cmd/compile, runtime: simplify multiway select implementation

This commit reworks multiway select statements to use normal control
flow primitives instead of the previous setjmp/longjmp-like behavior.
This simplifies liveness analysis and should prevent issues around
"returns twice" function calls within SSA passes.

test/live.go is updated because liveness analysis's CFG is more
representative of actual control flow. The case bodies are the only
real successors of the selectgo call, but previously the selectsend,
selectrecv, etc. calls were included in the successors list too.

Updates #19331.

Change-Id: I7f879b103a4b85e62fc36a270d812f54c0aa3e83
Reviewed-on: https://go-review.googlesource.com/37661
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/builtin.go

@@ -100,11 +100,10 @@ var runtimeDecls = [...]struct {
 	{"selectnbrecv", funcTag, 82},
 	{"selectnbrecv2", funcTag, 84},
 	{"newselect", funcTag, 85},
-	{"selectsend", funcTag, 81},
-	{"selectrecv", funcTag, 72},
-	{"selectrecv2", funcTag, 86},
-	{"selectdefault", funcTag, 87},
-	{"selectgo", funcTag, 56},
+	{"selectsend", funcTag, 74},
+	{"selectrecv", funcTag, 86},
+	{"selectdefault", funcTag, 56},
+	{"selectgo", funcTag, 87},
 	{"block", funcTag, 5},
 	{"makeslice", funcTag, 89},
 	{"makeslice64", funcTag, 90},
@@ -227,8 +226,8 @@ func runtimeTypes() []*Type {
 	typs[83] = typPtr(typs[11])
 	typs[84] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[3]), anonfield(typs[83]), anonfield(typs[70])}, []*Node{anonfield(typs[11])})
 	typs[85] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[8])}, nil)
-	typs[86] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[70]), anonfield(typs[3]), anonfield(typs[83])}, []*Node{anonfield(typs[11])})
-	typs[87] = functype(nil, []*Node{anonfield(typs[1])}, []*Node{anonfield(typs[11])})
+	typs[86] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[70]), anonfield(typs[3]), anonfield(typs[83])}, nil)
+	typs[87] = functype(nil, []*Node{anonfield(typs[1])}, []*Node{anonfield(typs[32])})
 	typs[88] = typSlice(typs[2])
 	typs[89] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[32]), anonfield(typs[32])}, []*Node{anonfield(typs[88])})
 	typs[90] = functype(nil, []*Node{anonfield(typs[1]), anonfield(typs[15]), anonfield(typs[15])}, []*Node{anonfield(typs[88])})

src/cmd/compile/internal/gc/builtin/runtime.go

@@ -134,11 +134,10 @@ func selectnbrecv(chanType *byte, elem *any, hchan <-chan any) bool
 func selectnbrecv2(chanType *byte, elem *any, received *bool, hchan <-chan any) bool
 
 func newselect(sel *byte, selsize int64, size int32)
-func selectsend(sel *byte, hchan chan<- any, elem *any) (selected bool)
-func selectrecv(sel *byte, hchan <-chan any, elem *any) (selected bool)
-func selectrecv2(sel *byte, hchan <-chan any, elem *any, received *bool) (selected bool)
-func selectdefault(sel *byte) (selected bool)
-func selectgo(sel *byte)
+func selectsend(sel *byte, hchan chan<- any, elem *any)
+func selectrecv(sel *byte, hchan <-chan any, elem *any, received *bool)
+func selectdefault(sel *byte)
+func selectgo(sel *byte) int
 func block()
 
 func makeslice(typ *byte, len int, cap int) (ary []any)

src/cmd/compile/internal/gc/plive.go

@@ -306,49 +306,6 @@ func iscall(prog *obj.Prog, name *obj.LSym) bool {
 	return name == prog.To.Sym
 }
 
