go/src/cmd/compile/internal/walk/expr.go

// 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 (
	"fmt"
	"go/constant"
	"internal/buildcfg"
	"strings"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/reflectdata"
	"cmd/compile/internal/staticdata"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/internal/obj"
)

// The result of walkExpr MUST be assigned back to n, e.g.
// 	n.Left = walkExpr(n.Left, init)
func walkExpr(n ir.Node, init *ir.Nodes) ir.Node {
	if n == nil {
		return n
	}

	if n, ok := n.(ir.InitNode); ok && init == n.PtrInit() {
		// not okay to use n->ninit when walking n,
		// because we might replace n with some other node
		// and would lose the init list.
		base.Fatalf("walkExpr init == &n->ninit")
	}

	if len(n.Init()) != 0 {
		walkStmtList(n.Init())
		init.Append(ir.TakeInit(n)...)
	}

	lno := ir.SetPos(n)

	if base.Flag.LowerW > 1 {
		ir.Dump("before walk expr", n)
	}

	if n.Typecheck() != 1 {
		base.Fatalf("missed typecheck: %+v", n)
	}

	if n.Type().IsUntyped() {
		base.Fatalf("expression has untyped type: %+v", n)
	}

	n = walkExpr1(n, init)

	// Eagerly compute sizes of all expressions for the back end.
	if typ := n.Type(); typ != nil && typ.Kind() != types.TBLANK && !typ.IsFuncArgStruct() {
		types.CheckSize(typ)
	}
	if n, ok := n.(*ir.Name); ok && n.Heapaddr != nil {
		types.CheckSize(n.Heapaddr.Type())
	}
	if ir.IsConst(n, constant.String) {
		// Emit string symbol now to avoid emitting
		// any concurrently during the backend.
		_ = staticdata.StringSym(n.Pos(), constant.StringVal(n.Val()))
	}

	if base.Flag.LowerW != 0 && n != nil {
		ir.Dump("after walk expr", n)
	}

	base.Pos = lno
	return n
}

func walkExpr1(n ir.Node, init *ir.Nodes) ir.Node {
	switch n.Op() {
	default:
		ir.Dump("walk", n)
		base.Fatalf("walkExpr: switch 1 unknown op %+v", n.Op())
		panic("unreachable")

	case ir.ONONAME, ir.OGETG:
		return n

	case ir.OTYPE, ir.ONAME, ir.OLITERAL, ir.ONIL, ir.OLINKSYMOFFSET:
		// TODO(mdempsky): Just return n; see discussion on CL 38655.
		// Perhaps refactor to use Node.mayBeShared for these instead.
		// If these return early, make sure to still call
		// StringSym for constant strings.
		return n

	case ir.OMETHEXPR:
		// TODO(mdempsky): Do this right after type checking.
		n := n.(*ir.SelectorExpr)
		return n.FuncName()

	case ir.ONOT, ir.ONEG, ir.OPLUS, ir.OBITNOT, ir.OREAL, ir.OIMAG, ir.OSPTR, ir.OITAB, ir.OIDATA:
		n := n.(*ir.UnaryExpr)
		n.X = walkExpr(n.X, init)
		return n

	case ir.ODOTMETH, ir.ODOTINTER:
		n := n.(*ir.SelectorExpr)
		n.X = walkExpr(n.X, init)
		return n

	case ir.OADDR:
		n := n.(*ir.AddrExpr)
		n.X = walkExpr(n.X, init)
		return n

	case ir.ODEREF:
		n := n.(*ir.StarExpr)
		n.X = walkExpr(n.X, init)
		return n

	case ir.OEFACE, ir.OAND, ir.OANDNOT, ir.OSUB, ir.OMUL, ir.OADD, ir.OOR, ir.OXOR, ir.OLSH, ir.ORSH:
		n := n.(*ir.BinaryExpr)
		n.X = walkExpr(n.X, init)
		n.Y = walkExpr(n.Y, init)
		return n

	case ir.ODOT, ir.ODOTPTR:
		n := n.(*ir.SelectorExpr)
		return walkDot(n, init)

	case ir.ODOTTYPE, ir.ODOTTYPE2:
		n := n.(*ir.TypeAssertExpr)
		return walkDotType(n, init)

