feat: async functions with return values

- runtime: bux_async_return(value, size) copies result to task->result
- runtime: bux_async_await returns void* result pointer
- runtime: bux_async_result(handle) to retrieve result
- sema: ekAwait returns *void
- HIR: await lowered to bux_async_await returning *void
- Example: async Compute() -> int with interleaved execution and result await

examples/async.bux

@@ -1,32 +1,42 @@
-import Std::Io::PrintLine;
+import Std::Io::{PrintLine, PrintInt};
 
 extern func bux_async_yield();
 extern func bux_async_run();
 extern func bux_async_spawn(fn: *void) -> *void;
-extern func bux_async_await(handle: *void);
+extern func bux_async_await(handle: *void) -> *void;
+extern func bux_async_return(value: *void, size: int64);
 
-async func WorkA() {
-    PrintLine("WorkA: step 1");
+async func Compute() -> int {
+    PrintLine("Compute: step 1");
     bux_async_yield();
-    PrintLine("WorkA: step 2");
-    bux_async_yield();
-    PrintLine("WorkA: done");
+    PrintLine("Compute: step 2");
+    let result: int = 42;
+    bux_async_return((&result) as *void, sizeof(int));
+    return result;
 }
 
-async func WorkB() {
-    PrintLine("WorkB: step 1");
+async func Double() -> int {
+    PrintLine("Double: step 1");
     bux_async_yield();
-    PrintLine("WorkB: step 2");
-    bux_async_yield();
-    PrintLine("WorkB: done");
+    PrintLine("Double: step 2");
+    let result: int = 84;
+    bux_async_return((&result) as *void, sizeof(int));
+    return result;
 }
 
 func Main() -> int {
     PrintLine("Main: start");
-    let h1: *void = spawn WorkA();
-    let h2: *void = spawn WorkB();
-    bux_async_await(h1);
-    bux_async_await(h2);
-    PrintLine("Main: all done");
+    let h1: *void = spawn Compute();
+    let h2: *void = spawn Double();
+    let r1Ptr: *int = h1.await as *int;
+    let r2Ptr: *int = h2.await as *int;
+    let r1: int = *r1Ptr;
+    let r2: int = *r2Ptr;
+    PrintLine("Results:");
+    PrintInt(r1);
+    PrintLine("");
+    PrintInt(r2);
+    PrintLine("");
+    PrintLine("Main: done");
     return 0;
 }

src/hir_lower.nim

@@ -770,7 +770,7 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
 
   of ekAwait:
     let lowered = ctx.lowerExpr(expr.exprAwaitOperand)
-    return hirCall("bux_async_await", @[lowered], makeVoid(), loc)
+    return hirCall("bux_async_await", @[lowered], makePointer(makeVoid()), loc)
 
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),

src/sema.nim

@@ -1010,8 +1010,8 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
     return makePointer(makeVoid())
   of ekAwait:
     let operand = sema.checkExpr(expr.exprAwaitOperand, scope)
-    # await on a task handle returns void for now
-    return makeVoid()
+    # await on a task handle returns *void (result pointer)
+    return makePointer(makeVoid())
   of ekSpread:
     return sema.checkExpr(expr.exprSpreadOperand, scope)
 

stdlib/runtime.c

@@ -808,6 +808,7 @@ typedef struct bux_async_task {
     uint8_t* stack;
     int state;           /* 0 = ready, 1 = running, 2 = done */
     void (*entry)(void);
+    void* result;        /* pointer to heap-allocated result */
     struct bux_async_task* next;
 } bux_async_task_t;
 
@@ -883,8 +884,8 @@ void bux_async_yield(void) {
     }
 }
 
-void bux_async_await(void* handle) {
-    if (!handle) return;
+void* bux_async_await(void* handle) {
+    if (!handle) return NULL;
     bux_async_task_t* target = (bux_async_task_t*)handle;
     while (target->state != 2) {
         if (bux_current_task != NULL) {
@@ -897,6 +898,7 @@ void bux_async_await(void* handle) {
             }
         }
     }
+    return target->result;
 }
 
 void bux_async_run(void) {
@@ -931,3 +933,17 @@ void bux_async_sleep(int64_t ms) {
         bux_async_yield();
     }
 }
+
+void bux_async_return(void* value, size_t size) {
+    if (bux_current_task != NULL && value != NULL && size > 0) {
+        void* copy = malloc(size);
+        if (copy) memcpy(copy, value, size);
+        bux_current_task->result = copy;
+    }
+}
+
+void* bux_async_result(void* handle) {
+    if (!handle) return NULL;
+    bux_async_task_t* task = (bux_async_task_t*)handle;
+    return task->result;
+}