-// Returns true for instructions that call a runtime function implementing a
-// select communication clause.
-
-var selectNames [4]*obj.LSym
-
-func isselectcommcasecall(prog *obj.Prog) bool {
-	if selectNames[0] == nil {
-		selectNames[0] = Linksym(Pkglookup("selectsend", Runtimepkg))
-		selectNames[1] = Linksym(Pkglookup("selectrecv", Runtimepkg))
-		selectNames[2] = Linksym(Pkglookup("selectrecv2", Runtimepkg))
-		selectNames[3] = Linksym(Pkglookup("selectdefault", Runtimepkg))
-	}
-
-	for _, name := range selectNames {
-		if iscall(prog, name) {
-			return true
-		}
-	}
-	return false
-}
-
-// Returns true for call instructions that target runtime·newselect.
-
-var isnewselect_sym *obj.LSym
-
-func isnewselect(prog *obj.Prog) bool {
-	if isnewselect_sym == nil {
-		isnewselect_sym = Linksym(Pkglookup("newselect", Runtimepkg))
-	}
-	return iscall(prog, isnewselect_sym)
-}
-
-// Returns true for call instructions that target runtime·selectgo.
-
-var isselectgocall_sym *obj.LSym
-
-func isselectgocall(prog *obj.Prog) bool {
-	if isselectgocall_sym == nil {
-		isselectgocall_sym = Linksym(Pkglookup("selectgo", Runtimepkg))
-	}
-	return iscall(prog, isselectgocall_sym)
-}
-
 var isdeferreturn_sym *obj.LSym
 
 func isdeferreturn(prog *obj.Prog) bool {
@@ -358,52 +315,6 @@ func isdeferreturn(prog *obj.Prog) bool {
 	return iscall(prog, isdeferreturn_sym)
 }
 
-// Walk backwards from a runtime·selectgo call up to its immediately dominating
-// runtime·newselect call. Any successor nodes of communication clause nodes
-// are implicit successors of the runtime·selectgo call node. The goal of this
-// analysis is to add these missing edges to complete the control flow graph.
-func addselectgosucc(selectgo *BasicBlock) {
-	pred := selectgo
-	for {
-		if len(pred.pred) == 0 {
-			Fatalf("selectgo does not have a newselect")
-		}
-		pred = pred.pred[0]
-		if blockany(pred, isselectcommcasecall) {
-			// A select comm case block should have exactly one
-			// successor.
-			if len(pred.succ) != 1 {
-				Fatalf("select comm case has too many successors")
-			}
-			succ := pred.succ[0]
-
-			// Its successor should have exactly two successors.
-			// The drop through should flow to the selectgo block
-			// and the branch should lead to the select case
-			// statements block.
-			if len(succ.succ) != 2 {
-				Fatalf("select comm case successor has too many successors")
-			}
-
-			// Add the block as a successor of the selectgo block.
-			addedge(selectgo, succ)
-		}
-
-		if blockany(pred, isnewselect) {
-			// Reached the matching newselect.
-			break
-		}
-	}
-}
-
-// The entry point for the missing selectgo control flow algorithm. Takes a
-// slice of *BasicBlocks containing selectgo calls.
-func fixselectgo(selectgo []*BasicBlock) {
-	for _, bb := range selectgo {
-		addselectgosucc(bb)
-	}
-}
-
 // Constructs a control flow graph from a sequence of instructions. This
 // procedure is complicated by various sources of implicit control flow that are
 // not accounted for using the standard cfg construction algorithm. Returns a
@@ -418,10 +329,6 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 		p.Opt = nil
 	}
 
-	// Allocate a slice to remember where we have seen selectgo calls.
-	// These blocks will be revisited to add successor control flow edges.
-	var selectgo []*BasicBlock
-
 	// Loop through all instructions identifying branch targets
 	// and fall-throughs and allocate basic blocks.
 	var cfg []*BasicBlock
@@ -442,12 +349,6 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 				p.Link.Opt = newblock(p.Link)
 				cfg = append(cfg, p.Link.Opt.(*BasicBlock))
 			}
-		} else if isselectcommcasecall(p) || isselectgocall(p) {
-			// Accommodate implicit selectgo control flow.
-			if p.Link.Opt == nil {
-				p.Link.Opt = newblock(p.Link)
-				cfg = append(cfg, p.Link.Opt.(*BasicBlock))
-			}
 		}
 	}
 
@@ -468,11 +369,6 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 				// generate any unreachable RET instructions.
 				break
 			}
-
-			// Collect basic blocks with selectgo calls.
-			if isselectgocall(p) {
-				selectgo = append(selectgo, bb)
-			}
 		}
 
 		if bb.last.To.Type == obj.TYPE_BRANCH {
@@ -502,11 +398,6 @@ func newcfg(firstp *obj.Prog) []*BasicBlock {
 		}
 	}
 