	case ir.OLEN, ir.OCAP:
		n := n.(*ir.UnaryExpr)
		return walkLenCap(n, init)

	case ir.OCOMPLEX:
		n := n.(*ir.BinaryExpr)
		n.X = walkExpr(n.X, init)
		n.Y = walkExpr(n.Y, init)
		return n

	case ir.OEQ, ir.ONE, ir.OLT, ir.OLE, ir.OGT, ir.OGE:
		n := n.(*ir.BinaryExpr)
		return walkCompare(n, init)

	case ir.OANDAND, ir.OOROR:
		n := n.(*ir.LogicalExpr)
		return walkLogical(n, init)

	case ir.OPRINT, ir.OPRINTN:
		return walkPrint(n.(*ir.CallExpr), init)

	case ir.OPANIC:
		n := n.(*ir.UnaryExpr)
		return mkcall("gopanic", nil, init, n.X)

	case ir.ORECOVER:
		return walkRecover(n.(*ir.CallExpr), init)

	case ir.OCFUNC:
		return n

	case ir.OCALLINTER, ir.OCALLFUNC, ir.OCALLMETH:
		n := n.(*ir.CallExpr)
		return walkCall(n, init)

	case ir.OAS, ir.OASOP:
		return walkAssign(init, n)

	case ir.OAS2:
		n := n.(*ir.AssignListStmt)
		return walkAssignList(init, n)

	// a,b,... = fn()
	case ir.OAS2FUNC:
		n := n.(*ir.AssignListStmt)
		return walkAssignFunc(init, n)

	// x, y = <-c
	// order.stmt made sure x is addressable or blank.
	case ir.OAS2RECV:
		n := n.(*ir.AssignListStmt)
		return walkAssignRecv(init, n)

	// a,b = m[i]
	case ir.OAS2MAPR:
		n := n.(*ir.AssignListStmt)
		return walkAssignMapRead(init, n)

	case ir.ODELETE:
		n := n.(*ir.CallExpr)
		return walkDelete(init, n)

	case ir.OAS2DOTTYPE:
		n := n.(*ir.AssignListStmt)
		return walkAssignDotType(n, init)

	case ir.OCONVIFACE:
		n := n.(*ir.ConvExpr)
		return walkConvInterface(n, init)

	case ir.OCONV, ir.OCONVNOP:
		n := n.(*ir.ConvExpr)
		return walkConv(n, init)

	case ir.ODIV, ir.OMOD:
		n := n.(*ir.BinaryExpr)
		return walkDivMod(n, init)

	case ir.OINDEX:
		n := n.(*ir.IndexExpr)
		return walkIndex(n, init)

	case ir.OINDEXMAP:
		n := n.(*ir.IndexExpr)
		return walkIndexMap(n, init)

	case ir.ORECV:
		base.Fatalf("walkExpr ORECV") // should see inside OAS only
		panic("unreachable")

	case ir.OSLICEHEADER:
		n := n.(*ir.SliceHeaderExpr)
		return walkSliceHeader(n, init)

	case ir.OSLICE, ir.OSLICEARR, ir.OSLICESTR, ir.OSLICE3, ir.OSLICE3ARR:
		n := n.(*ir.SliceExpr)
		return walkSlice(n, init)

	case ir.ONEW:
		n := n.(*ir.UnaryExpr)
		return walkNew(n, init)

	case ir.OADDSTR:
		return walkAddString(n.(*ir.AddStringExpr), init)

	case ir.OAPPEND:
		// order should make sure we only see OAS(node, OAPPEND), which we handle above.
		base.Fatalf("append outside assignment")
		panic("unreachable")

	case ir.OCOPY:
		return walkCopy(n.(*ir.BinaryExpr), init, base.Flag.Cfg.Instrumenting && !base.Flag.CompilingRuntime)

	case ir.OCLOSE:
		n := n.(*ir.UnaryExpr)
		return walkClose(n, init)

	case ir.OMAKECHAN:
		n := n.(*ir.MakeExpr)
		return walkMakeChan(n, init)

