src/cmd/compile/internal/gc/range.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package gc

 import (
 	"cmd/compile/internal/types"
 	"cmd/internal/sys"
 	"unicode/utf8"
 )

 // range
 func typecheckrange(n *Node) {
 	// Typechecking order is important here:
 	// 0. first typecheck range expression (slice/map/chan),
 	//	it is evaluated only once and so logically it is not part of the loop.
 	// 1. typecheck produced values,
 	//	this part can declare new vars and so it must be typechecked before body,
 	//	because body can contain a closure that captures the vars.
 	// 2. decldepth++ to denote loop body.
 	// 3. typecheck body.
 	// 4. decldepth--.
 	typecheckrangeExpr(n)

 	// second half of dance, the first half being typecheckrangeExpr
 	n.SetTypecheck(1)
 	ls := n.List.Slice()
 	for i1, n1 := range ls {
 		if n1.Typecheck() == 0 {
 			ls[i1] = typecheck(ls[i1], ctxExpr|ctxAssign)
 		}
 	}

 	decldepth++
 	typecheckslice(n.Nbody.Slice(), ctxStmt)
 	decldepth--
 }

 func typecheckrangeExpr(n *Node) {
 	n.Right = typecheck(n.Right, ctxExpr)

 	t := n.Right.Type
 	if t == nil {
 		return
 	}
 	// delicate little dance.  see typecheckas2
 	ls := n.List.Slice()
 	for i1, n1 := range ls {
 		if n1.Name == nil || n1.Name.Defn != n {
 			ls[i1] = typecheck(ls[i1], ctxExpr|ctxAssign)
 		}
 	}

 	if t.IsPtr() && t.Elem().IsArray() {
 		t = t.Elem()
 	}
 	n.Type = t

 	var t1, t2 *types.Type
 	toomany := false
 	switch t.Etype {
 	default:
 		yyerrorl(n.Pos, "cannot range over %L", n.Right)
 		return

 	case TARRAY, TSLICE:
 		t1 = types.Types[TINT]
 		t2 = t.Elem()

 	case TMAP:
 		t1 = t.Key()
 		t2 = t.Elem()

 	case TCHAN:
 		if !t.ChanDir().CanRecv() {
 			yyerrorl(n.Pos, "invalid operation: range %v (receive from send-only type %v)", n.Right, n.Right.Type)
 			return
 		}

 		t1 = t.Elem()
 		t2 = nil
 		if n.List.Len() == 2 {
 			toomany = true
 		}

 	case TSTRING:
 		t1 = types.Types[TINT]
 		t2 = types.Runetype
 	}

 	if n.List.Len() > 2 || toomany {
 		yyerrorl(n.Pos, "too many variables in range")
 	}

 	var v1, v2 *Node
 	if n.List.Len() != 0 {
 		v1 = n.List.First()
 	}
 	if n.List.Len() > 1 {
 		v2 = n.List.Second()
 	}

 	// this is not only an optimization but also a requirement in the spec.
 	// "if the second iteration variable is the blank identifier, the range
 	// clause is equivalent to the same clause with only the first variable
 	// present."
 	if v2.isBlank() {
 		if v1 != nil {
 			n.List.Set1(v1)
 		}
 		v2 = nil
 	}

 	var why string
 	if v1 != nil {
 		if v1.Name != nil && v1.Name.Defn == n {
 			v1.Type = t1
 		} else if v1.Type != nil && assignop(t1, v1.Type, &why) == 0 {
 			yyerrorl(n.Pos, "cannot assign type %v to %L in range%s", t1, v1, why)
 		}
 		checkassign(n, v1)
 	}

 	if v2 != nil {
 		if v2.Name != nil && v2.Name.Defn == n {
 			v2.Type = t2
 		} else if v2.Type != nil && assignop(t2, v2.Type, &why) == 0 {
 			yyerrorl(n.Pos, "cannot assign type %v to %L in range%s", t2, v2, why)
 		}
 		checkassign(n, v2)
 	}
 }

 func cheapComputableIndex(width int64) bool {
 	switch thearch.LinkArch.Family {
 	// MIPS does not have R+R addressing
 	// Arm64 may lack ability to generate this code in our assembler,
 	// but the architecture supports it.
 	case sys.PPC64, sys.S390X:
 		return width == 1
 	case sys.AMD64, sys.I386, sys.ARM64, sys.ARM:
 		switch width {
 		case 1, 2, 4, 8:
 			return true
 		}
 	}
 	return false
 }

 // walkrange transforms various forms of ORANGE into
 // simpler forms.  The result must be assigned back to n.
 // Node n may also be modified in place, and may also be
 // the returned node.
 func walkrange(n *Node) *Node {
 	if isMapClear(n) {
 		m := n.Right
 		lno := setlineno(m)
 		n = mapClear(m)
 		lineno = lno
 		return n
 	}