-	// Add missing successor edges to the selectgo blocks.
-	if len(selectgo) != 0 {
-		fixselectgo(selectgo)
-	}
-
 	// Find a depth-first order and assign a depth-first number to
 	// all basic blocks.
 	for _, bb := range cfg {

src/cmd/compile/internal/gc/select.go

@@ -101,6 +101,7 @@ func walkselect(sel *Node) {
 	var n *Node
 	var var_ *Node
 	var selv *Node
+	var chosen *Node
 	if i == 0 {
 		sel.Nbody.Set1(mkcall("block", nil, nil))
 		goto out
@@ -165,6 +166,7 @@ func walkselect(sel *Node) {
 		}
 
 		l = append(l, cas.Nbody.Slice()...)
+		l = append(l, nod(OBREAK, nil, nil))
 		sel.Nbody.Set(l)
 		goto out
 	}
@@ -220,24 +222,21 @@ func walkselect(sel *Node) {
 		default:
 			Fatalf("select %v", n.Op)
 
-			// if selectnbsend(c, v) { body } else { default body }
 		case OSEND:
+			// if selectnbsend(c, v) { body } else { default body }
 			ch := n.Left
-
 			r.Left = mkcall1(chanfn("selectnbsend", 2, ch.Type), Types[TBOOL], &r.Ninit, typename(ch.Type), ch, n.Right)
 
-			// if c != nil && selectnbrecv(&v, c) { body } else { default body }
 		case OSELRECV:
+			// if c != nil && selectnbrecv(&v, c) { body } else { default body }
 			r = nod(OIF, nil, nil)
-
 			r.Ninit.Set(cas.Ninit.Slice())
 			ch := n.Right.Left
 			r.Left = mkcall1(chanfn("selectnbrecv", 2, ch.Type), Types[TBOOL], &r.Ninit, typename(ch.Type), n.Left, ch)
 
-			// if c != nil && selectnbrecv2(&v, c) { body } else { default body }
 		case OSELRECV2:
+			// if c != nil && selectnbrecv2(&v, c) { body } else { default body }
 			r = nod(OIF, nil, nil)
-
 			r.Ninit.Set(cas.Ninit.Slice())
 			ch := n.Right.Left
 			r.Left = mkcall1(chanfn("selectnbrecv2", 2, ch.Type), Types[TBOOL], &r.Ninit, typename(ch.Type), n.Left, n.List.First(), ch)
@@ -246,7 +245,7 @@ func walkselect(sel *Node) {
 		r.Left = typecheck(r.Left, Erv)
 		r.Nbody.Set(cas.Nbody.Slice())
 		r.Rlist.Set(append(dflt.Ninit.Slice(), dflt.Nbody.Slice()...))
-		sel.Nbody.Set1(r)
+		sel.Nbody.Set2(r, nod(OBREAK, nil, nil))
 		goto out
 	}
 
@@ -255,7 +254,6 @@ func walkselect(sel *Node) {
 
 	// generate sel-struct
 	setlineno(sel)
-
 	selv = temp(selecttype(int32(sel.Xoffset)))
 	r = nod(OAS, selv, nil)
 	r = typecheck(r, Etop)
@@ -264,52 +262,62 @@ func walkselect(sel *Node) {
 	r = mkcall("newselect", nil, nil, var_, nodintconst(selv.Type.Width), nodintconst(sel.Xoffset))
 	r = typecheck(r, Etop)
 	init = append(init, r)
+
 	// register cases
 	for _, cas := range sel.List.Slice() {
 		setlineno(cas)
-		n = cas.Left
-		r = nod(OIF, nil, nil)
-		r.Ninit.Set(cas.Ninit.Slice())
-		cas.Ninit.Set(nil)
-		if n != nil {
-			r.Ninit.AppendNodes(&n.Ninit)
-			n.Ninit.Set(nil)
-		}
 
