Invert event loop

compiler/src/emit/calls.rs

@@ -97,6 +97,36 @@ impl<'a> Emitter<'a> {
                 let value = operand(self, FIRST_ARG as usize, self.rep(&self.cache.protos[kind as usize]));
                 (format!("({} = {value}, (undefined_t){{0}})", proto_global(kind)), Type::Undefined)
             }
+
+            // Keeps the fn where `jsrt_complete` finds it
+            Code::HostLink => {
+                let value = operand(self, FIRST_ARG as usize, self.rep(&self.cache.host_complete));
+                (format!("({HOST_COMPLETE_GLOBAL} = {value}, (undefined_t){{0}})"), Type::Undefined)
+            }
         }
     }
+
+    /// The body of `jsrt_complete(h)`: a call of the fn that the stdlib links
+    /// for the host, with op `h` (see `infer::host_calls()`)
+    pub(super) fn host_complete(&mut self) -> String {
+        let ty = self.cache.host_complete.clone();
+        let mut out = String::new();
+
+        if ty == Type::Unknown {
+            return out;
+        }
+
+        let rep = self.rep(&ty);
+        let operands = [(HOST_COMPLETE_GLOBAL.to_string(), rep), ("(undefined_t){0}".into(), Rep::Undefined), ("h".into(), Rep::Int)];
+
+        self.dispatch(HOST_COMPLETE_GLOBAL, &ty, "jsrt_type_error();", &mut out, |e, member, callee, rep| {
+            let Type::Obj(cell) = member else { return None };
+            let mut operands = operands.clone();
+            operands[0] = (callee.into(), rep);
+            let arg_types = [member.clone(), Type::Undefined, Type::Int];
+            Some(format!("{};", e.call_code(cell.code()?, HOST_COMPLETE, &arg_types, &operands).0))
+        });
+
+        out
+    }
 }

compiler/src/emit/classes.rs

@@ -121,6 +121,7 @@ pub(super) fn cell_sig(cell: &Cell, class: &HashMap<Cell, CellNum>, sig: &mut Ve
             Code::Fn(id) => [1, id as u32],
             Code::Native(id) => [2, id as u32],
             Code::ProtoLink(kind) => [3, kind as u32],
+            Code::HostLink => [5, 0],
         }),
         ObjKind::Array { elems } => {
             sig.push(4);

compiler/src/emit/instrs.rs

@@ -69,6 +69,7 @@ impl<'a> Emitter<'a> {
                 JsValue::String(s) => self.string_lit(&js_string_text(s)),
                 JsValue::NativeFn(id) => format!("&{}", builtin_name(self.num(self.cache.builtin(Code::Native(*id))))),
                 JsValue::ProtoLink(kind) => format!("&{}", builtin_name(self.num(self.cache.builtin(Code::ProtoLink(*kind))))),
+                JsValue::HostLink => format!("&{}", builtin_name(self.num(self.cache.builtin(Code::HostLink)))),
                 _ => format!("({}){{0}}", self.c_type(rep)),
             },
             Undefined | Null => format!("({}){{0}}", self.c_type(rep)),

compiler/src/emit/mod.rs

@@ -18,7 +18,7 @@ use std::{
 
 use crate::{
     infer::{
-        HOST, InferResult, Place, TypeCache,
+        HOST, HOST_COMPLETE, InferResult, Place, TypeCache,
         cells::{Cell, Code, LENGTH_FIELD, ObjKind, read_value},
         types::Type,
     },
@@ -94,11 +94,16 @@ pub fn emit(fns: &[RegFn], cache: &TypeCache, atoms: &[Atom]) -> String {
         }
     }
 
