src/cmd/compile/internal/walk/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 walk

 import (
 	"unicode/utf8"

 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
 	"cmd/compile/internal/reflectdata"
 	"cmd/compile/internal/ssagen"
 	"cmd/compile/internal/typecheck"
 	"cmd/compile/internal/types"
 	"cmd/internal/sys"
 )

 func cheapComputableIndex(width int64) bool {
 	switch ssagen.Arch.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(nrange *ir.RangeStmt) ir.Node {
 	if isMapClear(nrange) {
 		m := nrange.X
 		lno := ir.SetPos(m)
 		n := mapClear(m)
 		base.Pos = lno
 		return n
 	}

 	nfor := ir.NewForStmt(nrange.Pos(), nil, nil, nil, nil)
 	nfor.SetInit(nrange.Init())
 	nfor.Label = nrange.Label

 	// 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

 	a := nrange.X
 	t := typecheck.RangeExprType(a.Type())
 	lno := ir.SetPos(a)

 	v1, v2 := nrange.Key, nrange.Value

 	if ir.IsBlank(v2) {
 		v2 = nil
 	}

 	if ir.IsBlank(v1) && v2 == nil {
 		v1 = nil
 	}

 	if v1 == nil && v2 != nil {
 		base.Fatalf("walkRange: v2 != nil while v1 == nil")
 	}

 	var ifGuard *ir.IfStmt

 	var body []ir.Node
 	var init []ir.Node
 	switch t.Kind() {
 	default:
 		base.Fatalf("walkRange")

 	case types.TARRAY, types.TSLICE:
 		if nn := arrayClear(nrange, v1, v2, a); nn != nil {
 			base.Pos = lno
 			return nn
 		}

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

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

 		init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))
 		init = append(init, ir.NewAssignStmt(base.Pos, hn, ir.NewUnaryExpr(base.Pos, ir.OLEN, ha)))

 		nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, hn)
 		nfor.Post = ir.NewAssignStmt(base.Pos, hv1, ir.NewBinaryExpr(base.Pos, ir.OADD, hv1, ir.NewInt(1)))

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

 		// for v1 := range ha { body }
 		if v2 == nil {
 			body = []ir.Node{ir.NewAssignStmt(base.Pos, v1, hv1)}
 			break
 		}

 		// for v1, v2 := range ha { body }
 		if cheapComputableIndex(t.Elem().Width) {
 			// v1, v2 = hv1, ha[hv1]
 			tmp := ir.NewIndexExpr(base.Pos, ha, hv1)
 			tmp.SetBounded(true)
 			// Use OAS2 to correctly handle assignments
 			// of the form "v1, a[v1] := range".
 			a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1, tmp})
 			body = []ir.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 = ir.NewIfStmt(base.Pos, nil, nil, nil)
 		ifGuard.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, hn)
 		nfor.SetOp(ir.OFORUNTIL)

 		hp := typecheck.Temp(types.NewPtr(t.Elem()))
 		tmp := ir.NewIndexExpr(base.Pos, ha, ir.NewInt(0))
 		tmp.SetBounded(true)
 		init = append(init, ir.NewAssignStmt(base.Pos, hp, typecheck.NodAddr(tmp)))

 		// Use OAS2 to correctly handle assignments
 		// of the form "v1, a[v1] := range".
 		a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1, ir.NewStarExpr(base.Pos, hp)})
 		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.
 		as := ir.NewAssignStmt(base.Pos, hp, addptr(hp, t.Elem().Width))
 		nfor.Late = []ir.Node{typecheck.Stmt(as)}

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

 		hit := nrange.Prealloc
 		th := hit.Type()
 		// depends on layout of iterator struct.
 		// See cmd/compile/internal/reflectdata/reflect.go:MapIterType
 		keysym := th.Field(0).Sym
 		elemsym := th.Field(1).Sym // ditto

 		fn := typecheck.LookupRuntime("mapiterinit")

 		fn = typecheck.SubstArgTypes(fn, t.Key(), t.Elem(), th)
 		init = append(init, mkcallstmt1(fn, reflectdata.TypePtr(t), ha, typecheck.NodAddr(hit)))
 		nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, keysym), typecheck.NodNil())

 		fn = typecheck.LookupRuntime("mapiternext")
 		fn = typecheck.SubstArgTypes(fn, th)
 		nfor.Post = mkcallstmt1(fn, typecheck.NodAddr(hit))

 		key := ir.NewStarExpr(base.Pos, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, keysym))
 		if v1 == nil {
 			body = nil
 		} else if v2 == nil {
 			body = []ir.Node{ir.NewAssignStmt(base.Pos, v1, key)}
 		} else {
 			elem := ir.NewStarExpr(base.Pos, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, elemsym))
 			a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{key, elem})
 			body = []ir.Node{a}
 		}

