe.pdb
+
+# The bare-metal firmware builds into its own target dir (it is not a
+# workspace member, so the root /target rule does not cover it).
+bare-metal/*/target/

CHANGELOG.md

@@ -6,6 +6,33 @@ based on [Keep a Changelog](https://keepachangelog.com/); this project uses
 
 ## [Unreleased]
 
+### Added -- the bare-metal claim, run on silicon (ESP32-S3)
+
+CI proves rusty_zstd COMPILES for Cortex-M4F and RV32. That is a different
+claim from running, so this one was run: `bare-metal/esp32s3` is a `no_std +
+alloc` firmware that compresses and decompresses on the part and compares the
+result byte for byte against the source.
+
+```
+census64::CENSUS_LIVE = false  (false is expected here: no 64-bit atomics)
+source            8260 bytes
+L1  8260 ->  468 bytes  (17.65x)  round trip OK
+L3  8260 ->  426 bytes  (19.39x)  round trip OK
+L5  8260 ->  263 bytes  (31.41x)  round trip OK
+RESULT: PASS -- compressed and decompressed on the board
+```
+
+Three levels because they are three different match finders: Fast, DFast and
+Greedy. Xtensa LX7 is 32-bit, so every census counter on that part is the
+`census64` stub -- which makes the board the exact configuration the fix
+created, and the `PASS` the evidence that stubbing the instrument left the
+codec alone.
+
+It is excluded from the workspace and hand-run: it needs Espressif's Rust fork,
+which CI does not have. It claims no timing and no heap floor; 192 KiB was
+enough for 8 KiB of source at levels 1 to 5, and the crate sizes its tables
+from the source length. See `bare-metal/esp32s3/README.md`.
+
 ### Fixed -- the census counters cost the crate BARE METAL; they no longer do
 
 `cargo check -p rusty_zstd --no-default-features --features alloc` failed with

Cargo.toml

@@ -7,6 +7,11 @@ members = [
     "crates/rzstd-alloc",
 ]
 
+# The on-board proof (bare-metal/esp32s3) is a firmware crate for Xtensa: it
+# needs Espressif's Rust fork and its own linker script, so it must not be a
+# workspace member. Excluded, a plain `cargo build` at the root never sees it.
+exclude = ["bare-metal/esp32s3"]
+
 [workspace.package]
 version = "0.2.5"
 edition = "2021"

README.md

@@ -370,6 +370,7 @@ Fetch the oracle with `pwsh scripts/fetch-oracle.ps1`, or point
 | `no_std + alloc` | ✅ builds |
 | `thumbv7em-none-eabihf` (Cortex-M4F) | ✅ builds (`no_std + alloc`) |
 | `riscv32imac-unknown-none-elf` | ✅ builds (`no_std + alloc`) |
+| ESP32-S3 (Xtensa LX7) | ✅ **round-trips on the board** ([proof](bare-metal/esp32s3/README.md)) |
 
 AVX2 and NEON kernels are selected at **runtime**. On a CPU without them, the
 scalar twins run and the output is identical.

bare-metal/esp32s3/.cargo/config.toml

@@ -0,0 +1,15 @@
+[target.xtensa-esp32s3-none-elf]
+runner = "espflash flash --monitor "
+rustflags = [
+  "-C", "link-arg=-nostartfiles",
+  "-C", "link-arg=-Wl,-Tlinkall.x",
+]
+
+[build]
+target = "xtensa-esp32s3-none-elf"
+
+[env]
+ESP_LOG = "info"
+
+[unstable]
+build-std = ["core", "alloc"]