-		if n == nil {
-			// selectdefault(sel *byte);
-			r.Left = mkcall("selectdefault", Types[TBOOL], &r.Ninit, var_)
-		} else {
+		init = append(init, cas.Ninit.Slice()...)
+		cas.Ninit.Set(nil)
+
+		var x *Node
+		if n := cas.Left; n != nil {
+			init = append(init, n.Ninit.Slice()...)
+
 			switch n.Op {
 			default:
 				Fatalf("select %v", n.Op)
-
-				// selectsend(sel *byte, hchan *chan any, elem *any) (selected bool);
 			case OSEND:
-				r.Left = mkcall1(chanfn("selectsend", 2, n.Left.Type), Types[TBOOL], &r.Ninit, var_, n.Left, n.Right)
-
-				// selectrecv(sel *byte, hchan *chan any, elem *any) (selected bool);
+				// selectsend(sel *byte, hchan *chan any, elem *any)
+				x = mkcall1(chanfn("selectsend", 2, n.Left.Type), nil, nil, var_, n.Left, n.Right)
 			case OSELRECV:
-				r.Left = mkcall1(chanfn("selectrecv", 2, n.Right.Left.Type), Types[TBOOL], &r.Ninit, var_, n.Right.Left, n.Left)
-
-				// selectrecv2(sel *byte, hchan *chan any, elem *any, received *bool) (selected bool);
+				// selectrecv(sel *byte, hchan *chan any, elem *any, received *bool)
+				x = mkcall1(chanfn("selectrecv", 2, n.Right.Left.Type), nil, nil, var_, n.Right.Left, n.Left, nodnil())
 			case OSELRECV2:
-				r.Left = mkcall1(chanfn("selectrecv2", 2, n.Right.Left.Type), Types[TBOOL], &r.Ninit, var_, n.Right.Left, n.Left, n.List.First())
+				// selectrecv(sel *byte, hchan *chan any, elem *any, received *bool)
+				x = mkcall1(chanfn("selectrecv", 2, n.Right.Left.Type), nil, nil, var_, n.Right.Left, n.Left, n.List.First())
 			}
+		} else {
+			// selectdefault(sel *byte)
+			x = mkcall("selectdefault", nil, nil, var_)
 		}
 
-		// selv is no longer alive after use.
-		r.Nbody.Append(nod(OVARKILL, selv, nil))
+		init = append(init, x)
+	}
 
+	// run the select
+	setlineno(sel)
+	chosen = temp(Types[TINT])
+	r = nod(OAS, chosen, mkcall("selectgo", Types[TINT], nil, var_))
+	r = typecheck(r, Etop)
+	init = append(init, r)
+
+	// selv is no longer alive after selectgo.
+	init = append(init, nod(OVARKILL, selv, nil))
+
+	// dispatch cases
+	for i, cas := range sel.List.Slice() {
+		setlineno(cas)
+
+		cond := nod(OEQ, chosen, nodintconst(int64(i)))
+		cond = typecheck(cond, Erv)
+
+		r = nod(OIF, cond, nil)
 		r.Nbody.AppendNodes(&cas.Nbody)
 		r.Nbody.Append(nod(OBREAK, nil, nil))
 		init = append(init, r)
 	}
 
-	// run the select
-	setlineno(sel)
-
-	init = append(init, mkcall("selectgo", nil, nil, var_))
 	sel.Nbody.Set(init)
 
 out:
@@ -328,7 +336,6 @@ func selecttype(size int32) *Type {
 	scase.List.Append(nod(ODCLFIELD, newname(lookup("chan")), typenod(ptrto(Types[TUINT8]))))
 	scase.List.Append(nod(ODCLFIELD, newname(lookup("pc")), typenod(Types[TUINTPTR])))
 	scase.List.Append(nod(ODCLFIELD, newname(lookup("kind")), typenod(Types[TUINT16])))
-	scase.List.Append(nod(ODCLFIELD, newname(lookup("so")), typenod(Types[TUINT16])))
 	scase.List.Append(nod(ODCLFIELD, newname(lookup("receivedp")), typenod(ptrto(Types[TUINT8]))))
 	scase.List.Append(nod(ODCLFIELD, newname(lookup("releasetime")), typenod(Types[TUINT64])))
 	scase = typecheck(scase, Etype)

src/cmd/compile/internal/gc/ssa.go

@@ -527,7 +527,7 @@ func (s *state) stmt(n *Node) {
 		s.call(n, callNormal)
 		if n.Op == OCALLFUNC && n.Left.Op == ONAME && n.Left.Class == PFUNC {
 			if fn := n.Left.Sym.Name; compiling_runtime && fn == "throw" ||
-				n.Left.Sym.Pkg == Runtimepkg && (fn == "throwinit" || fn == "gopanic" || fn == "panicwrap" || fn == "selectgo" || fn == "block") {
+				n.Left.Sym.Pkg == Runtimepkg && (fn == "throwinit" || fn == "gopanic" || fn == "panicwrap" || fn == "block") {
 				m := s.mem()
 				b := s.endBlock()
 				b.Kind = ssa.BlockExit
@@ -921,12 +921,13 @@ func (s *state) stmt(n *Node) {
 			lab.breakTarget = nil
 		}
 