-    // The walks that the code calls, from the entry. The host calls the entry
+    if cache.host_complete != Type::Unknown {
-    // with nothing: its env and `this` are undefined
+        let _ = writeln!(statics, "static {} {HOST_COMPLETE_GLOBAL};", e.c_type(e.rep(&cache.host_complete)));
+    }
+
+    // The walks that the code calls, from the host's calls. The host calls the
+    // entry with nothing: its env and `this` are undefined
     let entry = cache.find(fns.len() - 1, HOST, &[], fns);
     let entry_args = vec!["(undefined_t){0}"; cache.walks[entry].req.arg_types.len()].join(", ");
     e.use_walk(entry);
+    let complete = e.host_complete();
 
     let mut fn_protos = String::new();
     let mut bodies = String::new();
@@ -127,7 +132,9 @@ pub fn emit(fns: &[RegFn], cache: &TypeCache, atoms: &[Atom]) -> String {
         e.helper_protos,
         e.helper_defs
     );
-    let _ = writeln!(out, "int main(void) {{\n{init}    {}({entry_args});\n    return 0;\n}}", e.fn_name(entry));
+    // What the host calls (see `lib/rt/jsrt.h`)
+    let _ = writeln!(out, "void jsrt_start(void) {{\n{init}    {}({entry_args});\n}}\n", e.fn_name(entry));
+    let _ = writeln!(out, "void jsrt_complete(int32_t h) {{\n{complete}}}");
     out
 }
 
@@ -287,6 +294,9 @@ fn builtin_name(id: CellNum) -> String {
     format!("b{id}")
 }
 
+/// The global that holds what the host calls when an IO op completes
+const HOST_COMPLETE_GLOBAL: &str = "jsrt_host_complete";
+
 fn proto_global(kind: ProtoOf) -> String {
     format!("proto_{kind:?}").to_lowercase()
 }

compiler/src/infer/cells.rs

@@ -27,6 +27,7 @@ pub enum Code {
     Fn(usize),
     Native(usize),
     ProtoLink(ProtoOf),
+    HostLink,
 }
 
 /// What the objects of a cell are, besides objects with named fields

compiler/src/infer/mod.rs

@@ -29,6 +29,10 @@ pub type Place = (usize, usize);
 /// The call place of the entry fn, which the host calls
 pub const HOST: Place = (usize::MAX, 0);
 
+/// The call place of the fn that the host calls when an IO op completes (see
+/// `Code::HostLink`)
+pub const HOST_COMPLETE: Place = (usize::MAX, 1);
+
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]
 pub struct InferRequest {
     pub fn_id: usize,
@@ -60,6 +64,7 @@ pub struct TypeCache {
     objects: BTreeMap<(usize, usize, Place), Cell>, // the cell of each name: the fn and instr that make the objects, and the call place
     builtins: Vec<Cell>, // the cells of the natives, then of the proto links
     pub protos: Protos,  // the prototype of each `ProtoOf`, as the stdlib links it
+    pub host_complete: Type, // what the host calls when an IO op completes, as the stdlib links it
     version: usize,      // goes up on every change of a cell, a return or a proto, and on every new cell or walk
 }
 
@@ -83,6 +88,7 @@ impl TypeCache {
         match code {
             Code::Native(id) => &self.builtins[id],
             Code::ProtoLink(kind) => &self.builtins[NATIVES.len() + kind as usize],
+            Code::HostLink => &self.builtins[NATIVES.len() + ProtoOf::ALL.len()],
             Code::Fn(_) => unreachable!(),
         }
     }
@@ -184,6 +190,28 @@ fn call_code(cache: &mut TypeCache, code: Code, arg_types: &[Type], fns: &[RegFn
             cache.protos[kind as usize] = joined;
             Type::Undefined
         }
+        Code::HostLink => {
+            let joined = join(&cache.host_complete, args.first().unwrap_or(&Type::Undefined));
+            cache.changed(joined != cache.host_complete);
+            cache.host_complete = joined;
+            Type::Undefined
+        }
+    }
+}
+
+/// The calls that the host makes: the entry, with no args, then the fn that
+/// it calls when an IO op completes, with the op's handle. Each member of its
+/// type is its own env, and `this` is undefined
+fn host_calls(cache: &mut TypeCache, fns: &[RegFn]) {
+    // The synthetic program entry is appended last by `flatten`
+    cache.call(fns.len() - 1, HOST, &[], fns);
+
+    for member in cache.host_complete.clone().members() {
+        if let Type::Obj(cell) = member
+            && let Some(Code::Fn(fn_id)) = cell.code()
+        {
+            cache.call(fn_id, HOST_COMPLETE, &[member.clone(), Type::Undefined, Type::Int], fns);
+        }
     }
 }
 
