runtime: eliminate scase.kind field

Currently, we include a "kind" field on scase to distinguish the three
kinds of cases in a select statement: sends, receives, and defaults.

This commit removes by kind field by instead arranging for the
compiler to always place sends before receives, and to provide their
counts separately. It also passes an explicit "block bool" parameter
to avoid needing to include a default case in the array.

It's safe to shuffle cases like this because the runtime will
randomize the order they're polled in anyway.

Fixes #40410.

Change-Id: Iaeaed4cf7bddd576d78f2c863bd91a03a5c82df2
Reviewed-on: https://go-review.googlesource.com/c/go/+/245125
Reviewed-by: Keith Randall <khr@golang.org>

5 files changed, 123 insertions, 101 deletions

diff --git a/src/cmd/compile/internal/gc/builtin.go b/src/cmd/compile/internal/gc/builtin.go
index eafdb0ebe7..861ffaaa5b 100644
--- a/src/cmd/compile/internal/gc/builtin.go
+++ b/src/cmd/compile/internal/gc/builtin.go
@@ -302,7 +302,7 @@ func runtimeTypes() []*types.Type {
 	typs[96] = types.NewPtr(typs[6])
 	typs[97] = functype(nil, []*Node{anonfield(typs[3]), anonfield(typs[96]), anonfield(typs[84])}, []*Node{anonfield(typs[6])})
 	typs[98] = functype(nil, []*Node{anonfield(typs[63])}, nil)
-	typs[99] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[63]), anonfield(typs[15])}, []*Node{anonfield(typs[15]), anonfield(typs[6])})
+	typs[99] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[1]), anonfield(typs[63]), anonfield(typs[15]), anonfield(typs[15]), anonfield(typs[6])}, []*Node{anonfield(typs[15]), anonfield(typs[6])})
 	typs[100] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[7])})
 	typs[101] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[22]), anonfield(typs[22])}, []*Node{anonfield(typs[7])})
 	typs[102] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[15]), anonfield(typs[7])}, []*Node{anonfield(typs[7])})
diff --git a/src/cmd/compile/internal/gc/builtin/runtime.go b/src/cmd/compile/internal/gc/builtin/runtime.go
index 25f86efdd6..635da80f7c 100644
--- a/src/cmd/compile/internal/gc/builtin/runtime.go
+++ b/src/cmd/compile/internal/gc/builtin/runtime.go
@@ -170,7 +170,7 @@ func selectnbrecv(elem *any, hchan <-chan any) bool
 func selectnbrecv2(elem *any, received *bool, hchan <-chan any) bool
 
 func selectsetpc(pc *uintptr)
-func selectgo(cas0 *byte, order0 *byte, pc0 *uintptr, ncases int) (int, bool)
+func selectgo(cas0 *byte, order0 *byte, pc0 *uintptr, nsends int, nrecvs int, block bool) (int, bool)
 func block()
 
 func makeslice(typ *byte, len int, cap int) unsafe.Pointer
diff --git a/src/cmd/compile/internal/gc/select.go b/src/cmd/compile/internal/gc/select.go
index 8eb31eb5c1..bae7ed30e2 100644
--- a/src/cmd/compile/internal/gc/select.go
+++ b/src/cmd/compile/internal/gc/select.go
@@ -106,18 +106,16 @@ func walkselect(sel *Node) {
 }
 
 func walkselectcases(cases *Nodes) []*Node {
-	n := cases.Len()
+	ncas := cases.Len()
 	sellineno := lineno
 
 	// optimization: zero-case select
-	if n == 0 {
+	if ncas == 0 {
 		return []*Node{mkcall("block", nil, nil)}
 	}
 
 	// optimization: one-case select: single op.
-	// TODO(rsc): Reenable optimization once order.go can handle it.
-	// golang.org/issue/7672.
-	if n == 1 {
+	if ncas == 1 {
 		cas := cases.First()
 		setlineno(cas)
 		l := cas.Ninit.Slice()
@@ -178,10 +176,12 @@ func walkselectcases(cases *Nodes) []*Node {
 
 	// convert case value arguments to addresses.
 	// this rewrite is used by both the general code and the next optimization.
+	var dflt *Node
 	for _, cas := range cases.Slice() {
 		setlineno(cas)
 		n := cas.Left
 		if n == nil {
+			dflt = cas
 			continue
 		}
 		switch n.Op {
@@ -202,15 +202,10 @@ func walkselectcases(cases *Nodes) []*Node {
 	}
 