-		// OSWITCH never falls through (s.curBlock == nil here).
-		// OSELECT does not fall through if we're calling selectgo.
-		// OSELECT does fall through if we're calling selectnb{send,recv}[2].
-		// In those latter cases, go to the code after the select.
-		if b := s.endBlock(); b != nil {
-			b.AddEdgeTo(bEnd)
+		// walk adds explicit OBREAK nodes to the end of all reachable code paths.
+		// If we still have a current block here, then mark it unreachable.
+		if s.curBlock != nil {
+			m := s.mem()
+			b := s.endBlock()
+			b.Kind = ssa.BlockExit
+			b.SetControl(m)
 		}
 		s.startBlock(bEnd)
 

src/runtime/asm_386.s

@@ -799,12 +799,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVL	AX, ret+4(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
-	MOVL	argp+0(FP),AX		// addr of first arg
-	MOVL	pc+4(FP), BX
-	MOVL	BX, -4(AX)		// set calling pc
-	RET
-
 // func cputicks() int64
 TEXT runtime·cputicks(SB),NOSPLIT,$0-8
 	TESTL	$0x4000000, runtime·cpuid_edx(SB) // no sse2, no mfence

src/runtime/asm_amd64.s

@@ -822,12 +822,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVQ	AX, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
-	MOVQ	argp+0(FP),AX		// addr of first arg
-	MOVQ	pc+8(FP), BX
-	MOVQ	BX, -8(AX)		// set calling pc
-	RET
-
 // func cputicks() int64
 TEXT runtime·cputicks(SB),NOSPLIT,$0-0
 	CMPB	runtime·lfenceBeforeRdtsc(SB), $1

src/runtime/asm_amd64p32.s

@@ -507,12 +507,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-12
 	MOVL	AX, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-8
-	MOVL	argp+0(FP),AX		// addr of first arg
-	MOVL	pc+4(FP), BX		// pc to set
-	MOVQ	BX, -8(AX)		// set calling pc
-	RET
-
 // int64 runtime·cputicks(void)
 TEXT runtime·cputicks(SB),NOSPLIT,$0-0
 	RDTSC

src/runtime/asm_arm.s

@@ -682,11 +682,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVW	R0, ret+4(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
-	MOVW	pc+4(FP), R0
-	MOVW	R0, 8(R13)		// set LR in caller
-	RET
-
 TEXT runtime·emptyfunc(SB),0,$0-0
 	RET
 

src/runtime/asm_arm64.s

@@ -709,11 +709,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	R0, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
-	MOVD	pc+8(FP), R0
-	MOVD	R0, 16(RSP)		// set LR in caller
-	RET
-
 TEXT runtime·abort(SB),NOSPLIT,$-8-0
 	B	(ZR)
 	UNDEF

src/runtime/asm_mips64x.s

@@ -621,11 +621,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVV	R1, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
-	MOVV	pc+8(FP), R1
-	MOVV	R1, 16(R29)		// set LR in caller
-	RET
-
 TEXT runtime·abort(SB),NOSPLIT,$-8-0
 	MOVW	(R0), R0
 	UNDEF

src/runtime/asm_mipsx.s

@@ -624,11 +624,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$4-8
 	MOVW	R1, ret+4(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$4-8
-	MOVW	pc+4(FP), R1
-	MOVW	R1, 8(R29)	// set LR in caller
-	RET
-
 TEXT runtime·abort(SB),NOSPLIT,$0-0
 	UNDEF
 

src/runtime/asm_ppc64x.s

@@ -719,11 +719,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	R3, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
-	MOVD	pc+8(FP), R3
-	MOVD	R3, FIXED_FRAME+8(R1)		// set LR in caller
-	RET
-
 TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
 	MOVW	(R0), R0
 	UNDEF

src/runtime/asm_s390x.s

@@ -661,11 +661,6 @@ TEXT runtime·getcallerpc(SB),NOSPLIT,$8-16
 	MOVD	R3, ret+8(FP)
 	RET
 
