onnx_runtime_loader/

function_inline.rs

//! Model-local function inlining (ONNX function expansion) at load time.
//!
//! An ONNX `ModelProto` may declare reusable subgraphs as `FunctionProto`s in
//! `ModelProto.functions`. A node whose `(domain, op_type, overload)` matches a
//! declared function's `(domain, name, overload)` is a *function call*: it is
//! semantically equivalent to the function body with the call's actual inputs,
//! outputs, and attributes substituted in.
//!
//! Our executor only has kernels for primitive ops, so this module rewrites the
//! `ModelProto` at the proto level — **before** [`crate::graph_builder`] runs —
//! so the rest of the pipeline never sees a function call. Because the rewrite
//! is proto-level, the existing `NodeProto → IR` conversion (attributes,
//! control-flow subgraphs) is reused unchanged.
//!
//! ## Algorithm (standard ONNX function expansion)
//!
//! For each function-call node, we splice in a fresh copy of the matched
//! function body:
//!
//! 1. **Value remapping.** Formal `input[i]`/`output[j]` names are mapped to the
//!    call's actual argument names (positionally). Every *other* value name in
//!    the body (an intermediate result) is renamed to a globally-fresh unique
//!    name (`__fn{K}_{orig}`, bumped until unused) so instantiations never
//!    collide with each other or with pre-existing model names. The empty name
//!    `""` (ONNX "absent optional") is never remapped. A pass-through output
//!    whose formal name aliases an input is wired via a boundary `Identity`.
//!
//! 2. **Attribute binding.** A body-node attribute with a non-empty
//!    `ref_attr_name = A` is a reference to the function's formal attribute `A`.
//!    It is resolved from the call site (the call node's attribute `A`), else the
//!    function's declared default (`attribute_proto` entry named `A`), else — if
//!    `A` is a required attribute (`FunctionProto.attribute`) — an error; else
//!    the attribute is dropped. Literal (non-`ref`) attributes are kept as-is.
//!
//! 3. **Recursion + fixpoint.** A function body may call other functions; those
//!    calls are expanded recursively to a fixpoint. True recursion (a function
//!    that transitively calls itself) is rejected rather than looped forever.
//!
//! 4. **Control-flow subgraphs.** Function calls may appear inside If/Loop/Scan
//!    subgraph bodies, and function bodies may themselves contain control flow;
//!    both are handled by recursing into every node's `Graph`/`Graphs`
//!    attributes. Attribute binding and value remapping are scope-aware: nested
//!    `ref_attr_name` references are bound at every depth, and a subgraph's own
//!    locals (inputs, initializers, node outputs) shadow outer captures.
//!
//! ## Opset policy
//!
//! `FunctionProto.opset_import` domains/versions are merged into the model's
//! `opset_import`, taking the highest version per domain. Per the ONNX spec the
//! operator schemas for a shared domain must be compatible across the two opset
//! lists, so a version difference is not treated as a conflict; a domain the
//! model does not yet declare is added.
//!
//! ## Overload policy
//!
//! Matching is exact on the full `(domain, name, overload)` triple, so an
//! overload set is disambiguated by the node's `overload` field.

use std::borrow::Cow;
use std::collections::{HashMap, HashSet};

use crate::proto::onnx::{
    AttributeProto, FunctionProto, GraphProto, ModelProto, NodeProto, OperatorSetIdProto,
};
use crate::LoaderError;

/// Unique identity of a model-local function: `(domain, name, overload)`.
type FnKey = (String, String, String);

fn fn_key_of_function(f: &FunctionProto) -> FnKey {
    (f.domain.clone(), f.name.clone(), f.overload.clone())
}

fn fn_key_of_call(n: &NodeProto) -> FnKey {
    (n.domain.clone(), n.op_type.clone(), n.overload.clone())
}