 	// optimization: two-case select but one is default: single non-blocking op.
-	if n == 2 && (cases.First().Left == nil || cases.Second().Left == nil) {
-		var cas *Node
-		var dflt *Node
-		if cases.First().Left == nil {
+	if ncas == 2 && dflt != nil {
+		cas := cases.First()
+		if cas == dflt {
 			cas = cases.Second()
-			dflt = cases.First()
-		} else {
-			dflt = cases.Second()
-			cas = cases.First()
 		}
 
 		n := cas.Left
@@ -257,74 +252,73 @@ func walkselectcases(cases *Nodes) []*Node {
 		return []*Node{r, nod(OBREAK, nil, nil)}
 	}
 
+	if dflt != nil {
+		ncas--
+	}
+	casorder := make([]*Node, ncas)
+	nsends, nrecvs := 0, 0
+
 	var init []*Node
 
 	// generate sel-struct
 	lineno = sellineno
-	selv := temp(types.NewArray(scasetype(), int64(n)))
+	selv := temp(types.NewArray(scasetype(), int64(ncas)))
 	r := nod(OAS, selv, nil)
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
-	order := temp(types.NewArray(types.Types[TUINT16], 2*int64(n)))
+	order := temp(types.NewArray(types.Types[TUINT16], 2*int64(ncas)))
 	r = nod(OAS, order, nil)
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
 	var pc0, pcs *Node
 	if flag_race {
-		pcs = temp(types.NewArray(types.Types[TUINTPTR], int64(n)))
+		pcs = temp(types.NewArray(types.Types[TUINTPTR], int64(ncas)))
 		pc0 = typecheck(nod(OADDR, nod(OINDEX, pcs, nodintconst(0)), nil), ctxExpr)
 	} else {
 		pc0 = nodnil()
 	}
 
 	// register cases
-	for i, cas := range cases.Slice() {
+	for _, cas := range cases.Slice() {
 		setlineno(cas)
 
 		init = append(init, cas.Ninit.Slice()...)
 		cas.Ninit.Set(nil)
 
-		// Keep in sync with runtime/select.go.
-		const (
-			caseNil = iota
-			caseRecv
-			caseSend
-			caseDefault
-		)
+		n := cas.Left
+		if n == nil { // default:
+			continue
+		}
 
+		var i int
 		var c, elem *Node
-		var kind int64 = caseDefault
-
-		if n := cas.Left; n != nil {
-			init = append(init, n.Ninit.Slice()...)
-
-			switch n.Op {
-			default:
-				Fatalf("select %v", n.Op)
-			case OSEND:
-				kind = caseSend
-				c = n.Left
-				elem = n.Right
-			case OSELRECV, OSELRECV2:
-				kind = caseRecv
-				c = n.Right.Left
-				elem = n.Left
-			}
+		switch n.Op {
+		default:
+			Fatalf("select %v", n.Op)
+		case OSEND:
+			i = nsends
+			nsends++
+			c = n.Left
+			elem = n.Right
+		case OSELRECV, OSELRECV2:
+			nrecvs++
+			i = ncas - nrecvs
+			c = n.Right.Left
+			elem = n.Left
 		}
 
+		casorder[i] = cas
+
 		setField := func(f string, val *Node) {
 			r := nod(OAS, nodSym(ODOT, nod(OINDEX, selv, nodintconst(int64(i))), lookup(f)), val)
 			r = typecheck(r, ctxStmt)
 			init = append(init, r)
 		}
 
-		setField("kind", nodintconst(kind))
-		if c != nil {
-			c = convnop(c, types.Types[TUNSAFEPTR])
-			setField("c", c)
-		}
+		c = convnop(c, types.Types[TUNSAFEPTR])
+		setField("c", c)
 		if elem != nil {
 			elem = convnop(elem, types.Types[TUNSAFEPTR])
 			setField("elem", elem)
@@ -337,6 +331,9 @@ func walkselectcases(cases *Nodes) []*Node {
 			init = append(init, r)
 		}
 	}
+	if nsends+nrecvs != ncas {
+		Fatalf("walkselectcases: miscount: %v + %v != %v", nsends, nrecvs, ncas)
+	}
 
 	// run the select
 	lineno = sellineno
@@ -345,7 +342,7 @@ func walkselectcases(cases *Nodes) []*Node {
 	r = nod(OAS2, nil, nil)
 	r.List.Set2(chosen, recvOK)
 	fn := syslook("selectgo")
-	r.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, bytePtrToIndex(selv, 0), bytePtrToIndex(order, 0), pc0, nodintconst(int64(n))))
+	r.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, bytePtrToIndex(selv, 0), bytePtrToIndex(order, 0), pc0, nodintconst(int64(nsends)), nodintconst(int64(nrecvs)), nodbool(dflt == nil)))
 	r = typecheck(r, ctxStmt)
 	init = append(init, r)
 