@@ -206,19 +234,18 @@ pub fn infer_types(fns: &[RegFn]) -> TypeCache {
         objects: BTreeMap::new(),
         builtins: vec![],
         protos: std::array::from_fn(|_| Type::Unknown),
+        host_complete: Type::Unknown,
         version: 0,
     };
 
-    let codes = (0..NATIVES.len()).map(Code::Native).chain(ProtoOf::ALL.map(Code::ProtoLink));
+    let codes = (0..NATIVES.len()).map(Code::Native).chain(ProtoOf::ALL.map(Code::ProtoLink)).chain([Code::HostLink]);
     cache.builtins = codes.map(|code| Cell::new(ObjKind::Fn(code))).collect();
 
     loop {
         let version = cache.version;
         cache.walks.iter_mut().for_each(|w| w.result = None);
 
-        // The synthetic program entry is appended last by `flatten`. It is
+        host_calls(&mut cache, fns);
-        // called by the host with no args
-        cache.call(fns.len() - 1, HOST, &[], fns);
 
         if cache.version == version {
             return cache;
@@ -302,6 +329,7 @@ fn walk_fn(w: usize, reg_fns: &[RegFn], cache: &mut TypeCache) -> InferResult {
                         JsValue::Undefined => Type::Undefined,
                         JsValue::NativeFn(id) => Type::Obj(cache.builtin(Code::Native(id)).clone()),
                         JsValue::ProtoLink(kind) => Type::Obj(cache.builtin(Code::ProtoLink(kind)).clone()),
+                        JsValue::HostLink => Type::Obj(cache.builtin(Code::HostLink).clone()),
                         _ => unreachable!(),
                     },
 

compiler/src/libqjs/parse.rs

@@ -21,8 +21,8 @@ pub mod opcodes {
 
 #[derive(Debug)]
 pub struct QuickJSBinary {
-    pub version: u8,
+    pub _version: u8,
-    pub checksum: u32,
+    pub _checksum: u32,
     pub atoms: Vec<Atom>,
     pub root_mod: Module,
 }
@@ -42,8 +42,8 @@ impl QuickJSBinary {
         Ok((
             bin,
             QuickJSBinary {
-                version,
+                _version: version,
-                checksum,
+                _checksum: checksum,
                 atoms,
                 root_mod,
             },
@@ -164,8 +164,8 @@ pub enum JsValue {
     /// array body; `raw` is the extra trailing value stored as `.raw`
     /// (`Undefined` when the template has no raw form).
     TemplateObject {
-        cooked: Vec<JsValue>,
+        _cooked: Vec<JsValue>,
-        raw: Box<JsValue>,
+        _raw: Box<JsValue>,
     },
     Function(Function),
     FnById(usize),
@@ -173,6 +173,8 @@ pub enum JsValue {
     NativeFn(usize),
     /// Makes its arg the prototype of a primitive kind (see `native`)
     ProtoLink(crate::native::ProtoOf),
+    /// `__link_host_complete`: its arg is what the host calls when an op completes
+    HostLink,
     Module(Module),
     TypedArray,
     ArrayBuffer,
@@ -240,8 +242,8 @@ impl JsValue {
                 (
                     bin,
                     Self::TemplateObject {
-                        cooked,
+                        _cooked: cooked,
-                        raw: Box::new(raw),
+                        _raw: Box::new(raw),
                     },
                 )
             }
@@ -271,16 +273,16 @@ pub enum FuncKind {
 