 	// variable name conventions:
 	//	ohv1, hv1, hv2: hidden (old) val 1, 2
 	//	ha, hit: hidden aggregate, iterator
 	//	hn, hp: hidden len, pointer
 	//	hb: hidden bool
 	//	a, v1, v2: not hidden aggregate, val 1, 2

 	t := n.Type

 	a := n.Right
 	lno := setlineno(a)
 	n.Right = nil

 	var v1, v2 *Node
 	l := n.List.Len()
 	if l > 0 {
 		v1 = n.List.First()
 	}

 	if l > 1 {
 		v2 = n.List.Second()
 	}

 	if v2.isBlank() {
 		v2 = nil
 	}

 	if v1.isBlank() && v2 == nil {
 		v1 = nil
 	}

 	if v1 == nil && v2 != nil {
 		Fatalf("walkrange: v2 != nil while v1 == nil")
 	}

 	// n.List has no meaning anymore, clear it
 	// to avoid erroneous processing by racewalk.
 	n.List.Set(nil)

 	var ifGuard *Node

 	translatedLoopOp := OFOR

 	var body []*Node
 	var init []*Node
 	switch t.Etype {
 	default:
 		Fatalf("walkrange")

 	case TARRAY, TSLICE:
 		if arrayClear(n, v1, v2, a) {
 			lineno = lno
 			return n
 		}

 		// order.stmt arranged for a copy of the array/slice variable if needed.
 		ha := a

 		hv1 := temp(types.Types[TINT])
 		hn := temp(types.Types[TINT])

 		init = append(init, nod(OAS, hv1, nil))
 		init = append(init, nod(OAS, hn, nod(OLEN, ha, nil)))

 		n.Left = nod(OLT, hv1, hn)
 		n.Right = nod(OAS, hv1, nod(OADD, hv1, nodintconst(1)))

 		// for range ha { body }
 		if v1 == nil {
 			break
 		}

 		// for v1 := range ha { body }
 		if v2 == nil {
 			body = []*Node{nod(OAS, v1, hv1)}
 			break
 		}

 		// for v1, v2 := range ha { body }
 		if cheapComputableIndex(n.Type.Elem().Width) {
 			// v1, v2 = hv1, ha[hv1]
 			tmp := nod(OINDEX, ha, hv1)
 			tmp.SetBounded(true)
 			// Use OAS2 to correctly handle assignments
 			// of the form "v1, a[v1] := range".
 			a := nod(OAS2, nil, nil)
 			a.List.Set2(v1, v2)
 			a.Rlist.Set2(hv1, tmp)
 			body = []*Node{a}
 			break
 		}

 		// TODO(austin): OFORUNTIL is a strange beast, but is
 		// necessary for expressing the control flow we need
 		// while also making "break" and "continue" work. It
 		// would be nice to just lower ORANGE during SSA, but
 		// racewalk needs to see many of the operations
 		// involved in ORANGE's implementation. If racewalk
 		// moves into SSA, consider moving ORANGE into SSA and
 		// eliminating OFORUNTIL.

 		// TODO(austin): OFORUNTIL inhibits bounds-check
 		// elimination on the index variable (see #20711).
 		// Enhance the prove pass to understand this.
 		ifGuard = nod(OIF, nil, nil)
 		ifGuard.Left = nod(OLT, hv1, hn)
 		translatedLoopOp = OFORUNTIL

 		hp := temp(types.NewPtr(n.Type.Elem()))
 		tmp := nod(OINDEX, ha, nodintconst(0))
 		tmp.SetBounded(true)
 		init = append(init, nod(OAS, hp, nod(OADDR, tmp, nil)))

 		// Use OAS2 to correctly handle assignments
 		// of the form "v1, a[v1] := range".
 		a := nod(OAS2, nil, nil)
 		a.List.Set2(v1, v2)
 		a.Rlist.Set2(hv1, nod(ODEREF, hp, nil))
 		body = append(body, a)

 		// Advance pointer as part of the late increment.
 		//
 		// This runs *after* the condition check, so we know
 		// advancing the pointer is safe and won't go past the
 		// end of the allocation.
 		a = nod(OAS, hp, addptr(hp, t.Elem().Width))
 		a = typecheck(a, ctxStmt)
 		n.List.Set1(a)

 	case TMAP:
 		// order.stmt allocated the iterator for us.
 		// we only use a once, so no copy needed.
 		ha := a

 		hit := prealloc[n]
 		th := hit.Type
 		n.Left = nil
 		keysym := th.Field(0).Sym  // depends on layout of iterator struct.  See reflect.go:hiter
 		elemsym := th.Field(1).Sym // ditto

 		fn := syslook("mapiterinit")

 		fn = substArgTypes(fn, t.Key(), t.Elem(), th)
 		init = append(init, mkcall1(fn, nil, nil, typename(t), ha, nod(OADDR, hit, nil)))
 		n.Left = nod(ONE, nodSym(ODOT, hit, keysym), nodnil())