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

 		hv1 := typecheck.Temp(t.Elem())
 		hv1.SetTypecheck(1)
 		if t.Elem().HasPointers() {
 			init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))
 		}
 		hb := typecheck.Temp(types.Types[types.TBOOL])

 		nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, hb, ir.NewBool(false))
 		lhs := []ir.Node{hv1, hb}
 		rhs := []ir.Node{ir.NewUnaryExpr(base.Pos, ir.ORECV, ha)}
 		a := ir.NewAssignListStmt(base.Pos, ir.OAS2RECV, lhs, rhs)
 		a.SetTypecheck(1)
 		nfor.Cond = ir.InitExpr([]ir.Node{a}, nfor.Cond)
 		if v1 == nil {
 			body = nil
 		} else {
 			body = []ir.Node{ir.NewAssignStmt(base.Pos, 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, ir.NewAssignStmt(base.Pos, hv1, nil))

 	case types.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 := typecheck.Temp(types.Types[types.TINT])
 		hv1t := typecheck.Temp(types.Types[types.TINT])
 		hv2 := typecheck.Temp(types.RuneType)

 		// hv1 := 0
 		init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))

 		// hv1 < len(ha)
 		nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, ir.NewUnaryExpr(base.Pos, ir.OLEN, ha))

 		if v1 != nil {
 			// hv1t = hv1
 			body = append(body, ir.NewAssignStmt(base.Pos, hv1t, hv1))
 		}

 		// hv2 := rune(ha[hv1])
 		nind := ir.NewIndexExpr(base.Pos, ha, hv1)
 		nind.SetBounded(true)
 		body = append(body, ir.NewAssignStmt(base.Pos, hv2, typecheck.Conv(nind, types.RuneType)))

 		// if hv2 < utf8.RuneSelf
 		nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
 		nif.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv2, ir.NewInt(utf8.RuneSelf))

 		// hv1++
 		nif.Body = []ir.Node{ir.NewAssignStmt(base.Pos, hv1, ir.NewBinaryExpr(base.Pos, ir.OADD, hv1, ir.NewInt(1)))}

 		// } else {
 		// hv2, hv1 = decoderune(ha, hv1)
 		fn := typecheck.LookupRuntime("decoderune")
 		call := mkcall1(fn, fn.Type().Results(), &nif.Else, ha, hv1)
 		a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{hv2, hv1}, []ir.Node{call})
 		nif.Else.Append(a)

 		body = append(body, nif)

 		if v1 != nil {
 			if v2 != nil {
 				// v1, v2 = hv1t, hv2
 				a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1t, hv2})
 				body = append(body, a)
 			} else {
 				// v1 = hv1t
 				body = append(body, ir.NewAssignStmt(base.Pos, v1, hv1t))
 			}
 		}
 	}

 	typecheck.Stmts(init)

 	if ifGuard != nil {
 		ifGuard.PtrInit().Append(init...)
 		ifGuard = typecheck.Stmt(ifGuard).(*ir.IfStmt)
 	} else {
 		nfor.PtrInit().Append(init...)
 	}

 	typecheck.Stmts(nfor.Cond.Init())

 	nfor.Cond = typecheck.Expr(nfor.Cond)
 	nfor.Cond = typecheck.DefaultLit(nfor.Cond, nil)
 	nfor.Post = typecheck.Stmt(nfor.Post)
 	typecheck.Stmts(body)
 	nfor.Body.Append(body...)
 	nfor.Body.Append(nrange.Body...)