 #[derive(Debug)]
 pub struct FunctionFlags {
-    pub has_prototype: bool,
+    pub _has_prototype: bool,
-    pub has_simple_param_list: bool,
+    pub _has_simple_param_list: bool,
-    pub is_derived_class_constructor: bool,
+    pub _is_derived_class_constructor: bool,
-    pub need_home_object: bool,
+    pub _need_home_object: bool,
-    pub func_kind: FuncKind,
+    pub _func_kind: FuncKind,
-    pub new_target_allowed: bool,
+    pub _new_target_allowed: bool,
-    pub super_call_allowed: bool,
+    pub _super_call_allowed: bool,
-    pub super_allowed: bool,
+    pub _super_allowed: bool,
-    pub arguments_allowed: bool,
+    pub _arguments_allowed: bool,
-    pub backtrace_barrier: bool,
+    pub _backtrace_barrier: bool,
     pub has_debug: bool,
 }
 
@@ -291,16 +293,16 @@ impl FunctionFlags {
         Ok((
             bin,
             Self {
-                has_prototype: raw & (1 << 0) != 0,
+                _has_prototype: raw & (1 << 0) != 0,
-                has_simple_param_list: raw & (1 << 1) != 0,
+                _has_simple_param_list: raw & (1 << 1) != 0,
-                is_derived_class_constructor: raw & (1 << 2) != 0,
+                _is_derived_class_constructor: raw & (1 << 2) != 0,
-                need_home_object: raw & (1 << 3) != 0,
+                _need_home_object: raw & (1 << 3) != 0,
-                func_kind: FuncKind::try_from_primitive((raw >> 4) & 0b11).unwrap(),
+                _func_kind: FuncKind::try_from_primitive((raw >> 4) & 0b11).unwrap(),
-                new_target_allowed: raw & (1 << 6) != 0,
+                _new_target_allowed: raw & (1 << 6) != 0,
-                super_call_allowed: raw & (1 << 7) != 0,
+                _super_call_allowed: raw & (1 << 7) != 0,
-                super_allowed: raw & (1 << 8) != 0,
+                _super_allowed: raw & (1 << 8) != 0,
-                arguments_allowed: raw & (1 << 9) != 0,
+                _arguments_allowed: raw & (1 << 9) != 0,
-                backtrace_barrier: raw & (1 << 10) != 0,
+                _backtrace_barrier: raw & (1 << 10) != 0,
                 has_debug: raw & (1 << 11) != 0,
             },
         ))
@@ -326,9 +328,9 @@ pub enum VarKind {
 
 #[derive(Debug)]
 pub struct VarDefFlags {
-    pub kind: VarKind,
+    pub _kind: VarKind,
-    pub is_const: bool,
+    pub _is_const: bool,
-    pub is_lexical: bool,
+    pub _is_lexical: bool,
     pub is_captured: bool,
 }
 
@@ -344,9 +346,9 @@ impl VarDefFlags {
         Ok((
             bin,
             Self {
-                kind,
+                _kind: kind,
-                is_const,
+                _is_const: is_const,
-                is_lexical,
+                _is_lexical: is_lexical,
                 is_captured,
             },
         ))
@@ -355,11 +357,11 @@ impl VarDefFlags {
 