/// Expand every call to a model-local function in `model` into the function's
/// body, so the returned `ModelProto`'s graph (and all nested subgraphs) contain
/// only calls to ops the runtime has kernels for.
///
/// When `model.functions` is empty this is a no-op and the input is borrowed
/// back unchanged (`Cow::Borrowed`). Otherwise a rewritten owned `ModelProto`
/// is returned with `functions` cleared and function opset imports merged in.
pub fn inline_functions(model: &ModelProto) -> Result<Cow<'_, ModelProto>, LoaderError> {
    if model.functions.is_empty() {
        return Ok(Cow::Borrowed(model));
    }

    let mut funcs: HashMap<FnKey, &FunctionProto> = HashMap::new();
    for f in &model.functions {
        funcs.insert(fn_key_of_function(f), f);
    }

    let graph = model
        .graph
        .as_ref()
        .ok_or_else(|| LoaderError::GraphBuild("ModelProto has no graph".into()))?;

    let mut counter: usize = 0;
    let mut stack: Vec<FnKey> = Vec::new();
    // Every value name already in use model-wide, so generated internal names
    // can be allocated to be globally fresh (BUG 4). Updated as inlining adds
    // new node outputs.
    let mut used: HashSet<String> = HashSet::new();
    collect_used_names(graph, &mut used);
    // Set when inlining synthesizes any default-domain (`""`/`ai.onnx`) node,
    // e.g. a boundary `Identity` alias, so we can guarantee the model declares a
    // default-domain opset import for it (BUG 3 regression).
    let mut synthesized_default = false;
    let new_graph = inline_graph(
        graph,
        &funcs,
        &mut counter,
        &mut stack,
        &mut used,
        &mut synthesized_default,
    )?;

    let mut out = model.clone();
    out.graph = Some(new_graph);
    out.opset_import = merged_opset_imports(model);
    if synthesized_default {
        ensure_default_opset_import(&mut out.opset_import);
    }
    out.functions.clear();
    Ok(Cow::Owned(out))
}

/// Conservative default `ai.onnx` opset version used only when inlining
/// synthesizes a default-domain node but the model (and its functions) declared
/// no default-domain opset import at all — a valid ONNX model that, e.g., only
/// called custom-domain functions. Any version ≥ 1 satisfies loader validation.
const DEFAULT_ONNX_OPSET_VERSION: i64 = 17;

/// The ONNX default operator-set domain has two equivalent spellings: the empty
/// string `""` and the explicit `"ai.onnx"`. They denote the SAME domain, so any
/// opset-import comparison/dedup must treat them as one.
fn is_default_domain(domain: &str) -> bool {
    domain.is_empty() || domain == "ai.onnx"
}

/// Canonical map key for an opset-import domain: every spelling of the default
/// domain collapses to a single key so duplicates cannot survive merging.
const DEFAULT_DOMAIN_KEY: &str = "";

fn domain_key(domain: &str) -> String {
    if is_default_domain(domain) {
        DEFAULT_DOMAIN_KEY.to_string()
    } else {
        domain.to_string()
    }
}

/// Ensure `imports` contains a default-domain (`""`/`ai.onnx`) opset entry so a
/// synthesized default-domain node (e.g. a boundary `Identity`) passes loader
/// validation. An existing default-domain import (under either spelling) is left
/// untouched — never downgraded, never duplicated.
fn ensure_default_opset_import(imports: &mut Vec<OperatorSetIdProto>) {
    let has_default = imports.iter().any(|o| is_default_domain(&o.domain));
    if !has_default {
        imports.push(OperatorSetIdProto {
            domain: String::new(),
            version: DEFAULT_ONNX_OPSET_VERSION,
        });
    }
}

/// Merge every function's `opset_import` into the model's, taking the highest
/// version per domain. Preserves the model's original import ordering, then
/// appends any domains introduced solely by functions (in first-seen order).
///
/// The default domain is canonicalized: `""` and `"ai.onnx"` collapse to a
/// SINGLE entry at the highest contributed version, so a model importing
/// `"ai.onnx"` plus a function (or synthesized Identity path) contributing `""`
/// never yields two logically-duplicate default-domain imports. The emitted
/// default entry keeps the model's original spelling if it declared one (so we
/// don't gratuitously rewrite `"ai.onnx"`→`""`); otherwise it is spelled `""`.
fn merged_opset_imports(model: &ModelProto) -> Vec<OperatorSetIdProto> {
    let mut order: Vec<String> = Vec::new();
    let mut best: HashMap<String, i64> = HashMap::new();
    // Preferred spelling for the emitted default-domain entry: the model's
    // original spelling if it imported the default domain, else `""`.
    let mut default_spelling: Option<String> = None;
    let mut note = |domain: &str, version: i64, from_model: bool| {
        if from_model && is_default_domain(domain) && default_spelling.is_none() {
            default_spelling = Some(domain.to_string());
        }
        let key = domain_key(domain);
        match best.entry(key.clone()) {
            std::collections::hash_map::Entry::Occupied(mut e) => {
                if version > *e.get() {
                    *e.get_mut() = version;
                }
            }
            std::collections::hash_map::Entry::Vacant(e) => {
                order.push(key);
                e.insert(version);
            }
        }
    };
    for o in &model.opset_import {
        note(&o.domain, o.version, true);
    }
    for f in &model.functions {
        for o in &f.opset_import {
            note(&o.domain, o.version, false);
        }
    }
    order
        .into_iter()
        .map(|key| {
            let version = best[&key];
            let domain = if key == DEFAULT_DOMAIN_KEY {
                default_spelling.clone().unwrap_or_default()
            } else {
                key
            };
            OperatorSetIdProto { domain, version }
        })
        .collect()
}