-TEXT runtime·setcallerpc(SB),NOSPLIT,$8-16
-	MOVD	pc+8(FP), R3
-	MOVD	R3, 16(R15)		// set LR in caller
-	RET
-
 TEXT runtime·abort(SB),NOSPLIT|NOFRAME,$0-0
 	MOVW	(R0), R0
 	UNDEF

src/runtime/select.go

@@ -11,11 +11,12 @@ import (
 	"unsafe"
 )
 
-const (
-	debugSelect = false
+const debugSelect = false
 
+const (
 	// scase.kind
-	caseRecv = iota
+	caseNil = iota
+	caseRecv
 	caseSend
 	caseDefault
 )
@@ -37,10 +38,9 @@ type hselect struct {
 type scase struct {
 	elem        unsafe.Pointer // data element
 	c           *hchan         // chan
-	pc          uintptr        // return pc
+	pc          uintptr        // return pc (for race detector / msan)
 	kind        uint16
-	so          uint16 // vararg of selected bool
-	receivedp   *bool  // pointer to received bool (recv2)
+	receivedp   *bool // pointer to received bool, if any
 	releasetime int64
 }
 
@@ -72,92 +72,63 @@ func newselect(sel *hselect, selsize int64, size int32) {
 	}
 }
 
-//go:nosplit
-func selectsend(sel *hselect, c *hchan, elem unsafe.Pointer) (selected bool) {
-	// nil cases do not compete
-	if c != nil {
-		selectsendImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
-	}
-	return
-}
-
-// cut in half to give stack a chance to split
-func selectsendImpl(sel *hselect, c *hchan, pc uintptr, elem unsafe.Pointer, so uintptr) {
+func selectsend(sel *hselect, c *hchan, elem unsafe.Pointer) {
+	pc := getcallerpc(unsafe.Pointer(&sel))
 	i := sel.ncase
 	if i >= sel.tcase {
 		throw("selectsend: too many cases")
 	}
 	sel.ncase = i + 1
+	if c == nil {
+		return
+	}
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
-
 	cas.pc = pc
 	cas.c = c
-	cas.so = uint16(so)
 	cas.kind = caseSend
 	cas.elem = elem
 
 	if debugSelect {
-		print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, " so=", cas.so, "\n")
+		print("selectsend s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, "\n")
 	}
 }
 
-//go:nosplit
-func selectrecv(sel *hselect, c *hchan, elem unsafe.Pointer) (selected bool) {
-	// nil cases do not compete
-	if c != nil {
-		selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, nil, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
-	}
-	return
-}
-
-//go:nosplit
-func selectrecv2(sel *hselect, c *hchan, elem unsafe.Pointer, received *bool) (selected bool) {
-	// nil cases do not compete
-	if c != nil {
-		selectrecvImpl(sel, c, getcallerpc(unsafe.Pointer(&sel)), elem, received, uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
-	}
-	return
-}
-
-func selectrecvImpl(sel *hselect, c *hchan, pc uintptr, elem unsafe.Pointer, received *bool, so uintptr) {
+func selectrecv(sel *hselect, c *hchan, elem unsafe.Pointer, received *bool) {
+	pc := getcallerpc(unsafe.Pointer(&sel))
 	i := sel.ncase
 	if i >= sel.tcase {
 		throw("selectrecv: too many cases")
 	}
 	sel.ncase = i + 1
+	if c == nil {
+		return
+	}
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
 	cas.pc = pc
 	cas.c = c
-	cas.so = uint16(so)
 	cas.kind = caseRecv
 	cas.elem = elem
 	cas.receivedp = received
 
 	if debugSelect {
-		print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, " so=", cas.so, "\n")
+		print("selectrecv s=", sel, " pc=", hex(cas.pc), " chan=", cas.c, "\n")
 	}
 }
 