 #[derive(Debug)]
 pub struct VarDef {
-    pub name: AtomRef,
+    pub _name: AtomRef,
-    pub scope_level: usize,
+    pub _scope_level: usize,
-    pub scope_next_plus_1: usize,
+    pub _scope_next_plus_1: usize,
-    pub flags: VarDefFlags,
+    pub _flags: VarDefFlags,
-    pub var_ref_idx: Option<usize>,
+    pub _var_ref_idx: Option<usize>,
 }
 
 impl VarDef {
@@ -379,11 +381,11 @@ impl VarDef {
         Ok((
             bin,
             Self {
-                name,
+                _name: name,
-                scope_level,
+                _scope_level: scope_level,
-                scope_next_plus_1,
+                _scope_next_plus_1: scope_next_plus_1,
-                flags,
+                _flags: flags,
-                var_ref_idx,
+                _var_ref_idx: var_ref_idx,
             },
         ))
     }
@@ -405,9 +407,9 @@ pub enum ClosureKind {
 #[derive(Debug)]
 pub struct ClosureVarFlags {
     pub closure_kind: ClosureKind,
-    pub is_const: bool,
+    pub _is_const: bool,
     pub is_lexical: bool,
-    pub var_kind: VarKind,
+    pub _var_kind: VarKind,
 }
 
 impl ClosureVarFlags {
@@ -424,9 +426,9 @@ impl ClosureVarFlags {
             bin,
             Self {
                 closure_kind,
-                is_const,
+                _is_const: is_const,
                 is_lexical,
-                var_kind,
+                _var_kind: var_kind,
             },
         ))
     }
@@ -434,7 +436,7 @@ impl ClosureVarFlags {
 
 #[derive(Debug)]
 pub struct ClosureVar {
-    pub name: AtomRef,
+    pub _name: AtomRef,
     pub var_idx: usize,
     pub flags: ClosureVarFlags,
 }
@@ -448,7 +450,7 @@ impl ClosureVar {
         Ok((
             bin,
             Self {
-                name,
+                _name: name,
                 var_idx,
                 flags,
             },
@@ -487,7 +489,7 @@ pub enum ImmOperand {
     /// (the scope chain entry point, FORMAT.md §3.3).
     NpopU16 {
         npop: u16,
-        scope: u16,
+        _scope: u16,
     },
     /// Jump. The wire value is a signed offset from the operand
     /// field's own position; `target` is already resolved.
@@ -501,21 +503,21 @@ pub enum ImmOperand {
     },
     AtomU16 {
         atom: AtomRef,
-        arg: u16,
+        _arg: u16,
     },
     AtomLabelU8 {
-        atom: AtomRef,
+        _atom: AtomRef,
-        target: usize,
+        _target: usize,
-        arg: u8,
+        _arg: u8,
     },
     AtomLabelU16 {
-        atom: AtomRef,
+        _atom: AtomRef,
-        target: usize,
+        _target: usize,
-        arg: u16,
+        _arg: u16,
     },
     LabelU16 {
-        target: usize,
+        _target: usize,
-        arg: u16,
+        _arg: u16,
     },
 }
 
@@ -618,7 +620,7 @@ fn parse_bytecode(code: &[u8]) -> IResult<&[u8], Vec<Instr>> {
             opcodes::OpFmt::Npop => ImmOperand::Npop(u16_at(0)),
             opcodes::OpFmt::NpopU16 => ImmOperand::NpopU16 {
                 npop: u16_at(0),
-                scope: u16_at(2),
+                _scope: u16_at(2),
             },
             opcodes::OpFmt::Loc => ImmOperand::Loc(u16_at(0)),
             opcodes::OpFmt::Arg => ImmOperand::Arg(u16_at(0)),
@@ -639,21 +641,21 @@ fn parse_bytecode(code: &[u8]) -> IResult<&[u8], Vec<Instr>> {
             },
             opcodes::OpFmt::AtomU16 => ImmOperand::AtomU16 {
                 atom: AtomRef::from_code_u32(u32_at(0)),
-                arg: u16_at(4),
+                _arg: u16_at(4),
             },
             opcodes::OpFmt::AtomLabelU8 => ImmOperand::AtomLabelU8 {
-                atom: AtomRef::from_code_u32(u32_at(0)),
+                _atom: AtomRef::from_code_u32(u32_at(0)),
-                target: jump(atom_label_base, u32_at(4) as i32 as i64),
+                _target: jump(atom_label_base, u32_at(4) as i32 as i64),
-                arg: u8_at(8),
+                _arg: u8_at(8),
             },
             opcodes::OpFmt::AtomLabelU16 => ImmOperand::AtomLabelU16 {
-                atom: AtomRef::from_code_u32(u32_at(0)),
+                _atom: AtomRef::from_code_u32(u32_at(0)),
-                target: jump(atom_label_base, u32_at(4) as i32 as i64),
+                _target: jump(atom_label_base, u32_at(4) as i32 as i64),
-                arg: u16_at(8),
+                _arg: u16_at(8),
             },
             opcodes::OpFmt::LabelU16 => ImmOperand::LabelU16 {
-                target: jump(pc + 1, u32_at(0) as i32 as i64),
+                _target: jump(pc + 1, u32_at(0) as i32 as i64),
-                arg: u16_at(4),
+                _arg: u16_at(4),
             },
         };
 