 		fn = syslook("mapiternext")
 		fn = substArgTypes(fn, th)
 		n.Right = mkcall1(fn, nil, nil, nod(OADDR, hit, nil))

 		key := nodSym(ODOT, hit, keysym)
 		key = nod(ODEREF, key, nil)
 		if v1 == nil {
 			body = nil
 		} else if v2 == nil {
 			body = []*Node{nod(OAS, v1, key)}
 		} else {
 			elem := nodSym(ODOT, hit, elemsym)
 			elem = nod(ODEREF, elem, nil)
 			a := nod(OAS2, nil, nil)
 			a.List.Set2(v1, v2)
 			a.Rlist.Set2(key, elem)
 			body = []*Node{a}
 		}

 	case TCHAN:
 		// order.stmt arranged for a copy of the channel variable.
 		ha := a

 		n.Left = nil

 		hv1 := temp(t.Elem())
 		hv1.SetTypecheck(1)
 		if t.Elem().HasPointers() {
 			init = append(init, nod(OAS, hv1, nil))
 		}
 		hb := temp(types.Types[TBOOL])

 		n.Left = nod(ONE, hb, nodbool(false))
 		a := nod(OAS2RECV, nil, nil)
 		a.SetTypecheck(1)
 		a.List.Set2(hv1, hb)
 		a.Right = nod(ORECV, ha, nil)
 		n.Left.Ninit.Set1(a)
 		if v1 == nil {
 			body = nil
 		} else {
 			body = []*Node{nod(OAS, v1, hv1)}
 		}
 		// Zero hv1. This prevents hv1 from being the sole, inaccessible
 		// reference to an otherwise GC-able value during the next channel receive.
 		// See issue 15281.
 		body = append(body, nod(OAS, hv1, nil))

 	case TSTRING:
 		// Transform string range statements like "for v1, v2 = range a" into
 		//
 		// ha := a
 		// for hv1 := 0; hv1 < len(ha); {
 		//   hv1t := hv1
 		//   hv2 := rune(ha[hv1])
 		//   if hv2 < utf8.RuneSelf {
 		//      hv1++
 		//   } else {
 		//      hv2, hv1 = decoderune(ha, hv1)
 		//   }
 		//   v1, v2 = hv1t, hv2
 		//   // original body
 		// }

 		// order.stmt arranged for a copy of the string variable.
 		ha := a

 		hv1 := temp(types.Types[TINT])
 		hv1t := temp(types.Types[TINT])
 		hv2 := temp(types.Runetype)

 		// hv1 := 0
 		init = append(init, nod(OAS, hv1, nil))

 		// hv1 < len(ha)
 		n.Left = nod(OLT, hv1, nod(OLEN, ha, nil))

 		if v1 != nil {
 			// hv1t = hv1
 			body = append(body, nod(OAS, hv1t, hv1))
 		}

 		// hv2 := rune(ha[hv1])
 		nind := nod(OINDEX, ha, hv1)
 		nind.SetBounded(true)
 		body = append(body, nod(OAS, hv2, conv(nind, types.Runetype)))

 		// if hv2 < utf8.RuneSelf
 		nif := nod(OIF, nil, nil)
 		nif.Left = nod(OLT, hv2, nodintconst(utf8.RuneSelf))

 		// hv1++
 		nif.Nbody.Set1(nod(OAS, hv1, nod(OADD, hv1, nodintconst(1))))

 		// } else {
 		eif := nod(OAS2, nil, nil)
 		nif.Rlist.Set1(eif)

 		// hv2, hv1 = decoderune(ha, hv1)
 		eif.List.Set2(hv2, hv1)
 		fn := syslook("decoderune")
 		eif.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, ha, hv1))

 		body = append(body, nif)

 		if v1 != nil {
 			if v2 != nil {
 				// v1, v2 = hv1t, hv2
 				a := nod(OAS2, nil, nil)
 				a.List.Set2(v1, v2)
 				a.Rlist.Set2(hv1t, hv2)
 				body = append(body, a)
 			} else {
 				// v1 = hv1t
 				body = append(body, nod(OAS, v1, hv1t))
 			}
 		}
 	}

 	n.Op = translatedLoopOp
 	typecheckslice(init, ctxStmt)

 	if ifGuard != nil {
 		ifGuard.Ninit.Append(init...)
 		ifGuard = typecheck(ifGuard, ctxStmt)
 	} else {
 		n.Ninit.Append(init...)
 	}

 	typecheckslice(n.Left.Ninit.Slice(), ctxStmt)

 	n.Left = typecheck(n.Left, ctxExpr)
 	n.Left = defaultlit(n.Left, nil)
 	n.Right = typecheck(n.Right, ctxStmt)
 	typecheckslice(body, ctxStmt)
 	n.Nbody.Prepend(body...)