/// Rewrite `gp` so its node list contains no calls to any declared function.
/// Regular nodes are kept (with their control-flow subgraphs recursively
/// inlined); function-call nodes are replaced by their expanded bodies.
fn inline_graph(
    gp: &GraphProto,
    funcs: &HashMap<FnKey, &FunctionProto>,
    counter: &mut usize,
    stack: &mut Vec<FnKey>,
    used: &mut HashSet<String>,
    synthesized_default: &mut bool,
) -> Result<GraphProto, LoaderError> {
    let mut out = gp.clone();
    out.node = Vec::with_capacity(gp.node.len());
    for node in &gp.node {
        expand_node(node, funcs, counter, stack, used, synthesized_default, &mut out.node)?;
    }
    Ok(out)
}

/// Append the fully-inlined form of `node` to `sink`. If `node` calls a
/// function, its body (recursively inlined) is appended; otherwise the node is
/// appended with its subgraph attributes recursively inlined.
fn expand_node(
    node: &NodeProto,
    funcs: &HashMap<FnKey, &FunctionProto>,
    counter: &mut usize,
    stack: &mut Vec<FnKey>,
    used: &mut HashSet<String>,
    synthesized_default: &mut bool,
    sink: &mut Vec<NodeProto>,
) -> Result<(), LoaderError> {
    if let Some(func) = funcs.get(&fn_key_of_call(node)) {
        instantiate(node, func, funcs, counter, stack, used, synthesized_default, sink)?;
    } else {
        sink.push(inline_subgraph_attrs(
            node,
            funcs,
            counter,
            stack,
            used,
            synthesized_default,
        )?);
    }
    Ok(())
}

/// Return a copy of `node` whose `Graph`/`Graphs` attribute bodies have had any
/// function calls inside them inlined.
fn inline_subgraph_attrs(
    node: &NodeProto,
    funcs: &HashMap<FnKey, &FunctionProto>,
    counter: &mut usize,
    stack: &mut Vec<FnKey>,
    used: &mut HashSet<String>,
    synthesized_default: &mut bool,
) -> Result<NodeProto, LoaderError> {
    let mut out = node.clone();
    for attr in &mut out.attribute {
        if let Some(g) = attr.g.as_mut() {
            *g = inline_graph(g, funcs, counter, stack, used, synthesized_default)?;
        }
        for g in &mut attr.graphs {
            *g = inline_graph(g, funcs, counter, stack, used, synthesized_default)?;
        }
    }
    Ok(out)
}

/// Expand a single function call: substitute actual arguments and attributes
/// into a fresh copy of the function body, then recursively inline any calls the
/// body itself makes. Appends the resulting primitive nodes to `sink`.
#[allow(clippy::too_many_arguments)]
fn instantiate(
    call: &NodeProto,
    func: &FunctionProto,
    funcs: &HashMap<FnKey, &FunctionProto>,
    counter: &mut usize,
    stack: &mut Vec<FnKey>,
    used: &mut HashSet<String>,
    synthesized_default: &mut bool,
    sink: &mut Vec<NodeProto>,
) -> Result<(), LoaderError> {
    let key = fn_key_of_function(func);

    if stack.contains(&key) {
        let mut chain: Vec<String> = stack.iter().map(fmt_key).collect();
        chain.push(fmt_key(&key));
        return Err(LoaderError::RecursiveFunction {
            function: fmt_key(&key),
            chain: chain.join(" -> "),
        });
    }