@@ -671,18 +673,18 @@ fn parse_bytecode(code: &[u8]) -> IResult<&[u8], Vec<Instr>> {
 
 #[derive(Debug)]
 pub struct Function {
-    pub flags: FunctionFlags,
+    pub _flags: FunctionFlags,
-    pub is_strict_mode: bool,
+    pub _is_strict_mode: bool,
     pub name: AtomRef,
     pub arg_count: usize,
     /// Num locals
     pub var_count: usize,
     /// Parameters before the first default or rest, this is Function.prototype.length
-    pub defined_arg_count: usize,
+    pub _defined_arg_count: usize,
-    pub stack_size: usize,
+    pub _stack_size: usize,
-    pub var_ref_count: usize,
+    pub _var_ref_count: usize,
     /// arguments first, then locals
-    pub var_defs: Vec<VarDef>,
+    pub _var_defs: Vec<VarDef>,
     pub closure_vars: Vec<ClosureVar>,
     pub cpool: Vec<JsValue>,
     /// LLM GENERATED, TENTATIVE
@@ -728,15 +730,15 @@ impl Function {
         Ok((
             bin,
             Self {
-                flags,
+                _flags: flags,
-                is_strict_mode: strict_mode == 1,
+                _is_strict_mode: strict_mode == 1,
                 name,
                 arg_count,
                 var_count,
-                defined_arg_count,
+                _defined_arg_count: defined_arg_count,
-                stack_size,
+                _stack_size: stack_size,
-                var_ref_count,
+                _var_ref_count: var_ref_count,
-                var_defs,
+                _var_defs: var_defs,
                 closure_vars,
                 cpool,
                 byte_code,
@@ -747,12 +749,12 @@ impl Function {
 
 #[derive(Debug)]
 pub struct Module {
-    pub name: AtomRef,
+    pub _name: AtomRef,
-    pub import_modules: Vec<AtomRef>,
+    pub _import_modules: Vec<AtomRef>,
-    pub exports: Vec<ExportInfo>,
+    pub _exports: Vec<ExportInfo>,
-    pub star_exports: Vec<usize>,
+    pub _star_exports: Vec<usize>,
-    pub imports: Vec<ImportInfo>,
+    pub _imports: Vec<ImportInfo>,
-    pub has_top_level_await: bool,
+    pub _has_top_level_await: bool,
     pub root_fn: Function,
 }
 