-//go:nosplit
-func selectdefault(sel *hselect) (selected bool) {
-	selectdefaultImpl(sel, getcallerpc(unsafe.Pointer(&sel)), uintptr(unsafe.Pointer(&selected))-uintptr(unsafe.Pointer(&sel)))
-	return
-}
-
-func selectdefaultImpl(sel *hselect, callerpc uintptr, so uintptr) {
+func selectdefault(sel *hselect) {
+	pc := getcallerpc(unsafe.Pointer(&sel))
 	i := sel.ncase
 	if i >= sel.tcase {
 		throw("selectdefault: too many cases")
 	}
 	sel.ncase = i + 1
 	cas := (*scase)(add(unsafe.Pointer(&sel.scase), uintptr(i)*unsafe.Sizeof(sel.scase[0])))
-	cas.pc = callerpc
+	cas.pc = pc
 	cas.c = nil
-	cas.so = uint16(so)
 	cas.kind = caseDefault
 
 	if debugSelect {
-		print("selectdefault s=", sel, " pc=", hex(cas.pc), " so=", cas.so, "\n")
+		print("selectdefault s=", sel, " pc=", hex(cas.pc), "\n")
 	}
 }
 
@@ -181,14 +152,11 @@ func selunlock(scases []scase, lockorder []uint16) {
 	// the G that calls select runnable again and schedules it for execution.
 	// When the G runs on another M, it locks all the locks and frees sel.
 	// Now if the first M touches sel, it will access freed memory.
-	n := len(scases)
-	r := 0
-	// skip the default case
-	if n > 0 && scases[lockorder[0]].c == nil {
-		r = 1
-	}
-	for i := n - 1; i >= r; i-- {
+	for i := len(scases) - 1; i >= 0; i-- {
 		c := scases[lockorder[i]].c
+		if c == nil {
+			break
+		}
 		if i > 0 && c == scases[lockorder[i-1]].c {
 			continue // will unlock it on the next iteration
 		}
@@ -229,23 +197,21 @@ func block() {
 // *sel is on the current goroutine's stack (regardless of any
 // escaping in selectgo).
 //
-// selectgo does not return. Instead, it overwrites its return PC and
-// returns directly to the triggered select case. Because of this, it
-// cannot appear at the top of a split stack.
-//
+// selectgo returns the index of the chosen scase, which matches the
+// ordinal position of its respective select{recv,send,default} call.
 //go:nosplit
-func selectgo(sel *hselect) {
-	pc, offset := selectgoImpl(sel)
-	*(*bool)(add(unsafe.Pointer(&sel), uintptr(offset))) = true
-	setcallerpc(unsafe.Pointer(&sel), pc)
+func selectgo(sel *hselect) int {
+	return selectgoImpl(sel)
 }
 
-// selectgoImpl returns scase.pc and scase.so for the select
-// case which fired.
-func selectgoImpl(sel *hselect) (uintptr, uint16) {
+// Separate function to keep runtime/trace.TestTraceSymbolize happy.
+func selectgoImpl(sel *hselect) int {
 	if debugSelect {
 		print("select: sel=", sel, "\n")
 	}
+	if sel.ncase != sel.tcase {
+		throw("selectgo: case count mismatch")
+	}
 
 	scaseslice := slice{unsafe.Pointer(&sel.scase), int(sel.ncase), int(sel.ncase)}
 	scases := *(*[]scase)(unsafe.Pointer(&scaseslice))
@@ -338,13 +304,19 @@ func selectgoImpl(sel *hselect) (uintptr, uint16) {
 
 loop:
 	// pass 1 - look for something already waiting
+	var dfli int
 	var dfl *scase
+	var casi int
 	var cas *scase
 	for i := 0; i < int(sel.ncase); i++ {
-		cas = &scases[pollorder[i]]
+		casi = int(pollorder[i])
+		cas = &scases[casi]
 		c = cas.c
 
 		switch cas.kind {
+		case caseNil:
+			continue
+
 		case caseRecv:
 			sg = c.sendq.dequeue()
 			if sg != nil {
@@ -373,12 +345,14 @@ loop:
 			}
 
 		case caseDefault:
+			dfli = casi
 			dfl = cas
 		}
 	}
 
 	if dfl != nil {
 		selunlock(scases, lockorder)
+		casi = dfli
 		cas = dfl
 		goto retc
 	}
@@ -391,7 +365,11 @@ loop:
 	}
 	nextp = &gp.waiting
 	for _, casei := range lockorder {
-		cas = &scases[casei]
+		casi = int(casei)
+		cas = &scases[casi]
+		if cas.kind == caseNil {
+			continue
+		}
 		c = cas.c
 		sg := acquireSudog()
 		sg.g = gp
@@ -485,6 +463,7 @@ loop:
 	// otherwise they stack up on quiet channels
 	// record the successful case, if any.
 	// We singly-linked up the SudoGs in lock order.
+	casi = -1
 	cas = nil
 	sglist = gp.waiting
 	// Clear all elem before unlinking from gp.waiting.
@@ -497,11 +476,15 @@ loop:
 