    // Arity: passing *more* actuals than the function declares is illegal;
    // passing fewer is allowed (trailing optionals omitted, mapped to absent).
    if call.input.len() > func.input.len() {
        return Err(LoaderError::FunctionArityMismatch {
            function: fmt_key(&key),
            node: node_label(call),
            kind: "input",
            formal: func.input.len(),
            actual: call.input.len(),
        });
    }
    if call.output.len() > func.output.len() {
        return Err(LoaderError::FunctionArityMismatch {
            function: fmt_key(&key),
            node: node_label(call),
            kind: "output",
            formal: func.output.len(),
            actual: call.output.len(),
        });
    }

    let inst_id = *counter;
    *counter += 1;

    // The set of formal names actually produced by a body node. A formal output
    // that is *not* produced is a pass-through of an input (or otherwise-defined
    // value) and needs a boundary alias rather than a rename (BUG 3).
    let produced: HashSet<&str> = func
        .node
        .iter()
        .flat_map(|n| n.output.iter())
        .filter(|o| !o.is_empty())
        .map(String::as_str)
        .collect();

    // 1. Value remapping: formals -> actuals, everything else -> globally fresh.
    let mut rename: HashMap<String, String> = HashMap::new();
    // Boundary `Identity` aliases (src_actual -> dst_actual) for pass-through
    // outputs whose name aliases an input/other output (BUG 3).
    let mut aliases: Vec<(String, String)> = Vec::new();

    for (i, formal) in func.input.iter().enumerate() {
        if formal.is_empty() {
            continue;
        }
        let actual = call.input.get(i).cloned().unwrap_or_default();
        rename.insert(formal.clone(), actual);
    }
    for (j, formal) in func.output.iter().enumerate() {
        if formal.is_empty() {
            continue;
        }
        let actual = call.output.get(j).cloned().unwrap_or_default();
        if produced.contains(formal.as_str()) {
            // Genuinely produced by the body: consumers read the output actual.
            rename.insert(formal.clone(), actual);
        } else if let Some(src) = rename.get(formal) {
            // Pass-through: the formal is already bound (e.g. it is also an
            // input, or an earlier output). Keep body references reading the
            // source, and emit a boundary alias to the output actual.
            if !actual.is_empty() && src != &actual {
                aliases.push((src.clone(), actual));
            }
        } else {
            // Output not produced and not otherwise bound: map it directly.
            rename.insert(formal.clone(), actual);
        }
    }
    // Fresh, globally-unique names for internal (non-formal) body value names.
    for bn in &func.node {
        for name in bn.input.iter().chain(bn.output.iter()) {
            if name.is_empty() || rename.contains_key(name) {
                continue;
            }
            let fresh = fresh_name(name, inst_id, used);
            rename.insert(name.clone(), fresh);
        }
    }

    // 2. Attribute binding + value renaming for each body node.
    stack.push(key.clone());
    let result = (|| {
        let mut instantiated: Vec<NodeProto> = Vec::with_capacity(func.node.len());
        for (idx, bn) in func.node.iter().enumerate() {
            let mut nn = bn.clone();

            // Rename node name to a fresh unique one to avoid duplicate-name
            // collisions between instantiations.
            nn.name = if bn.name.is_empty() {
                format!("__fn{inst_id}_n{idx}")
            } else {
                format!("__fn{inst_id}_{}", bn.name)
            };

            // Bind attributes (resolve ref_attr_name against the call site) at
            // every depth, including nodes inside control-flow subgraphs (BUG 1).
            bind_node_attributes(&mut nn, call, func, &key)?;

            // Rename value references (inputs/outputs + captured names inside
            // any control-flow subgraph attributes, scope-aware).
            rename_value_refs(&mut nn, &rename);

            instantiated.push(nn);
        }

        // Boundary `Identity` aliases for pass-through outputs (BUG 3). Appended
        // last so their source values are already produced. `Identity` is a
        // default-domain op, so record that we synthesized one to guarantee the
        // model declares a default-domain opset import (BUG 3 regression).
        for (k, (src, dst)) in aliases.iter().enumerate() {
            *synthesized_default = true;
            instantiated.push(NodeProto {
                op_type: "Identity".to_string(),
                input: vec![src.clone()],
                output: vec![dst.clone()],
                name: format!("__fn{inst_id}_alias{k}"),
                ..Default::default()
            });
        }