 	if ifGuard != nil {
 		ifGuard.Nbody.Set1(n)
 		n = ifGuard
 	}

 	n = walkstmt(n)

 	lineno = lno
 	return n
 }

 // isMapClear checks if n is of the form:
 //
 // for k := range m {
 //   delete(m, k)
 // }
 //
 // where == for keys of map m is reflexive.
 func isMapClear(n *Node) bool {
 	if Debug['N'] != 0 || instrumenting {
 		return false
 	}

 	if n.Op != ORANGE || n.Type.Etype != TMAP || n.List.Len() != 1 {
 		return false
 	}

 	k := n.List.First()
 	if k == nil || k.isBlank() {
 		return false
 	}

 	// Require k to be a new variable name.
 	if k.Name == nil || k.Name.Defn != n {
 		return false
 	}

 	if n.Nbody.Len() != 1 {
 		return false
 	}

 	stmt := n.Nbody.First() // only stmt in body
 	if stmt == nil || stmt.Op != ODELETE {
 		return false
 	}

 	m := n.Right
 	if !samesafeexpr(stmt.List.First(), m) || !samesafeexpr(stmt.List.Second(), k) {
 		return false
 	}

 	// Keys where equality is not reflexive can not be deleted from maps.
 	if !isreflexive(m.Type.Key()) {
 		return false
 	}

 	return true
 }

 // mapClear constructs a call to runtime.mapclear for the map m.
 func mapClear(m *Node) *Node {
 	t := m.Type

 	// instantiate mapclear(typ *type, hmap map[any]any)
 	fn := syslook("mapclear")
 	fn = substArgTypes(fn, t.Key(), t.Elem())
 	n := mkcall1(fn, nil, nil, typename(t), m)

 	n = typecheck(n, ctxStmt)
 	n = walkstmt(n)

 	return n
 }

 // Lower n into runtime·memclr if possible, for
 // fast zeroing of slices and arrays (issue 5373).
 // Look for instances of
 //
 // for i := range a {
 // 	a[i] = zero
 // }
 //
 // in which the evaluation of a is side-effect-free.
 //
 // Parameters are as in walkrange: "for v1, v2 = range a".
 func arrayClear(n, v1, v2, a *Node) bool {
 	if Debug['N'] != 0 || instrumenting {
 		return false
 	}

 	if v1 == nil || v2 != nil {
 		return false
 	}

 	if n.Nbody.Len() != 1 || n.Nbody.First() == nil {
 		return false
 	}

 	stmt := n.Nbody.First() // only stmt in body
 	if stmt.Op != OAS || stmt.Left.Op != OINDEX {
 		return false
 	}

 	if !samesafeexpr(stmt.Left.Left, a) || !samesafeexpr(stmt.Left.Right, v1) {
 		return false
 	}

 	elemsize := n.Type.Elem().Width
 	if elemsize <= 0 || !isZero(stmt.Right) {
 		return false
 	}

 	// Convert to
 	// if len(a) != 0 {
 	// 	hp = &a[0]
 	// 	hn = len(a)*sizeof(elem(a))
 	// 	memclr{NoHeap,Has}Pointers(hp, hn)
 	// 	i = len(a) - 1
 	// }
 	n.Op = OIF

 	n.Nbody.Set(nil)
 	n.Left = nod(ONE, nod(OLEN, a, nil), nodintconst(0))

 	// hp = &a[0]
 	hp := temp(types.Types[TUNSAFEPTR])

 	tmp := nod(OINDEX, a, nodintconst(0))
 	tmp.SetBounded(true)
 	tmp = nod(OADDR, tmp, nil)
 	tmp = convnop(tmp, types.Types[TUNSAFEPTR])
 	n.Nbody.Append(nod(OAS, hp, tmp))

 	// hn = len(a) * sizeof(elem(a))
 	hn := temp(types.Types[TUINTPTR])

 	tmp = nod(OLEN, a, nil)
 	tmp = nod(OMUL, tmp, nodintconst(elemsize))
 	tmp = conv(tmp, types.Types[TUINTPTR])
 	n.Nbody.Append(nod(OAS, hn, tmp))

 	var fn *Node
 	if a.Type.Elem().HasHeapPointer() {
 		// memclrHasPointers(hp, hn)
 		Curfn.Func.setWBPos(stmt.Pos)
 		fn = mkcall("memclrHasPointers", nil, nil, hp, hn)
 	} else {
 		// memclrNoHeapPointers(hp, hn)
 		fn = mkcall("memclrNoHeapPointers", nil, nil, hp, hn)
 	}

 	n.Nbody.Append(fn)

 	// i = len(a) - 1
 	v1 = nod(OAS, v1, nod(OSUB, nod(OLEN, a, nil), nodintconst(1)))

 	n.Nbody.Append(v1)

 	n.Left = typecheck(n.Left, ctxExpr)
 	n.Left = defaultlit(n.Left, nil)
 	typecheckslice(n.Nbody.Slice(), ctxStmt)
 	n = walkstmt(n)
 	return true
 }

