Select support (#36)

This PR adds support for `select` statements.

compiler/coroutine_test.go

@@ -99,6 +99,12 @@ func TestCoroutineYield(t *testing.T) {
 			coro:   func() { RangeOverMaps(5) },
 			yields: []int{0, 5, 5, 50, 5, 4, 3, 2, 1, 0},
 		},
+
+		{
+			name:   "select",
+			coro:   func() { Select(8) },
+			yields: []int{-1, 0, 0, 1, 10, 2, 20, 3, 30, 4, 40, 50, 0, 1, 2},
+		},
 	}
 
 	// TODO: remove me

compiler/desugar.go

@@ -22,6 +22,10 @@ import (
 // labels so that the desugaring pass (and other compilation passes) are able
 // to both decompose and introduce control flow.
 //
+// Nondeterministic control flow and iteration (e.g. select, for..range
+// over maps) is split into two parts so that yield points within can resume
+// from the same place.
+//
 // The desugaring pass works at the statement level (ast.Stmt) and does not
 // consider expressions (ast.Expr). This means that the pass does not
 // recurse into expressions that may contain statements. At this time, only
@@ -273,6 +277,12 @@ func (d *desugarer) desugar(stmt ast.Stmt, breakTo, continueTo, userLabel *ast.I
 			Body: d.desugarList(s.Body, breakTo, continueTo),
 		}
 
+	case *ast.CommClause:
+		stmt = &ast.CommClause{
+			Comm: d.desugar(s.Comm, nil, nil, nil),
+			Body: d.desugarList(s.Body, breakTo, continueTo),
+		}
+
 	case *ast.BranchStmt:
 		if s.Label != nil {
 			label := d.getUserLabel(s.Label)
@@ -299,8 +309,40 @@ func (d *desugarer) desugar(stmt ast.Stmt, breakTo, continueTo, userLabel *ast.I
 		// labels can be mapped.
 		stmt = d.desugar(s.Stmt, breakTo, continueTo, s.Label)
 
-	case *ast.SelectStmt, *ast.CommClause:
-		panic("not implemented")
+	case *ast.SelectStmt:
+		// Rewrite select statements into a select+switch statement. The
+		// select cases exist only to record the selection; the select
+		// case bodies are moved into the switch statement over that
+		// selection. This allows coroutines to jump back to the right
+		// case when resuming.
+		selection := d.newVar(types.Typ[types.Int])
+		switchBody := &ast.BlockStmt{List: make([]ast.Stmt, len(s.Body.List))}
+		for i, c := range s.Body.List {
+			cc := c.(*ast.CommClause)
+			id := &ast.BasicLit{Kind: token.INT, Value: strconv.Itoa(i + 1)}
+			switchBody.List[i] = &ast.CaseClause{
+				List: []ast.Expr{id},
+				Body: cc.Body,
+			}
+			cc.Body = []ast.Stmt{
+				&ast.AssignStmt{
+					Lhs: []ast.Expr{selection},
+					Tok: token.ASSIGN,
+					Rhs: []ast.Expr{id},
+				},
+			}
+		}
+		stmt = &ast.BlockStmt{
+			List: []ast.Stmt{
+				&ast.AssignStmt{
+					Lhs: []ast.Expr{selection},
+					Tok: token.DEFINE,
+					Rhs: []ast.Expr{&ast.BasicLit{Kind: token.INT, Value: "0"}},
+				},
+				s,
+				d.desugar(&ast.SwitchStmt{Tag: selection, Body: switchBody}, breakTo, continueTo, userLabel),
+			},
+		}
 
 	case *ast.AssignStmt, *ast.DeclStmt, *ast.DeferStmt, *ast.EmptyStmt,
 		*ast.ExprStmt, *ast.GoStmt, *ast.IncDecStmt, *ast.ReturnStmt, *ast.SendStmt:

compiler/dispatch.go

@@ -43,6 +43,12 @@ func trackDispatchSpans0(stmt ast.Stmt, dispatchSpans map[ast.Stmt]dispatchSpan,
 		for _, child := range s.Body {
 			nextID = trackDispatchSpans0(child, dispatchSpans, nextID)
 		}
+	case *ast.SelectStmt:
+		nextID = trackDispatchSpans0(s.Body, dispatchSpans, nextID)
+	case *ast.CommClause:
+		for _, child := range s.Body {
+			nextID = trackDispatchSpans0(child, dispatchSpans, nextID)
+		}
 	case *ast.LabeledStmt:
 		nextID = trackDispatchSpans0(s.Stmt, dispatchSpans, nextID)
 	default:
@@ -105,6 +111,10 @@ func compileDispatch(stmt ast.Stmt, dispatchSpans map[ast.Stmt]dispatchSpan) ast
 		for i, child := range s.Body.List {
 			s.Body.List[i] = compileDispatch(child, dispatchSpans)
 		}
+	case *ast.SelectStmt:
+		for i, child := range s.Body.List {
+			s.Body.List[i] = compileDispatch(child, dispatchSpans)
+		}
 	case *ast.CaseClause:
 		switch {
 		case len(s.Body) == 1:
@@ -113,6 +123,14 @@ func compileDispatch(stmt ast.Stmt, dispatchSpans map[ast.Stmt]dispatchSpan) ast
 		case len(s.Body) > 1:
 			s.Body = []ast.Stmt{compileDispatch0(s.Body, dispatchSpans)}
 		}
+	case *ast.CommClause:
+		switch {
+		case len(s.Body) == 1:
+			child := compileDispatch(s.Body[0], dispatchSpans)
+			s.Body[0] = unnestBlocks(child)
+		case len(s.Body) > 1:
+			s.Body = []ast.Stmt{compileDispatch0(s.Body, dispatchSpans)}
+		}
 	case *ast.LabeledStmt:
 		s.Stmt = compileDispatch(s.Stmt, dispatchSpans)
 	}

compiler/testdata/coroutine.go

@@ -3,6 +3,7 @@
 package testdata
 
 import (
+	"time"
 	"unsafe"
 
 	"github.com/stealthrocket/coroutine"
@@ -254,3 +255,38 @@ func RangeOverMaps(n int) {
 		coroutine.Yield[int, any](len(m2))
 	}
 }
+
+func Select(n int) {
+	select {
+	default:
+		coroutine.Yield[int, any](-1)
+	}
+
+	for i := 0; i < n; i++ {
+		select {
+		case <-time.After(0):
+			if i >= 5 {
+				break
+			}
+			coroutine.Yield[int, any](i)
+		case <-time.After(1 * time.Second):
+			panic("unreachable")
+		}
+
+	foo:
+		select {
+		case <-time.After(0):
+			if i >= 6 {
+				break foo
+			}
+			coroutine.Yield[int, any](i * 10)
+		}
+	}
+
+	select {
+	case <-time.After(0):
+		for j := 0; j < 3; j++ {
+			coroutine.Yield[int, any](j)
+		}
+	}
+}