        // 3. Recursively inline any function calls the body itself makes.
        let mut expanded: Vec<NodeProto> = Vec::new();
        for n in &instantiated {
            expand_node(n, funcs, counter, stack, used, synthesized_default, &mut expanded)?;
        }
        Ok::<Vec<NodeProto>, LoaderError>(expanded)
    })();
    stack.pop();

    sink.extend(result?);
    Ok(())
}

/// Bind a body node's attributes for a specific instantiation, recursing into
/// any control-flow subgraph so that `ref_attr_name` references carried by
/// nested nodes are resolved against the same call site (BUG 1).
fn bind_node_attributes(
    node: &mut NodeProto,
    call: &NodeProto,
    func: &FunctionProto,
    key: &FnKey,
) -> Result<(), LoaderError> {
    let mut bound: Vec<AttributeProto> = Vec::with_capacity(node.attribute.len());
    for attr in &node.attribute {
        if let Some(mut resolved) = bind_attribute(attr, call, func, key)? {
            if let Some(g) = resolved.g.as_mut() {
                for sub in &mut g.node {
                    bind_node_attributes(sub, call, func, key)?;
                }
            }
            for g in &mut resolved.graphs {
                for sub in &mut g.node {
                    bind_node_attributes(sub, call, func, key)?;
                }
            }
            bound.push(resolved);
        }
    }
    node.attribute = bound;
    Ok(())
}

/// Resolve a body-node attribute for a specific instantiation.
///
/// * Literal attribute (`ref_attr_name` empty): kept unchanged.
/// * Reference attribute (`ref_attr_name = A`): replaced by the call-site
///   attribute `A`, else the function's default for `A`, else dropped (if `A` is
///   optional) or an error (if `A` is required). The emitted attribute keeps the
///   body attribute's `name` and has `ref_attr_name` cleared.
///
/// Returns `Ok(None)` when the attribute should be omitted from the node.
fn bind_attribute(
    attr: &AttributeProto,
    call: &NodeProto,
    func: &FunctionProto,
    key: &FnKey,
) -> Result<Option<AttributeProto>, LoaderError> {
    if attr.ref_attr_name.is_empty() {
        return Ok(Some(attr.clone()));
    }
    let a = &attr.ref_attr_name;

    // Call-site value wins.
    if let Some(supplied) = call.attribute.iter().find(|ca| &ca.name == a) {
        let mut bound = supplied.clone();
        bound.name = attr.name.clone();
        bound.ref_attr_name.clear();
        return Ok(Some(bound));
    }
    // Otherwise the function's declared default, if any.
    if let Some(default) = func.attribute_proto.iter().find(|d| &d.name == a) {
        let mut bound = default.clone();
        bound.name = attr.name.clone();
        bound.ref_attr_name.clear();
        return Ok(Some(bound));
    }
    // No value and no default: an error if the attribute is required, else drop.
    if func.attribute.iter().any(|req| req == a) {
        return Err(LoaderError::MissingRequiredFunctionAttribute {
            function: fmt_key(key),
            node: node_label(call),
            attribute: a.clone(),
        });
    }
    Ok(None)
}

/// Apply `rename` to a node's value references: its inputs, its outputs, and any
/// value names captured inside its control-flow subgraph attributes. A name of
/// `""` (absent optional) is left untouched; a name absent from `rename` is left
/// as-is (subgraph-local names live in their own scope).
///
/// The node's own inputs/outputs live in the function-body scope, so they are
/// remapped directly. Subgraph attributes are remapped scope-aware
/// ([`rename_subgraph_refs`]).
fn rename_value_refs(node: &mut NodeProto, rename: &HashMap<String, String>) {
    for name in node.input.iter_mut().chain(node.output.iter_mut()) {
        if let Some(new) = rename.get(name.as_str()) {
            *name = new.clone();
        }
    }
    for attr in &mut node.attribute {
        if let Some(g) = attr.g.as_mut() {
            rename_subgraph_refs(g, rename);
        }
        for g in &mut attr.graphs {
            rename_subgraph_refs(g, rename);
        }
    }
}