 // addptr returns (*T)(uintptr(p) + n).
 func addptr(p *Node, n int64) *Node {
 	t := p.Type

 	p = nod(OCONVNOP, p, nil)
 	p.Type = types.Types[TUINTPTR]

 	p = nod(OADD, p, nodintconst(n))

 	p = nod(OCONVNOP, p, nil)
 	p.Type = t

 	return p
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package gc

	import (
	"cmd/compile/internal/types"
	"cmd/internal/sys"
	"unicode/utf8"
	)

	// range
	func typecheckrange(n *Node) {
	// Typechecking order is important here:
	// 0. first typecheck range expression (slice/map/chan),
	// it is evaluated only once and so logically it is not part of the loop.
	// 1. typecheck produced values,
	// this part can declare new vars and so it must be typechecked before body,
	// because body can contain a closure that captures the vars.
	// 2. decldepth++ to denote loop body.
	// 3. typecheck body.
	// 4. decldepth--.
	typecheckrangeExpr(n)

	// second half of dance, the first half being typecheckrangeExpr
	n.SetTypecheck(1)
	ls := n.List.Slice()
	for i1, n1 := range ls {
	if n1.Typecheck() == 0 {
	ls[i1] = typecheck(ls[i1], ctxExpr\|ctxAssign)
	}
	}

	decldepth++
	typecheckslice(n.Nbody.Slice(), ctxStmt)
	decldepth--
	}

	func typecheckrangeExpr(n *Node) {
	n.Right = typecheck(n.Right, ctxExpr)

	t := n.Right.Type
	if t == nil {
	return
	}
	// delicate little dance. see typecheckas2
	ls := n.List.Slice()
	for i1, n1 := range ls {
	if n1.Name == nil \|\| n1.Name.Defn != n {
	ls[i1] = typecheck(ls[i1], ctxExpr\|ctxAssign)
	}
	}

	if t.IsPtr() && t.Elem().IsArray() {
	t = t.Elem()
	}
	n.Type = t

	var t1, t2 *types.Type
	toomany := false
	switch t.Etype {
	default:
	yyerrorl(n.Pos, "cannot range over %L", n.Right)
	return

	case TARRAY, TSLICE:
	t1 = types.Types[TINT]
	t2 = t.Elem()

	case TMAP:
	t1 = t.Key()
	t2 = t.Elem()

	case TCHAN:
	if !t.ChanDir().CanRecv() {
	yyerrorl(n.Pos, "invalid operation: range %v (receive from send-only type %v)", n.Right, n.Right.Type)
	return
	}

	t1 = t.Elem()
	t2 = nil
	if n.List.Len() == 2 {
	toomany = true
	}

	case TSTRING:
	t1 = types.Types[TINT]
	t2 = types.Runetype
	}

	if n.List.Len() > 2 \|\| toomany {
	yyerrorl(n.Pos, "too many variables in range")
	}

	var v1, v2 *Node
	if n.List.Len() != 0 {
	v1 = n.List.First()
	}
	if n.List.Len() > 1 {
	v2 = n.List.Second()
	}

	// this is not only an optimization but also a requirement in the spec.
	// "if the second iteration variable is the blank identifier, the range
	// clause is equivalent to the same clause with only the first variable
	// present."
	if v2.isBlank() {
	if v1 != nil {
	n.List.Set1(v1)
	}
	v2 = nil
	}

	var why string
	if v1 != nil {
	if v1.Name != nil && v1.Name.Defn == n {
	v1.Type = t1
	} else if v1.Type != nil && assignop(t1, v1.Type, &why) == 0 {
	yyerrorl(n.Pos, "cannot assign type %v to %L in range%s", t1, v1, why)
	}
	checkassign(n, v1)
	}

	if v2 != nil {
	if v2.Name != nil && v2.Name.Defn == n {
	v2.Type = t2
	} else if v2.Type != nil && assignop(t2, v2.Type, &why) == 0 {
	yyerrorl(n.Pos, "cannot assign type %v to %L in range%s", t2, v2, why)
	}
	checkassign(n, v2)
	}
	}

	func cheapComputableIndex(width int64) bool {
	switch thearch.LinkArch.Family {
	// MIPS does not have R+R addressing
	// Arm64 may lack ability to generate this code in our assembler,
	// but the architecture supports it.
	case sys.PPC64, sys.S390X:
	return width == 1
	case sys.AMD64, sys.I386, sys.ARM64, sys.ARM:
	switch width {
	case 1, 2, 4, 8:
	return true
	}
	}
	return false
	}