@@ -357,14 +354,11 @@ func walkselectcases(cases *Nodes) []*Node {
 	}
 
 	// dispatch cases
-	for i, cas := range cases.Slice() {
-		setlineno(cas)
-
-		cond := nod(OEQ, chosen, nodintconst(int64(i)))
+	dispatch := func(cond, cas *Node) {
 		cond = typecheck(cond, ctxExpr)
 		cond = defaultlit(cond, nil)
 
-		r = nod(OIF, cond, nil)
+		r := nod(OIF, cond, nil)
 
 		if n := cas.Left; n != nil && n.Op == OSELRECV2 {
 			x := nod(OAS, n.List.First(), recvOK)
@@ -377,6 +371,15 @@ func walkselectcases(cases *Nodes) []*Node {
 		init = append(init, r)
 	}
 
+	if dflt != nil {
+		setlineno(dflt)
+		dispatch(nod(OLT, chosen, nodintconst(0)), dflt)
+	}
+	for i, cas := range casorder {
+		setlineno(cas)
+		dispatch(nod(OEQ, chosen, nodintconst(int64(i))), cas)
+	}
+
 	return init
 }
 
@@ -395,7 +398,6 @@ func scasetype() *types.Type {
 		scase = tostruct([]*Node{
 			namedfield("c", types.Types[TUNSAFEPTR]),
 			namedfield("elem", types.Types[TUNSAFEPTR]),
-			namedfield("kind", types.Types[TUINT16]),
 		})
 		scase.SetNoalg(true)
 	}
diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index ed2f225077..5a12699472 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -1725,6 +1725,14 @@ func TestSelectMaxCases(t *testing.T) {
 	_, _, _ = Select(sCases)
 }
 
+func TestSelectNop(t *testing.T) {
+	// "select { default: }" should always return the default case.
+	chosen, _, _ := Select([]SelectCase{{Dir: SelectDefault}})
+	if chosen != 0 {
+		t.Fatalf("expected Select to return 0, but got %#v", chosen)
+	}
+}
+
 func BenchmarkSelect(b *testing.B) {
 	channel := make(chan int)
 	close(channel)
diff --git a/src/runtime/select.go b/src/runtime/select.go
index d7c7d9f26f..80768b285b 100644
--- a/src/runtime/select.go
+++ b/src/runtime/select.go
@@ -12,23 +12,12 @@ import (
 
 const debugSelect = false
 
-// scase.kind values.
-// Known to compiler.
-// Changes here must also be made in src/cmd/compile/internal/gc/select.go's walkselectcases.
-const (
-	caseNil = iota
-	caseRecv
-	caseSend
-	caseDefault
-)
-
 // Select case descriptor.
 // Known to compiler.
 // Changes here must also be made in src/cmd/internal/gc/select.go's scasetype.
 type scase struct {
 	c    *hchan         // chan
 	elem unsafe.Pointer // data element
-	kind uint16
 }
 
 var (
@@ -115,7 +104,7 @@ func block() {
 // ordinal position of its respective select{recv,send,default} call.
 // Also, if the chosen scase was a receive operation, it reports whether
 // a value was received.
-func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool) {
+func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, nsends, nrecvs int, block bool) (int, bool) {
 	if debugSelect {
 		print("select: cas0=", cas0, "\n")
 	}
@@ -125,6 +114,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 	cas1 := (*[1 << 16]scase)(unsafe.Pointer(cas0))
 	order1 := (*[1 << 17]uint16)(unsafe.Pointer(order0))
 
+	ncases := nsends + nrecvs
 	scases := cas1[:ncases:ncases]
 	pollorder := order1[:ncases:ncases]
 	lockorder := order1[ncases:][:ncases:ncases]
@@ -158,16 +148,12 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 	// optimizing (and needing to test).
 
 	// generate permuted order
-	dfli := -1
 	norder := 0
 	for i := range scases {
 		cas := &scases[i]
 
 		// Omit cases without channels from the poll and lock orders.
 		if cas.c == nil {
-			if cas.kind == caseDefault {
-				dfli = i
-			}
 			cas.elem = nil // allow GC
 			continue
 		}
@@ -250,8 +236,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 		cas = &scases[casi]
 		c = cas.c
 
-		switch cas.kind {
-		case caseRecv:
+		if casi >= nsends {
 			sg = c.sendq.dequeue()
 			if sg != nil {
 				goto recv
@@ -262,8 +247,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 			if c.closed != 0 {
 				goto rclose
 			}
-
-		case caseSend:
+		} else {
 			if raceenabled {
 				racereadpc(c.raceaddr(), casePC(casi), chansendpc)
 			}
@@ -280,9 +264,9 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 		}
 	}
 