 	var n ir.Node = nfor
 	if ifGuard != nil {
 		ifGuard.Body = []ir.Node{n}
 		n = ifGuard
 	}

 	n = walkStmt(n)

 	base.Pos = 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 *ir.RangeStmt) bool {
 	if base.Flag.N != 0 || base.Flag.Cfg.Instrumenting {
 		return false
 	}

 	t := n.X.Type()
 	if n.Op() != ir.ORANGE || t.Kind() != types.TMAP || n.Key == nil || n.Value != nil {
 		return false
 	}

 	k := n.Key
 	// Require k to be a new variable name.
 	if !ir.DeclaredBy(k, n) {
 		return false
 	}

 	if len(n.Body) != 1 {
 		return false
 	}

 	stmt := n.Body[0] // only stmt in body
 	if stmt == nil || stmt.Op() != ir.ODELETE {
 		return false
 	}

 	m := n.X
 	if delete := stmt.(*ir.CallExpr); !ir.SameSafeExpr(delete.Args[0], m) || !ir.SameSafeExpr(delete.Args[1], k) {
 		return false
 	}

 	// Keys where equality is not reflexive can not be deleted from maps.
 	if !types.IsReflexive(t.Key()) {
 		return false
 	}

 	return true
 }

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

 	// instantiate mapclear(typ *type, hmap map[any]any)
 	fn := typecheck.LookupRuntime("mapclear")
 	fn = typecheck.SubstArgTypes(fn, t.Key(), t.Elem())
 	n := mkcallstmt1(fn, reflectdata.TypePtr(t), m)
 	return walkStmt(typecheck.Stmt(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(loop *ir.RangeStmt, v1, v2, a ir.Node) ir.Node {
 	if base.Flag.N != 0 || base.Flag.Cfg.Instrumenting {
 		return nil
 	}

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

 	if len(loop.Body) != 1 || loop.Body[0] == nil {
 		return nil
 	}

 	stmt1 := loop.Body[0] // only stmt in body
 	if stmt1.Op() != ir.OAS {
 		return nil
 	}
 	stmt := stmt1.(*ir.AssignStmt)
 	if stmt.X.Op() != ir.OINDEX {
 		return nil
 	}
 	lhs := stmt.X.(*ir.IndexExpr)

 	if !ir.SameSafeExpr(lhs.X, a) || !ir.SameSafeExpr(lhs.Index, v1) {
 		return nil
 	}

 	elemsize := typecheck.RangeExprType(loop.X.Type()).Elem().Width
 	if elemsize <= 0 || !ir.IsZero(stmt.Y) {
 		return nil
 	}

 	// 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 := ir.NewIfStmt(base.Pos, nil, nil, nil)
 	n.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(0))

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

 	ix := ir.NewIndexExpr(base.Pos, a, ir.NewInt(0))
 	ix.SetBounded(true)
 	addr := typecheck.ConvNop(typecheck.NodAddr(ix), types.Types[types.TUNSAFEPTR])
 	n.Body.Append(ir.NewAssignStmt(base.Pos, hp, addr))

 	// hn = len(a) * sizeof(elem(a))
 	hn := typecheck.Temp(types.Types[types.TUINTPTR])
 	mul := typecheck.Conv(ir.NewBinaryExpr(base.Pos, ir.OMUL, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(elemsize)), types.Types[types.TUINTPTR])
 	n.Body.Append(ir.NewAssignStmt(base.Pos, hn, mul))

 	var fn ir.Node
 	if a.Type().Elem().HasPointers() {
 		// memclrHasPointers(hp, hn)
 		ir.CurFunc.SetWBPos(stmt.Pos())
 		fn = mkcallstmt("memclrHasPointers", hp, hn)
 	} else {
 		// memclrNoHeapPointers(hp, hn)
 		fn = mkcallstmt("memclrNoHeapPointers", hp, hn)
 	}

 	n.Body.Append(fn)

 	// i = len(a) - 1
 	v1 = ir.NewAssignStmt(base.Pos, v1, ir.NewBinaryExpr(base.Pos, ir.OSUB, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(1)))

 	n.Body.Append(v1)

 	n.Cond = typecheck.Expr(n.Cond)
 	n.Cond = typecheck.DefaultLit(n.Cond, nil)
 	typecheck.Stmts(n.Body)
 	return walkStmt(n)
 }