	// walkrange transforms various forms of ORANGE into
	// simpler forms. The result must be assigned back to n.
	// Node n may also be modified in place, and may also be
	// the returned node.
	func walkrange(n Node) Node {
	if isMapClear(n) {
	m := n.Right
	lno := setlineno(m)
	n = mapClear(m)
	lineno = lno
	return n
	}

	// variable name conventions:
	// ohv1, hv1, hv2: hidden (old) val 1, 2
	// ha, hit: hidden aggregate, iterator
	// hn, hp: hidden len, pointer
	// hb: hidden bool
	// a, v1, v2: not hidden aggregate, val 1, 2

	t := n.Type

	a := n.Right
	lno := setlineno(a)
	n.Right = nil

	var v1, v2 *Node
	l := n.List.Len()
	if l > 0 {
	v1 = n.List.First()
	}

	if l > 1 {
	v2 = n.List.Second()
	}

	if v2.isBlank() {
	v2 = nil
	}

	if v1.isBlank() && v2 == nil {
	v1 = nil
	}

	if v1 == nil && v2 != nil {
	Fatalf("walkrange: v2 != nil while v1 == nil")
	}

	// n.List has no meaning anymore, clear it
	// to avoid erroneous processing by racewalk.
	n.List.Set(nil)

	var ifGuard *Node

	translatedLoopOp := OFOR

	var body []*Node
	var init []*Node
	switch t.Etype {
	default:
	Fatalf("walkrange")

	case TARRAY, TSLICE:
	if arrayClear(n, v1, v2, a) {
	lineno = lno
	return n
	}

	// order.stmt arranged for a copy of the array/slice variable if needed.
	ha := a

	hv1 := temp(types.Types[TINT])
	hn := temp(types.Types[TINT])

	init = append(init, nod(OAS, hv1, nil))
	init = append(init, nod(OAS, hn, nod(OLEN, ha, nil)))

	n.Left = nod(OLT, hv1, hn)
	n.Right = nod(OAS, hv1, nod(OADD, hv1, nodintconst(1)))

	// for range ha { body }
	if v1 == nil {
	break
	}

	// for v1 := range ha { body }
	if v2 == nil {
	body = []*Node{nod(OAS, v1, hv1)}
	break
	}

	// for v1, v2 := range ha { body }
	if cheapComputableIndex(n.Type.Elem().Width) {
	// v1, v2 = hv1, ha[hv1]
	tmp := nod(OINDEX, ha, hv1)
	tmp.SetBounded(true)
	// Use OAS2 to correctly handle assignments
	// of the form "v1, a[v1] := range".
	a := nod(OAS2, nil, nil)
	a.List.Set2(v1, v2)
	a.Rlist.Set2(hv1, tmp)
	body = []*Node{a}
	break
	}

	// TODO(austin): OFORUNTIL is a strange beast, but is
	// necessary for expressing the control flow we need
	// while also making "break" and "continue" work. It
	// would be nice to just lower ORANGE during SSA, but
	// racewalk needs to see many of the operations
	// involved in ORANGE's implementation. If racewalk
	// moves into SSA, consider moving ORANGE into SSA and
	// eliminating OFORUNTIL.

	// TODO(austin): OFORUNTIL inhibits bounds-check
	// elimination on the index variable (see #20711).
	// Enhance the prove pass to understand this.
	ifGuard = nod(OIF, nil, nil)
	ifGuard.Left = nod(OLT, hv1, hn)
	translatedLoopOp = OFORUNTIL

	hp := temp(types.NewPtr(n.Type.Elem()))
	tmp := nod(OINDEX, ha, nodintconst(0))
	tmp.SetBounded(true)
	init = append(init, nod(OAS, hp, nod(OADDR, tmp, nil)))

	// Use OAS2 to correctly handle assignments
	// of the form "v1, a[v1] := range".
	a := nod(OAS2, nil, nil)
	a.List.Set2(v1, v2)
	a.Rlist.Set2(hv1, nod(ODEREF, hp, nil))
	body = append(body, a)

	// Advance pointer as part of the late increment.
	//
	// This runs after the condition check, so we know
	// advancing the pointer is safe and won't go past the
	// end of the allocation.
	a = nod(OAS, hp, addptr(hp, t.Elem().Width))
	a = typecheck(a, ctxStmt)
	n.List.Set1(a)

	case TMAP:
	// order.stmt allocated the iterator for us.
	// we only use a once, so no copy needed.
	ha := a

	hit := prealloc[n]
	th := hit.Type
	n.Left = nil
	keysym := th.Field(0).Sym // depends on layout of iterator struct. See reflect.go:hiter
	elemsym := th.Field(1).Sym // ditto

	fn := syslook("mapiterinit")

	fn = substArgTypes(fn, t.Key(), t.Elem(), th)
	init = append(init, mkcall1(fn, nil, nil, typename(t), ha, nod(OADDR, hit, nil)))
	n.Left = nod(ONE, nodSym(ODOT, hit, keysym), nodnil())