/// Scope-aware renaming of outer-scope value captures inside a subgraph (BUG 2).
///
/// ONNX subgraphs have their own lexical scope. A subgraph's graph inputs,
/// initializers, and node outputs are *locals* that shadow any outer name, so
/// they must not be remapped. Only genuine captures of the enclosing scope —
/// node inputs, and `GraphProto.output` entries that directly name a captured
/// value — are rewritten to the outer actual. Shadowing is restored on descent
/// into deeper subgraphs by recomputing the local set at each level.
fn rename_subgraph_refs(gp: &mut GraphProto, rename: &HashMap<String, String>) {
    // Names locally bound in this subgraph shadow the outer scope.
    let mut locals: HashSet<&str> = HashSet::new();
    for i in &gp.input {
        if !i.name.is_empty() {
            locals.insert(i.name.as_str());
        }
    }
    for init in &gp.initializer {
        if !init.name.is_empty() {
            locals.insert(init.name.as_str());
        }
    }
    // Sparse initializers are also initializers (schema: GraphProto.
    // sparse_initializer), hence local bindings that shadow outer names.
    for sparse in &gp.sparse_initializer {
        if let Some(values) = &sparse.values
            && !values.name.is_empty()
        {
            locals.insert(values.name.as_str());
        }
    }    for n in &gp.node {
        for o in &n.output {
            if !o.is_empty() {
                locals.insert(o.as_str());
            }
        }
    }

    // Effective remap for this scope: outer captures minus anything shadowed.
    let effective: HashMap<String, String> = rename
        .iter()
        .filter(|(k, _)| !locals.contains(k.as_str()))
        .map(|(k, v)| (k.clone(), v.clone()))
        .collect();

    for n in &mut gp.node {
        for name in n.input.iter_mut().chain(n.output.iter_mut()) {
            if let Some(new) = effective.get(name.as_str()) {
                *name = new.clone();
            }
        }
        // Recurse into deeper subgraphs with this scope's effective map so a
        // name shadowed here stays shadowed, and is restored on the way out.
        for attr in &mut n.attribute {
            if let Some(g) = attr.g.as_mut() {
                rename_subgraph_refs(g, &effective);
            }
            for g in &mut attr.graphs {
                rename_subgraph_refs(g, &effective);
            }
        }
    }

    // A subgraph output that directly names a captured value must follow it.
    for o in &mut gp.output {
        if let Some(new) = effective.get(o.name.as_str()) {
            o.name = new.clone();
        }
    }
}

/// Collect every value name in use within `gp` (and its nested subgraphs):
/// graph inputs/outputs, initializers, value_info, and all node inputs/outputs.
/// Used to allocate globally-fresh generated names (BUG 4).
fn collect_used_names(gp: &GraphProto, used: &mut HashSet<String>) {
    for i in &gp.input {
        if !i.name.is_empty() {
            used.insert(i.name.clone());
        }
    }
    for o in &gp.output {
        if !o.name.is_empty() {
            used.insert(o.name.clone());
        }
    }
    for init in &gp.initializer {
        if !init.name.is_empty() {
            used.insert(init.name.clone());
        }
    }
    for sparse in &gp.sparse_initializer {
        if let Some(values) = &sparse.values
            && !values.name.is_empty()
        {
            used.insert(values.name.clone());
        }
    }
    for vi in &gp.value_info {
        if !vi.name.is_empty() {
            used.insert(vi.name.clone());
        }
    }
    for n in &gp.node {
        for name in n.input.iter().chain(n.output.iter()) {
            if !name.is_empty() {
                used.insert(name.clone());
            }
        }
        for attr in &n.attribute {
            if let Some(g) = &attr.g {
                collect_used_names(g, used);
            }
            for g in &attr.graphs {
                collect_used_names(g, used);
            }
        }
    }
}

/// Allocate a generated name for internal body value `base`, guaranteed unique
/// against every name already in use `used` (BUG 4). The chosen name is added to
/// `used` so subsequent allocations remain distinct.
fn fresh_name(base: &str, inst_id: usize, used: &mut HashSet<String>) -> String {
    let mut candidate = format!("__fn{inst_id}_{base}");
    let mut suffix = 0usize;
    while used.contains(&candidate) {
        suffix += 1;
        candidate = format!("__fn{inst_id}_{base}__{suffix}");
    }
    used.insert(candidate.clone());
    candidate
}

fn fmt_key(key: &FnKey) -> String {
    let (domain, name, overload) = key;
    let d = if domain.is_empty() { "ai.onnx" } else { domain };
    if overload.is_empty() {
        format!("{d}::{name}")
    } else {
        format!("{d}::{name}:{overload}")
    }
}

fn node_label(n: &NodeProto) -> String {
    if n.name.is_empty() {
        format!("<{}::{} (unnamed)>", n.domain, n.op_type)
    } else {
        n.name.clone()
    }
}