 	for _, casei := range lockorder {
 		k = &scases[casei]
+		if k.kind == caseNil {
+			continue
+		}
 		if sglist.releasetime > 0 {
 			k.releasetime = sglist.releasetime
 		}
 		if sg == sglist {
 			// sg has already been dequeued by the G that woke us up.
+			casi = int(casei)
 			cas = k
 		} else {
 			c = k.c
@@ -650,7 +633,7 @@ retc:
 	if cas.releasetime > 0 {
 		blockevent(cas.releasetime-t0, 2)
 	}
-	return cas.pc, cas.so
+	return casi
 
 sclose:
 	// send on closed channel
@@ -694,22 +677,15 @@ func reflect_rselect(cases []runtimeSelect) (chosen int, recvOK bool) {
 		rc := &cases[i]
 		switch rc.dir {
 		case selectDefault:
-			selectdefaultImpl(sel, uintptr(i), 0)
+			selectdefault(sel)
 		case selectSend:
-			if rc.ch == nil {
-				break
-			}
-			selectsendImpl(sel, rc.ch, uintptr(i), rc.val, 0)
+			selectsend(sel, rc.ch, rc.val)
 		case selectRecv:
-			if rc.ch == nil {
-				break
-			}
-			selectrecvImpl(sel, rc.ch, uintptr(i), rc.val, r, 0)
+			selectrecv(sel, rc.ch, rc.val, r)
 		}
 	}
 
-	pc, _ := selectgoImpl(sel)
-	chosen = int(pc)
+	chosen = selectgo(sel)
 	recvOK = *r
 	return
 }

src/runtime/stubs.go

@@ -192,9 +192,6 @@ func cgocallback_gofunc(fv uintptr, frame uintptr, framesize, ctxt uintptr)
 // data dependency ordering.
 func publicationBarrier()
 
-//go:noescape
-func setcallerpc(argp unsafe.Pointer, pc uintptr)
-
 // getcallerpc returns the program counter (PC) of its caller's caller.
 // getcallersp returns the stack pointer (SP) of its caller's caller.
 // For both, the argp must be a pointer to the caller's first function argument.

test/live.go

@@ -589,14 +589,14 @@ func f38(b bool) {
 	// we care that the println lines have no live variables
 	// and therefore no output.
 	if b {
-		select { // ERROR "live at call to newselect: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to selectgo: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$"
-		case <-fc38(): // ERROR "live at call to selectrecv: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$"
+		select { // ERROR "live at call to newselect:( .autotmp_[0-9]+)+$" "live at call to selectgo:( .autotmp_[0-9]+)+$"
+		case <-fc38(): // ERROR "live at call to selectrecv:( .autotmp_[0-9]+)+$"
 			printnl()
-		case fc38() <- *fi38(1): // ERROR "live at call to fc38: .autotmp_[0-9]+$" "live at call to fi38: .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to selectsend: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$"
+		case fc38() <- *fi38(1): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$" "live at call to selectsend:( .autotmp_[0-9]+)+$"
 			printnl()
-		case *fi38(2) = <-fc38(): // ERROR "live at call to fc38: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to fi38: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to selectrecv: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$"
+		case *fi38(2) = <-fc38(): // ERROR "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$" "live at call to selectrecv:( .autotmp_[0-9]+)+$"
 			printnl()
-		case *fi38(3), *fb38() = <-fc38(): // ERROR "live at call to fb38: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to fc38: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to fi38: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$" "live at call to selectrecv2: .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+ .autotmp_[0-9]+$"
+		case *fi38(3), *fb38() = <-fc38(): // ERROR "live at call to fb38:( .autotmp_[0-9]+)+$" "live at call to fc38:( .autotmp_[0-9]+)+$" "live at call to fi38:( .autotmp_[0-9]+)+$" "live at call to selectrecv:( .autotmp_[0-9]+)+$"
 			printnl()
 		}
 		printnl()