@@ -780,12 +782,12 @@ impl Module {
         Ok((
             bin,
             Self {
-                name,
+                _name: name,
-                import_modules,
+                _import_modules: import_modules,
-                exports,
+                _exports: exports,
-                star_exports,
+                _star_exports: star_exports,
-                imports,
+                _imports: imports,
-                has_top_level_await: has_tla == 1,
+                _has_top_level_await: has_tla == 1,
                 root_fn,
             },
         ))
@@ -795,7 +797,7 @@ impl Module {
 #[derive(Debug)]
 pub enum ExportInfo {
     /// All exports except aliases and star exports
-    Local { var_idx: usize, name: AtomRef },
+    Local { _var_idx: usize, _name: AtomRef },
     /// Alias exports, has `from`, eg: export { a } from "./dep.js";
     Alias,
 }
@@ -808,7 +810,7 @@ impl ExportInfo {
             0 => {
                 let (bin, var_idx) = leb128(bin)?;
                 let (bin, name) = AtomRef::parse(bin)?;
-                (bin, Self::Local { var_idx, name })
+                (bin, Self::Local { _var_idx: var_idx, _name: name })
             }
             1 => (bin, Self::Alias),
             _ => unreachable!(),
@@ -820,9 +822,9 @@ impl ExportInfo {
 
 #[derive(Debug)]
 pub struct ImportInfo {
-    pub var_idx: usize,
+    pub _var_idx: usize,
-    pub name: AtomRef,
+    pub _name: AtomRef,
-    pub req_module_idx: usize,
+    pub _req_module_idx: usize,
 }
 
 impl ImportInfo {
@@ -834,9 +836,9 @@ impl ImportInfo {
         Ok((
             bin,
             Self {
-                var_idx,
+                _var_idx: var_idx,
-                name,
+                _name: name,
-                req_module_idx,
+                _req_module_idx: req_module_idx,
             },
         ))
     }

compiler/src/native.rs

@@ -58,7 +58,6 @@ pub const NATIVES: &[NativeFn] = &[
     native!("__native_io_queue_close"(Int) -> Int),
     native!("__native_io_queue_stat"(String) -> Int),
     native!("__native_io_queue_cancel"(Int) -> Int),
-    native!("__native_io_wait"() -> Int),
     native!("__native_io_status"(Int) -> Int),
     native!("__native_io_result_string"(Int) -> String),
     native!("__native_io_result_number"(Int) -> Float),
@@ -110,6 +109,12 @@ static PROTO_LINKS: &[(&str, ProtoOf)] = &[
     ("__link_proto_array", ProtoOf::Array),
 ];
 
+/// `__link_host_complete(f)` makes `f` what the host calls when an IO op
+/// completes, with the op's handle (see `jsrt_complete` in `lib/rt/jsrt.h`).
+/// The host owns the event loop: a flavor of the runtime runs it, and the
+/// program only queues ops and handles their completions
+pub const HOST_LINK: &str = "__link_host_complete";
+
 /// The value of the global `name`. The only globals are the ones the stdlib
 /// may read, anything else is an error
 pub fn global(name: AtomRef, atoms: &[Atom]) -> JsValue {
@@ -124,6 +129,10 @@ pub fn global(name: AtomRef, atoms: &[Atom]) -> JsValue {
         return JsValue::NativeFn(idx);
     }
 
+    if name == HOST_LINK {
+        return JsValue::HostLink;
+    }
+
     match PROTO_LINKS.iter().find(|(link, _)| name == *link) {
         Some(&(_, kind)) => JsValue::ProtoLink(kind),
         None => panic!("unknown global `{name}`"),

lib/rt/io.h

@@ -2,9 +2,10 @@
 // io_uring (see uring.h)
 //
 // A `__native_io_queue_<op>` queues an op and gives its handle. It makes no
-// syscall. `__native_io_wait` puts queued ops into the SQ, submits them,
+// syscall, and no native waits. The host's loop (`main` below) puts queued
-// waits until one op is done, and gives its handle. The getters then read
+// ops into the SQ, submits them, waits until one op is done, and gives its
-// that op's result, until the next wait
+// handle to `jsrt_complete`. The getters read that op's result while
+// `jsrt_complete` runs
 //
 // At most `cq_entries` ops are in flight, so the CQ cannot overflow. The rest
 // stay queued until a later wait. So a queue never fails, and the program
@@ -392,8 +393,8 @@ int32_t __native_io_queue_cancel(int32_t target) {
 }
 