	case ir.OMAKEMAP:
		n := n.(*ir.MakeExpr)
		return walkMakeMap(n, init)

	case ir.OMAKESLICE:
		n := n.(*ir.MakeExpr)
		return walkMakeSlice(n, init)

	case ir.OMAKESLICECOPY:
		n := n.(*ir.MakeExpr)
		return walkMakeSliceCopy(n, init)

	case ir.ORUNESTR:
		n := n.(*ir.ConvExpr)
		return walkRuneToString(n, init)

	case ir.OBYTES2STR, ir.ORUNES2STR:
		n := n.(*ir.ConvExpr)
		return walkBytesRunesToString(n, init)

	case ir.OBYTES2STRTMP:
		n := n.(*ir.ConvExpr)
		return walkBytesToStringTemp(n, init)

	case ir.OSTR2BYTES:
		n := n.(*ir.ConvExpr)
		return walkStringToBytes(n, init)

	case ir.OSTR2BYTESTMP:
		n := n.(*ir.ConvExpr)
		return walkStringToBytesTemp(n, init)

	case ir.OSTR2RUNES:
		n := n.(*ir.ConvExpr)
		return walkStringToRunes(n, init)

	case ir.OARRAYLIT, ir.OSLICELIT, ir.OMAPLIT, ir.OSTRUCTLIT, ir.OPTRLIT:
		return walkCompLit(n, init)

	case ir.OSEND:
		n := n.(*ir.SendStmt)
		return walkSend(n, init)

	case ir.OCLOSURE:
		return walkClosure(n.(*ir.ClosureExpr), init)

	case ir.OCALLPART:
		return walkCallPart(n.(*ir.SelectorExpr), init)
	}

	// No return! Each case must return (or panic),
	// to avoid confusion about what gets returned
	// in the presence of type assertions.
}

// walk the whole tree of the body of an
// expression or simple statement.
// the types expressions are calculated.
// compile-time constants are evaluated.
// complex side effects like statements are appended to init
func walkExprList(s []ir.Node, init *ir.Nodes) {
	for i := range s {
		s[i] = walkExpr(s[i], init)
	}
}

func walkExprListCheap(s []ir.Node, init *ir.Nodes) {
	for i, n := range s {
		s[i] = cheapExpr(n, init)
		s[i] = walkExpr(s[i], init)
	}
}

func walkExprListSafe(s []ir.Node, init *ir.Nodes) {
	for i, n := range s {
		s[i] = safeExpr(n, init)
		s[i] = walkExpr(s[i], init)
	}
}

// return side-effect free and cheap n, appending side effects to init.
// result may not be assignable.
func cheapExpr(n ir.Node, init *ir.Nodes) ir.Node {
	switch n.Op() {
	case ir.ONAME, ir.OLITERAL, ir.ONIL:
		return n
	}

	return copyExpr(n, n.Type(), init)
}

// return side effect-free n, appending side effects to init.
// result is assignable if n is.
func safeExpr(n ir.Node, init *ir.Nodes) ir.Node {
	if n == nil {
		return nil
	}

	if len(n.Init()) != 0 {
		walkStmtList(n.Init())
		init.Append(ir.TakeInit(n)...)
	}

	switch n.Op() {
	case ir.ONAME, ir.OLITERAL, ir.ONIL, ir.OLINKSYMOFFSET:
		return n

	case ir.OLEN, ir.OCAP:
		n := n.(*ir.UnaryExpr)
		l := safeExpr(n.X, init)
		if l == n.X {
			return n
		}
		a := ir.Copy(n).(*ir.UnaryExpr)
		a.X = l
		return walkExpr(typecheck.Expr(a), init)

	case ir.ODOT, ir.ODOTPTR:
		n := n.(*ir.SelectorExpr)
		l := safeExpr(n.X, init)
		if l == n.X {
			return n
		}
		a := ir.Copy(n).(*ir.SelectorExpr)
		a.X = l
		return walkExpr(typecheck.Expr(a), init)

	case ir.ODEREF:
		n := n.(*ir.StarExpr)
		l := safeExpr(n.X, init)
		if l == n.X {
			return n
		}
		a := ir.Copy(n).(*ir.StarExpr)
		a.X = l
		return walkExpr(typecheck.Expr(a), init)