-	if dfli >= 0 {
+	if !block {
 		selunlock(scases, lockorder)
-		casi = dfli
+		casi = -1
 		goto retc
 	}
 
@@ -311,12 +295,10 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 		*nextp = sg
 		nextp = &sg.waitlink
 
-		switch cas.kind {
-		case caseRecv:
-			c.recvq.enqueue(sg)
-
-		case caseSend:
+		if casi < nsends {
 			c.sendq.enqueue(sg)
+		} else {
+			c.recvq.enqueue(sg)
 		}
 	}
 
@@ -359,7 +341,7 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 			}
 		} else {
 			c = k.c
-			if k.kind == caseSend {
+			if int(casei) < nsends {
 				c.sendq.dequeueSudoG(sglist)
 			} else {
 				c.recvq.dequeueSudoG(sglist)
@@ -378,27 +360,29 @@ func selectgo(cas0 *scase, order0 *uint16, pc0 *uintptr, ncases int) (int, bool)
 	c = cas.c
 
 	if debugSelect {
-		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " kind=", cas.kind, "\n")
+		print("wait-return: cas0=", cas0, " c=", c, " cas=", cas, " send=", casi < nsends, "\n")
 	}
 
-	if cas.kind == caseRecv {
+	if casi < nsends {
+		if !caseSuccess {
+			goto sclose
+		}
+	} else {
 		recvOK = caseSuccess
-	} else if cas.kind == caseSend && !caseSuccess {
-		goto sclose
 	}
 
 	if raceenabled {
-		if cas.kind == caseRecv && cas.elem != nil {
-			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
-		} else if cas.kind == caseSend {
+		if casi < nsends {
 			raceReadObjectPC(c.elemtype, cas.elem, casePC(casi), chansendpc)
+		} else if cas.elem != nil {
+			raceWriteObjectPC(c.elemtype, cas.elem, casePC(casi), chanrecvpc)
 		}
 	}
 	if msanenabled {
-		if cas.kind == caseRecv && cas.elem != nil {
-			msanwrite(cas.elem, c.elemtype.size)
-		} else if cas.kind == caseSend {
+		if casi < nsends {
 			msanread(cas.elem, c.elemtype.size)
+		} else if cas.elem != nil {
+			msanwrite(cas.elem, c.elemtype.size)
 		}
 	}
 
@@ -526,29 +510,57 @@ func reflect_rselect(cases []runtimeSelect) (int, bool) {
 		block()
 	}
 	sel := make([]scase, len(cases))
-	order := make([]uint16, 2*len(cases))
-	for i := range cases {
-		rc := &cases[i]
+	orig := make([]int, len(cases))
+	nsends, nrecvs := 0, 0
+	dflt := -1
+	for i, rc := range cases {
+		var j int
 		switch rc.dir {
 		case selectDefault:
-			sel[i] = scase{kind: caseDefault}
+			dflt = i
+			continue
 		case selectSend:
-			sel[i] = scase{kind: caseSend, c: rc.ch, elem: rc.val}
+			j = nsends
+			nsends++
 		case selectRecv:
-			sel[i] = scase{kind: caseRecv, c: rc.ch, elem: rc.val}
+			nrecvs++
+			j = len(cases) - nrecvs
 		}
+
+		sel[j] = scase{c: rc.ch, elem: rc.val}
+		orig[j] = i
 	}
 
+	// Only a default case.
+	if nsends+nrecvs == 0 {
+		return dflt, false
+	}
+
+	// Compact sel and orig if necessary.
+	if nsends+nrecvs < len(cases) {
+		copy(sel[nsends:], sel[len(cases)-nrecvs:])
+		copy(orig[nsends:], orig[len(cases)-nrecvs:])
+	}
+
+	order := make([]uint16, 2*(nsends+nrecvs))
 	var pc0 *uintptr
 	if raceenabled {
-		pcs := make([]uintptr, len(cases))
+		pcs := make([]uintptr, nsends+nrecvs)
 		for i := range pcs {
 			selectsetpc(&pcs[i])
 		}
 		pc0 = &pcs[0]
 	}
 
-	return selectgo(&sel[0], &order[0], pc0, len(cases))
+	chosen, recvOK := selectgo(&sel[0], &order[0], pc0, nsends, nrecvs, dflt == -1)
+
+	// Translate chosen back to caller's ordering.
+	if chosen < 0 {
+		chosen = dflt
+	} else {
+		chosen = orig[chosen]
+	}
+	return chosen, recvOK
 }
 
 func (q *waitq) dequeueSudoG(sgp *sudog) {