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

 	p = ir.NewConvExpr(base.Pos, ir.OCONVNOP, nil, p)
 	p.SetType(types.Types[types.TUINTPTR])

 	p = ir.NewBinaryExpr(base.Pos, ir.OADD, p, ir.NewInt(n))

 	p = ir.NewConvExpr(base.Pos, ir.OCONVNOP, nil, p)
 	p.SetType(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 walk

	import (
	"unicode/utf8"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/reflectdata"
	"cmd/compile/internal/ssagen"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/internal/sys"
	)

	func cheapComputableIndex(width int64) bool {
	switch ssagen.Arch.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(nrange *ir.RangeStmt) ir.Node {
	if isMapClear(nrange) {
	m := nrange.X
	lno := ir.SetPos(m)
	n := mapClear(m)
	base.Pos = lno
	return n
	}

	nfor := ir.NewForStmt(nrange.Pos(), nil, nil, nil, nil)
	nfor.SetInit(nrange.Init())
	nfor.Label = nrange.Label

	// 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

	a := nrange.X
	t := typecheck.RangeExprType(a.Type())
	lno := ir.SetPos(a)

	v1, v2 := nrange.Key, nrange.Value

	if ir.IsBlank(v2) {
	v2 = nil
	}

	if ir.IsBlank(v1) && v2 == nil {
	v1 = nil
	}

	if v1 == nil && v2 != nil {
	base.Fatalf("walkRange: v2 != nil while v1 == nil")
	}

	var ifGuard *ir.IfStmt

	var body []ir.Node
	var init []ir.Node
	switch t.Kind() {
	default:
	base.Fatalf("walkRange")

	case types.TARRAY, types.TSLICE:
	if nn := arrayClear(nrange, v1, v2, a); nn != nil {
	base.Pos = lno
	return nn
	}

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

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

	init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))
	init = append(init, ir.NewAssignStmt(base.Pos, hn, ir.NewUnaryExpr(base.Pos, ir.OLEN, ha)))

	nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, hn)
	nfor.Post = ir.NewAssignStmt(base.Pos, hv1, ir.NewBinaryExpr(base.Pos, ir.OADD, hv1, ir.NewInt(1)))

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

	// for v1 := range ha { body }
	if v2 == nil {
	body = []ir.Node{ir.NewAssignStmt(base.Pos, v1, hv1)}
	break
	}

	// for v1, v2 := range ha { body }
	if cheapComputableIndex(t.Elem().Width) {
	// v1, v2 = hv1, ha[hv1]
	tmp := ir.NewIndexExpr(base.Pos, ha, hv1)
	tmp.SetBounded(true)
	// Use OAS2 to correctly handle assignments
	// of the form "v1, a[v1] := range".
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1, tmp})
	body = []ir.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 = ir.NewIfStmt(base.Pos, nil, nil, nil)
	ifGuard.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, hn)
	nfor.SetOp(ir.OFORUNTIL)

	hp := typecheck.Temp(types.NewPtr(t.Elem()))
	tmp := ir.NewIndexExpr(base.Pos, ha, ir.NewInt(0))
	tmp.SetBounded(true)
	init = append(init, ir.NewAssignStmt(base.Pos, hp, typecheck.NodAddr(tmp)))

	// Use OAS2 to correctly handle assignments
	// of the form "v1, a[v1] := range".
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1, ir.NewStarExpr(base.Pos, hp)})
	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.
	as := ir.NewAssignStmt(base.Pos, hp, addptr(hp, t.Elem().Width))
	nfor.Late = []ir.Node{typecheck.Stmt(as)}

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

	hit := nrange.Prealloc
	th := hit.Type()
	// depends on layout of iterator struct.
	// See cmd/compile/internal/reflectdata/reflect.go:MapIterType
	keysym := th.Field(0).Sym
	elemsym := th.Field(1).Sym // ditto

	fn := typecheck.LookupRuntime("mapiterinit")

	fn = typecheck.SubstArgTypes(fn, t.Key(), t.Elem(), th)
	init = append(init, mkcallstmt1(fn, reflectdata.TypePtr(t), ha, typecheck.NodAddr(hit)))
	nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, keysym), typecheck.NodNil())

	fn = typecheck.LookupRuntime("mapiternext")
	fn = typecheck.SubstArgTypes(fn, th)
	nfor.Post = mkcallstmt1(fn, typecheck.NodAddr(hit))

	key := ir.NewStarExpr(base.Pos, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, keysym))
	if v1 == nil {
	body = nil
	} else if v2 == nil {
	body = []ir.Node{ir.NewAssignStmt(base.Pos, v1, key)}
	} else {
	elem := ir.NewStarExpr(base.Pos, ir.NewSelectorExpr(base.Pos, ir.ODOT, hit, elemsym))
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{key, elem})
	body = []ir.Node{a}
	}