	fn = syslook("mapiternext")
	fn = substArgTypes(fn, th)
	n.Right = mkcall1(fn, nil, nil, nod(OADDR, hit, nil))

	key := nodSym(ODOT, hit, keysym)
	key = nod(ODEREF, key, nil)
	if v1 == nil {
	body = nil
	} else if v2 == nil {
	body = []*Node{nod(OAS, v1, key)}
	} else {
	elem := nodSym(ODOT, hit, elemsym)
	elem = nod(ODEREF, elem, nil)
	a := nod(OAS2, nil, nil)
	a.List.Set2(v1, v2)
	a.Rlist.Set2(key, elem)
	body = []*Node{a}
	}

	case TCHAN:
	// order.stmt arranged for a copy of the channel variable.
	ha := a

	n.Left = nil

	hv1 := temp(t.Elem())
	hv1.SetTypecheck(1)
	if t.Elem().HasPointers() {
	init = append(init, nod(OAS, hv1, nil))
	}
	hb := temp(types.Types[TBOOL])

	n.Left = nod(ONE, hb, nodbool(false))
	a := nod(OAS2RECV, nil, nil)
	a.SetTypecheck(1)
	a.List.Set2(hv1, hb)
	a.Right = nod(ORECV, ha, nil)
	n.Left.Ninit.Set1(a)
	if v1 == nil {
	body = nil
	} else {
	body = []*Node{nod(OAS, v1, hv1)}
	}
	// Zero hv1. This prevents hv1 from being the sole, inaccessible
	// reference to an otherwise GC-able value during the next channel receive.
	// See issue 15281.
	body = append(body, nod(OAS, hv1, nil))

	case TSTRING:
	// Transform string range statements like "for v1, v2 = range a" into
	//
	// ha := a
	// for hv1 := 0; hv1 < len(ha); {
	// hv1t := hv1
	// hv2 := rune(ha[hv1])
	// if hv2 < utf8.RuneSelf {
	// hv1++
	// } else {
	// hv2, hv1 = decoderune(ha, hv1)
	// }
	// v1, v2 = hv1t, hv2
	// // original body
	// }

	// order.stmt arranged for a copy of the string variable.
	ha := a

	hv1 := temp(types.Types[TINT])
	hv1t := temp(types.Types[TINT])
	hv2 := temp(types.Runetype)

	// hv1 := 0
	init = append(init, nod(OAS, hv1, nil))

	// hv1 < len(ha)
	n.Left = nod(OLT, hv1, nod(OLEN, ha, nil))

	if v1 != nil {
	// hv1t = hv1
	body = append(body, nod(OAS, hv1t, hv1))
	}

	// hv2 := rune(ha[hv1])
	nind := nod(OINDEX, ha, hv1)
	nind.SetBounded(true)
	body = append(body, nod(OAS, hv2, conv(nind, types.Runetype)))

	// if hv2 < utf8.RuneSelf
	nif := nod(OIF, nil, nil)
	nif.Left = nod(OLT, hv2, nodintconst(utf8.RuneSelf))

	// hv1++
	nif.Nbody.Set1(nod(OAS, hv1, nod(OADD, hv1, nodintconst(1))))

	// } else {
	eif := nod(OAS2, nil, nil)
	nif.Rlist.Set1(eif)

	// hv2, hv1 = decoderune(ha, hv1)
	eif.List.Set2(hv2, hv1)
	fn := syslook("decoderune")
	eif.Rlist.Set1(mkcall1(fn, fn.Type.Results(), nil, ha, hv1))

	body = append(body, nif)

	if v1 != nil {
	if v2 != nil {
	// v1, v2 = hv1t, hv2
	a := nod(OAS2, nil, nil)
	a.List.Set2(v1, v2)
	a.Rlist.Set2(hv1t, hv2)
	body = append(body, a)
	} else {
	// v1 = hv1t
	body = append(body, nod(OAS, v1, hv1t))
	}
	}
	}

	n.Op = translatedLoopOp
	typecheckslice(init, ctxStmt)

	if ifGuard != nil {
	ifGuard.Ninit.Append(init...)
	ifGuard = typecheck(ifGuard, ctxStmt)
	} else {
	n.Ninit.Append(init...)
	}

	typecheckslice(n.Left.Ninit.Slice(), ctxStmt)

	n.Left = typecheck(n.Left, ctxExpr)
	n.Left = defaultlit(n.Left, nil)
	n.Right = typecheck(n.Right, ctxStmt)
	typecheckslice(body, ctxStmt)
	n.Nbody.Prepend(body...)