 // The handle of an op that is done, or -1 if no op is queued or in flight.
-// The op that the last wait gave is freed now
+// The op that the last call gave is freed now
-int32_t __native_io_wait(void) {
+static int32_t jsrt_io_next(void) {
     if (jsrt_io.given >= 0) {
         jsrt_io_release(jsrt_io.given);
         jsrt_io.given = -1;
@@ -452,4 +453,18 @@ double __native_io_result_number(int32_t h) {
     return op->res;
 }
 
+#ifndef JSRT_NO_HOST
+// The Linux flavor's host: run the program, then give it each op as it
+// completes, until none is left
+int main(void) {
+    jsrt_start();
+
+    for (int32_t h; (h = jsrt_io_next()) >= 0;) {
+        jsrt_complete(h);
+    }
+
+    return 0;
+}
+#endif
+
 #endif // JSRT_IMPLEMENTATION

lib/rt/jsrt.h

@@ -10,7 +10,15 @@
 // str.h      strings and the string natives
 // number.h   the number natives
 // any.h      any_t and JS's operators on it
-// io.h       IO natives, async on io_uring (uring.h)
+// io.h       IO natives, async on io_uring (uring.h), and the host's loop
+//
+// The host owns the event loop. The program exports two fns, which the host
+// calls, and a flavor of the runtime is the host. This one, for Linux, is the
+// `main` in io.h: define JSRT_NO_HOST to bring another
+//
+// jsrt_start()      runs the program's top level, then its microtasks
+// jsrt_complete(h)  op `h` is done: runs its callback, then the microtasks.
+//                   The op's result can be read only until this returns
 #pragma once
 
 // io.h needs Linux's syscall() and mmap(), which -std=c11 hides. This must
@@ -20,6 +28,12 @@
 #endif
 
 #include "core.h"
+
+#include <stdint.h>
+
+void jsrt_start(void);
+void jsrt_complete(int32_t h);
+
 #include "str.h"
 #include "number.h"
 #include "any.h"

lib/stdlib/end.js

@@ -1,2 +1,3 @@
-// Runs after the program: the event loop (see loop.js)
+// Runs after the program's top level: its microtasks. The host then runs
-__run_loop();
+// the event loop (see loop.js)
+__drain_microtasks();

lib/stdlib/loop.js

@@ -1,8 +1,8 @@
-// The event loop. A macrotask is an IO op or a timer, which completes in C
+// The event loop's JS side. The host owns the loop (see lib/rt/jsrt.h). A
-// (see lib/rt/io.h): `__native_io_wait()` gives the handle of the next one
+// macrotask is an IO op or a timer, which the host runs: it calls
-// that completes. A microtask is a promise job, in `__microtasks`. After the
+// `__complete(h)` when op `h` is done. A microtask is a promise job, in
-// program, end.js runs the loop: all microtasks, then one macrotask, then all
+// `__microtasks`. After the program's top level, end.js runs all microtasks,
-// microtasks again, until nothing is pending
+// and after each macrotask, `__complete` does
 
 // What to do when IO op `h` completes: `__pending[h]`, a fn that gets `h`.
 // Handles are small ints, and a handle is used again after its op completes
@@ -32,15 +32,12 @@ function __drain_microtasks() {
     q.head = 0;
 }
 
-function __run_loop() {
+// What the host calls when op `h` is done: its callback, then the microtasks
-    for (;;) {
+function __complete(h) {
-        __drain_microtasks();
+    const done = __pending[h];
-
+    __pending[h] = undefined;
-        const h = __native_io_wait();
+    if (done !== undefined) done(h);
-        if (h < 0) return;
+    __drain_microtasks();
-
-        const done = __pending[h];
-        __pending[h] = undefined;
-        if (done !== undefined) done(h);
-    }
 }
+
+__link_host_complete(__complete);