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

	hv1 := typecheck.Temp(t.Elem())
	hv1.SetTypecheck(1)
	if t.Elem().HasPointers() {
	init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))
	}
	hb := typecheck.Temp(types.Types[types.TBOOL])

	nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, hb, ir.NewBool(false))
	lhs := []ir.Node{hv1, hb}
	rhs := []ir.Node{ir.NewUnaryExpr(base.Pos, ir.ORECV, ha)}
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2RECV, lhs, rhs)
	a.SetTypecheck(1)
	nfor.Cond = ir.InitExpr([]ir.Node{a}, nfor.Cond)
	if v1 == nil {
	body = nil
	} else {
	body = []ir.Node{ir.NewAssignStmt(base.Pos, 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, ir.NewAssignStmt(base.Pos, hv1, nil))

	case types.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 := typecheck.Temp(types.Types[types.TINT])
	hv1t := typecheck.Temp(types.Types[types.TINT])
	hv2 := typecheck.Temp(types.RuneType)

	// hv1 := 0
	init = append(init, ir.NewAssignStmt(base.Pos, hv1, nil))

	// hv1 < len(ha)
	nfor.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv1, ir.NewUnaryExpr(base.Pos, ir.OLEN, ha))

	if v1 != nil {
	// hv1t = hv1
	body = append(body, ir.NewAssignStmt(base.Pos, hv1t, hv1))
	}

	// hv2 := rune(ha[hv1])
	nind := ir.NewIndexExpr(base.Pos, ha, hv1)
	nind.SetBounded(true)
	body = append(body, ir.NewAssignStmt(base.Pos, hv2, typecheck.Conv(nind, types.RuneType)))

	// if hv2 < utf8.RuneSelf
	nif := ir.NewIfStmt(base.Pos, nil, nil, nil)
	nif.Cond = ir.NewBinaryExpr(base.Pos, ir.OLT, hv2, ir.NewInt(utf8.RuneSelf))

	// hv1++
	nif.Body = []ir.Node{ir.NewAssignStmt(base.Pos, hv1, ir.NewBinaryExpr(base.Pos, ir.OADD, hv1, ir.NewInt(1)))}

	// } else {
	// hv2, hv1 = decoderune(ha, hv1)
	fn := typecheck.LookupRuntime("decoderune")
	call := mkcall1(fn, fn.Type().Results(), &nif.Else, ha, hv1)
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{hv2, hv1}, []ir.Node{call})
	nif.Else.Append(a)

	body = append(body, nif)

	if v1 != nil {
	if v2 != nil {
	// v1, v2 = hv1t, hv2
	a := ir.NewAssignListStmt(base.Pos, ir.OAS2, []ir.Node{v1, v2}, []ir.Node{hv1t, hv2})
	body = append(body, a)
	} else {
	// v1 = hv1t
	body = append(body, ir.NewAssignStmt(base.Pos, v1, hv1t))
	}
	}
	}

	typecheck.Stmts(init)

	if ifGuard != nil {
	ifGuard.PtrInit().Append(init...)
	ifGuard = typecheck.Stmt(ifGuard).(*ir.IfStmt)
	} else {
	nfor.PtrInit().Append(init...)
	}

	typecheck.Stmts(nfor.Cond.Init())

	nfor.Cond = typecheck.Expr(nfor.Cond)
	nfor.Cond = typecheck.DefaultLit(nfor.Cond, nil)
	nfor.Post = typecheck.Stmt(nfor.Post)
	typecheck.Stmts(body)
	nfor.Body.Append(body...)
	nfor.Body.Append(nrange.Body...)