	if ifGuard != nil {
	ifGuard.Nbody.Set1(n)
	n = ifGuard
	}

	n = walkstmt(n)

	lineno = lno
	return n
	}

	// isMapClear checks if n is of the form:
	//
	// for k := range m {
	// delete(m, k)
	// }
	//
	// where == for keys of map m is reflexive.
	func isMapClear(n *Node) bool {
	if Debug['N'] != 0 \|\| instrumenting {
	return false
	}

	if n.Op != ORANGE \|\| n.Type.Etype != TMAP \|\| n.List.Len() != 1 {
	return false
	}

	k := n.List.First()
	if k == nil \|\| k.isBlank() {
	return false
	}

	// Require k to be a new variable name.
	if k.Name == nil \|\| k.Name.Defn != n {
	return false
	}

	if n.Nbody.Len() != 1 {
	return false
	}

	stmt := n.Nbody.First() // only stmt in body
	if stmt == nil \|\| stmt.Op != ODELETE {
	return false
	}

	m := n.Right
	if !samesafeexpr(stmt.List.First(), m) \|\| !samesafeexpr(stmt.List.Second(), k) {
	return false
	}

	// Keys where equality is not reflexive can not be deleted from maps.
	if !isreflexive(m.Type.Key()) {
	return false
	}

	return true
	}

	// mapClear constructs a call to runtime.mapclear for the map m.
	func mapClear(m Node) Node {
	t := m.Type

	// instantiate mapclear(typ *type, hmap map[any]any)
	fn := syslook("mapclear")
	fn = substArgTypes(fn, t.Key(), t.Elem())
	n := mkcall1(fn, nil, nil, typename(t), m)

	n = typecheck(n, ctxStmt)
	n = walkstmt(n)

	return n
	}

	// Lower n into runtime·memclr if possible, for
	// fast zeroing of slices and arrays (issue 5373).
	// Look for instances of
	//
	// for i := range a {
	// a[i] = zero
	// }
	//
	// in which the evaluation of a is side-effect-free.
	//
	// Parameters are as in walkrange: "for v1, v2 = range a".
	func arrayClear(n, v1, v2, a *Node) bool {
	if Debug['N'] != 0 \|\| instrumenting {
	return false
	}

	if v1 == nil \|\| v2 != nil {
	return false
	}

	if n.Nbody.Len() != 1 \|\| n.Nbody.First() == nil {
	return false
	}

	stmt := n.Nbody.First() // only stmt in body
	if stmt.Op != OAS \|\| stmt.Left.Op != OINDEX {
	return false
	}

	if !samesafeexpr(stmt.Left.Left, a) \|\| !samesafeexpr(stmt.Left.Right, v1) {
	return false
	}

	elemsize := n.Type.Elem().Width
	if elemsize <= 0 \|\| !isZero(stmt.Right) {
	return false
	}

	// Convert to
	// if len(a) != 0 {
	// hp = &a[0]
	// hn = len(a)*sizeof(elem(a))
	// memclr{NoHeap,Has}Pointers(hp, hn)
	// i = len(a) - 1
	// }
	n.Op = OIF

	n.Nbody.Set(nil)
	n.Left = nod(ONE, nod(OLEN, a, nil), nodintconst(0))

	// hp = &a[0]
	hp := temp(types.Types[TUNSAFEPTR])

	tmp := nod(OINDEX, a, nodintconst(0))
	tmp.SetBounded(true)
	tmp = nod(OADDR, tmp, nil)
	tmp = convnop(tmp, types.Types[TUNSAFEPTR])
	n.Nbody.Append(nod(OAS, hp, tmp))

	// hn = len(a) * sizeof(elem(a))
	hn := temp(types.Types[TUINTPTR])

	tmp = nod(OLEN, a, nil)
	tmp = nod(OMUL, tmp, nodintconst(elemsize))
	tmp = conv(tmp, types.Types[TUINTPTR])
	n.Nbody.Append(nod(OAS, hn, tmp))

	var fn *Node
	if a.Type.Elem().HasHeapPointer() {
	// memclrHasPointers(hp, hn)
	Curfn.Func.setWBPos(stmt.Pos)
	fn = mkcall("memclrHasPointers", nil, nil, hp, hn)
	} else {
	// memclrNoHeapPointers(hp, hn)
	fn = mkcall("memclrNoHeapPointers", nil, nil, hp, hn)
	}

	n.Nbody.Append(fn)

	// i = len(a) - 1
	v1 = nod(OAS, v1, nod(OSUB, nod(OLEN, a, nil), nodintconst(1)))

	n.Nbody.Append(v1)

	n.Left = typecheck(n.Left, ctxExpr)
	n.Left = defaultlit(n.Left, nil)
	typecheckslice(n.Nbody.Slice(), ctxStmt)
	n = walkstmt(n)
	return true
	}

	// addptr returns (*T)(uintptr(p) + n).
	func addptr(p Node, n int64) Node {
	t := p.Type

	p = nod(OCONVNOP, p, nil)
	p.Type = types.Types[TUINTPTR]

	p = nod(OADD, p, nodintconst(n))

	p = nod(OCONVNOP, p, nil)
	p.Type = t

	return p
	}