12-a-new-device-language

12 — A device language the engine has never heard of

A rule package teaches mcpp to compile .toy, a small language with its own compiler, and a project uses it. No mcpp release is involved: .toy is not in the engine's built-in device-extension table and will never be.

cd examples/12-a-new-device-language/app
mcpp run

       Rules example.rules.toy (example:rules-toy)
    Building host tool toyc:toyc from toyc v0.1.0 (once per package version × host toolchain)
   Compiling toyapp v0.1.0 (.)
    Finished dev [unoptimized + debuginfo] in 0.65s

gcd(1071, 462) = 21
scale(21, 2)   = 42
answer()       = 42

The three packages

directory	what it is	who builds it
toyc/	the compiler for .toy: a lexer, a recursive-descent parser, semantic checks and a C++ emitter	mcpp, for the build machine, as a host tool
rules-toy/	the rule: it declares the extension, the module a consumer imports, and one action per .toy	the consumer's build program
app/	the project	mcpp, for the target

The language

.toy has integers, let, assignment, if / else, while, calls between kernels, and the arithmetic and comparison operators. Every kernel takes and returns an integer.

program := { kernel }
kernel  := 'kernel' ident '(' [ ident { ',' ident } ] ')' block
block   := '{' { stmt } '}'
stmt    := 'let' ident '=' expr ';'
         | ident '=' expr ';'
         | 'return' expr ';'
         | 'if' '(' expr ')' block [ 'else' block ]
         | 'while' '(' expr ')' block
expr    := cmp
cmp     := sum { ('<' | '>' | '<=' | '>=' | '==' | '!=') sum }
sum     := term { ('+' | '-') term }
term    := unary { ('*' | '/' | '%') unary }
unary   := [ '-' ] primary
primary := number | ident | ident '(' [ expr { ',' expr } ] ')' | '(' expr ')'

src/kernels/answer.toy is Euclid's algorithm and a caller:

kernel gcd(a, b) {
    while (b != 0) {
        let t = b;
        b = a % b;
        a = t;
    }
    return a;
}

kernel answer() {
    return scale(gcd(1071, 462), 2);
}

toyc emits one extern "C" function per kernel, and forward-declares them all first so kernels may call each other in any order:

extern "C" int toy_gcd(int v_a, int v_b) {
    while ((v_b != 0)) {
        int v_t = v_b;
        v_b = (v_a % v_b);
        v_a = v_t;
    }
    return v_a;
}

Three things in that output are decisions rather than accidents. extern "C", because the two sides are produced by different compilers and share no C++ ABI — the same reason a device island's boundary is. v_ on every local, because a kernel that names a variable class must not become a C++ file that fails to compile for a reason the toy source cannot express. Parentheses around every binary expression, because the AST already holds the grouping and the emitter does not reproduce C++'s precedence table.

The compiler rejects what it cannot compile, and says where. Each of these was produced by running toyc on a file with that one defect:

$ toyc bad.toy -o bad.cpp
bad.toy:1:1: error: kernel `scale` can reach its end without a `return`
bad2.toy:2:12: error: `b` is not a kernel in this file
bad3.toy:2:12: error: `x` is not declared
bad4.toy:3:5: error: expected `;`, found `return`

The first is the one worth the code it takes. A kernel returns an integer on every path, and a block satisfies that if it ends in a return or in an if/else whose branches both do — while never counts, because the language cannot state that a loop runs at all. Emitting return 0; at the end instead would have compiled everything and given a wrong answer for the kernel whose author forgot a branch.

The two manifest keys

rules-toy/mcpp.toml declares them on the feature that selects the rule:

[features.rules-toy]
sources           = ["src/rules-toy.cppm"]
rule_module       = "example.rules.toy"
device_extensions = [".toy"]

key	effect
device_extensions	a consumer that activates the feature gets .toy classified as a device source: never scanned for imports, never producing a BMI, and refused if no action claims it
rule_module	the module the consumer's build program imports. It implies host-module = true, so the consumer writes the feature and nothing else

Device extensions are not in the default source glob. A .toy is compiled because the manifest names it:

[dependencies]
rules-toy = { path = "../rules-toy", features = ["rules-toy"] }

[build]
sources = ["src/*.cpp", "src/kernels/*.toy"]

The compiler is a package, built for the build machine

The rule contains no compiler. toyc is an ordinary mcpp package with a kind = "bin" target, and one line brings it into the graph:

[feature-deps.rules-toy]
toyc = { path = "../toyc", tools = ["toyc"], reexport = true }

part	what it does
tools = ["toyc"]	mcpp builds that target for the build machine, even when the project around it is cross-compiling
reexport = true	the tool reaches whoever activated the feature. Without it the tool stays with this package, which is the supply-chain default: an arbitrary transitive dependency must not put entries in a build program's tool namespace
on [feature-deps]	a project that depends on this package without activating the rule builds no compiler

The rule reads the path back with mcpp::dep_bin("toyc", "toyc"), under the name of the manifest entry that declared the tool rather than the consumer's spelling of anything.

That gating was measured. With features = ["rules-toy"] removed from the consumer, the build stops before any tool is built:

error: build.mcpp imports 'example.rules.toy', and no dependency provides it as a host module.
       declared without `host-module = true`: rules-toy (in [dependencies])

and the tool store holds no toyc entry afterwards.

What this example is the first to demonstrate

Every other rule in this repository is built into mcpp:plugins and compiles an extension the engine already knows. This one adds a language from outside — the property the accelerator design is built on — and it is the tree's only example of a dependency that produces a host tool, which is how a rule package ships a real compiler rather than a script.

Criteria

criterion	measured
a .toy is compiled and its output joins the link, on an engine that does not know the extension	answer() = 42
the language is executed rather than pattern-matched	gcd(1071, 462) = 21, computed by the emitted loop
editing the .toy reaches the artifact	scale(…, 2) → scale(…, 3): 42 → 63
the compiler is built for the build machine, on demand, and only when the rule is active	the Building host tool line above; no store entry without the feature

The boundary this example measured: a host tool is cached by version

Four changes were made one at a time, each from the same starting state:

what changed	the artifact	how it was changed
the .toy source	follows: 42 → 63	scale(…, 2) → scale(…, 3)
the compiler's bytes, at the path the action names	follows: 42 → 168	overwriting the binary in the tool store
the compiler's sources, its version unchanged	does not follow: the previous answer stands	editing the emitter
the compiler's version	follows: 42 → 168, and the tool is rebuilt	0.1.0 → 0.1.1

Rows two and three are the whole finding, and they separate two things that are easy to merge. The action's input tracking works: rules-toy declares the compiler beside the source, and changing that file's bytes re-runs the edge. What does not happen is the rebuild that would change those bytes. The tool store's key is the tool package's identity, version, host triple, compiler identity, profile, features and the versions of its transitive dependencies — it holds no source content. For a package that arrives from an index the key is exact, because a published version is immutable; for a path dependency being edited it is not.

mcpp run prints Finished dev in 0.00s in row three, and that line is mcpp's own summary rather than evidence: row two prints it too, and the artifact changed.

Row four does not test row two, which is why the difference is worth stating. The tool's path is on the action's command line, so a new version re-runs the edge whether or not the compiler is also a declared input. Removing a.input(compiler) from rules-toy and bumping the version left the artifact following anyway. The isolating change is different bytes at the same path — row two — and it takes both of its directions: with the input removed, the artifact followed the overwrite and then stopped following the restore. CI runs that pair.

Two ways out: bump the tool package's version, or empty the build cache with mcpp cache clean — the tool store lives inside it, at <mcpp cache dir>/tool/<index>/<name>@<version>/.

One more trap sits behind them. Going back from 0.1.1 to 0.1.0, whose clean tool was still in the store, left the artifact at 168. The build program did not re-run, so the plan still named the 0.1.1 binary, which had not changed. rm -rf target cleared it. Iterating on a compiler means the version goes forward only.

Three things the first version got wrong

Recorded because each is a mistake a rule author will make once.

An action's command does not run from the package root. mcpp::device_sources() answers with package-root-relative paths, and the command runs from the build directory. The relative path reached toyc unchanged and the read failed there. The rule joins mcpp::manifest_dir() to each path.

The compiler was a shell script, and its exit status lied. The first toyc summed with sed … | grep … | paste -sd+ - | bc. A pipeline's status is its last command's: when an earlier stage produced nothing, bc still exited 0, so set -e never fired and the script wrote a program that compiled, linked, ran and printed 0. Replacing the script with a compiled program removed the class, not just the instance.

The strings must outlive the action. a.id = ("toy:" + stem).c_str() hands submit() a pointer into a temporary that is already gone.