	var n ir.Node = nfor
	if ifGuard != nil {
	ifGuard.Body = []ir.Node{n}
	n = ifGuard
	}

	n = walkStmt(n)

	base.Pos = 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 *ir.RangeStmt) bool {
	if base.Flag.N != 0 \|\| base.Flag.Cfg.Instrumenting {
	return false
	}

	t := n.X.Type()
	if n.Op() != ir.ORANGE \|\| t.Kind() != types.TMAP \|\| n.Key == nil \|\| n.Value != nil {
	return false
	}

	k := n.Key
	// Require k to be a new variable name.
	if !ir.DeclaredBy(k, n) {
	return false
	}

	if len(n.Body) != 1 {
	return false
	}

	stmt := n.Body[0] // only stmt in body
	if stmt == nil \|\| stmt.Op() != ir.ODELETE {
	return false
	}

	m := n.X
	if delete := stmt.(*ir.CallExpr); !ir.SameSafeExpr(delete.Args[0], m) \|\| !ir.SameSafeExpr(delete.Args[1], k) {
	return false
	}

	// Keys where equality is not reflexive can not be deleted from maps.
	if !types.IsReflexive(t.Key()) {
	return false
	}

	return true
	}

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

	// instantiate mapclear(typ *type, hmap map[any]any)
	fn := typecheck.LookupRuntime("mapclear")
	fn = typecheck.SubstArgTypes(fn, t.Key(), t.Elem())
	n := mkcallstmt1(fn, reflectdata.TypePtr(t), m)
	return walkStmt(typecheck.Stmt(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(loop *ir.RangeStmt, v1, v2, a ir.Node) ir.Node {
	if base.Flag.N != 0 \|\| base.Flag.Cfg.Instrumenting {
	return nil
	}

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

	if len(loop.Body) != 1 \|\| loop.Body[0] == nil {
	return nil
	}

	stmt1 := loop.Body[0] // only stmt in body
	if stmt1.Op() != ir.OAS {
	return nil
	}
	stmt := stmt1.(*ir.AssignStmt)
	if stmt.X.Op() != ir.OINDEX {
	return nil
	}
	lhs := stmt.X.(*ir.IndexExpr)

	if !ir.SameSafeExpr(lhs.X, a) \|\| !ir.SameSafeExpr(lhs.Index, v1) {
	return nil
	}

	elemsize := typecheck.RangeExprType(loop.X.Type()).Elem().Width
	if elemsize <= 0 \|\| !ir.IsZero(stmt.Y) {
	return nil
	}

	// 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 := ir.NewIfStmt(base.Pos, nil, nil, nil)
	n.Cond = ir.NewBinaryExpr(base.Pos, ir.ONE, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(0))

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

	ix := ir.NewIndexExpr(base.Pos, a, ir.NewInt(0))
	ix.SetBounded(true)
	addr := typecheck.ConvNop(typecheck.NodAddr(ix), types.Types[types.TUNSAFEPTR])
	n.Body.Append(ir.NewAssignStmt(base.Pos, hp, addr))

	// hn = len(a) * sizeof(elem(a))
	hn := typecheck.Temp(types.Types[types.TUINTPTR])
	mul := typecheck.Conv(ir.NewBinaryExpr(base.Pos, ir.OMUL, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(elemsize)), types.Types[types.TUINTPTR])
	n.Body.Append(ir.NewAssignStmt(base.Pos, hn, mul))

	var fn ir.Node
	if a.Type().Elem().HasPointers() {
	// memclrHasPointers(hp, hn)
	ir.CurFunc.SetWBPos(stmt.Pos())
	fn = mkcallstmt("memclrHasPointers", hp, hn)
	} else {
	// memclrNoHeapPointers(hp, hn)
	fn = mkcallstmt("memclrNoHeapPointers", hp, hn)
	}

	n.Body.Append(fn)

	// i = len(a) - 1
	v1 = ir.NewAssignStmt(base.Pos, v1, ir.NewBinaryExpr(base.Pos, ir.OSUB, ir.NewUnaryExpr(base.Pos, ir.OLEN, a), ir.NewInt(1)))

	n.Body.Append(v1)

	n.Cond = typecheck.Expr(n.Cond)
	n.Cond = typecheck.DefaultLit(n.Cond, nil)
	typecheck.Stmts(n.Body)
	return walkStmt(n)
	}

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

	p = ir.NewConvExpr(base.Pos, ir.OCONVNOP, nil, p)
	p.SetType(types.Types[types.TUINTPTR])

	p = ir.NewBinaryExpr(base.Pos, ir.OADD, p, ir.NewInt(n))

	p = ir.NewConvExpr(base.Pos, ir.OCONVNOP, nil, p)
	p.SetType(t)

	return p
	}