	case ir.OINDEX, ir.OINDEXMAP:
		n := n.(*ir.IndexExpr)
		l := safeExpr(n.X, init)
		r := safeExpr(n.Index, init)
		if l == n.X && r == n.Index {
			return n
		}
		a := ir.Copy(n).(*ir.IndexExpr)
		a.X = l
		a.Index = r
		return walkExpr(typecheck.Expr(a), init)

	case ir.OSTRUCTLIT, ir.OARRAYLIT, ir.OSLICELIT:
		n := n.(*ir.CompLitExpr)
		if isStaticCompositeLiteral(n) {
			return n
		}
	}

	// make a copy; must not be used as an lvalue
	if ir.IsAddressable(n) {
		base.Fatalf("missing lvalue case in safeExpr: %v", n)
	}
	return cheapExpr(n, init)
}

func copyExpr(n ir.Node, t *types.Type, init *ir.Nodes) ir.Node {
	l := typecheck.Temp(t)
	appendWalkStmt(init, ir.NewAssignStmt(base.Pos, l, n))
	return l
}

func walkAddString(n *ir.AddStringExpr, init *ir.Nodes) ir.Node {
	c := len(n.List)

	if c < 2 {
		base.Fatalf("walkAddString count %d too small", c)
	}

	buf := typecheck.NodNil()
	if n.Esc() == ir.EscNone {
		sz := int64(0)
		for _, n1 := range n.List {
			if n1.Op() == ir.OLITERAL {
				sz += int64(len(ir.StringVal(n1)))
			}
		}

		// Don't allocate the buffer if the result won't fit.
		if sz < tmpstringbufsize {
			// Create temporary buffer for result string on stack.
			buf = stackBufAddr(tmpstringbufsize, types.Types[types.TUINT8])
		}
	}

	// build list of string arguments
	args := []ir.Node{buf}
	for _, n2 := range n.List {
		args = append(args, typecheck.Conv(n2, types.Types[types.TSTRING]))
	}

	var fn string
	if c <= 5 {
		// small numbers of strings use direct runtime helpers.
		// note: order.expr knows this cutoff too.
		fn = fmt.Sprintf("concatstring%d", c)
	} else {
		// large numbers of strings are passed to the runtime as a slice.
		fn = "concatstrings"

		t := types.NewSlice(types.Types[types.TSTRING])
		// args[1:] to skip buf arg
		slice := ir.NewCompLitExpr(base.Pos, ir.OCOMPLIT, ir.TypeNode(t), args[1:])
		slice.Prealloc = n.Prealloc
		args = []ir.Node{buf, slice}
		slice.SetEsc(ir.EscNone)
	}

	cat := typecheck.LookupRuntime(fn)
	r := ir.NewCallExpr(base.Pos, ir.OCALL, cat, nil)
	r.Args = args
	r1 := typecheck.Expr(r)
	r1 = walkExpr(r1, init)
	r1.SetType(n.Type())

	return r1
}

// walkCall walks an OCALLFUNC, OCALLINTER, or OCALLMETH node.
func walkCall(n *ir.CallExpr, init *ir.Nodes) ir.Node {
	if n.Op() == ir.OCALLINTER || n.Op() == ir.OCALLMETH {
		// We expect both interface call reflect.Type.Method and concrete
		// call reflect.(*rtype).Method.
		usemethod(n)
	}
	if n.Op() == ir.OCALLINTER {
		reflectdata.MarkUsedIfaceMethod(n)
	}

	if n.Op() == ir.OCALLFUNC && n.X.Op() == ir.OCLOSURE {
		directClosureCall(n)
	}

	walkCall1(n, init)
	return n
}

func walkCall1(n *ir.CallExpr, init *ir.Nodes) {
	if n.Walked() {
		return // already walked
	}
	n.SetWalked(true)

	// If this is a method call t.M(...),
	// rewrite into a function call T.M(t, ...).
	// TODO(mdempsky): Do this right after type checking.
	if n.Op() == ir.OCALLMETH {
		withRecv := make([]ir.Node, len(n.Args)+1)
		dot := n.X.(*ir.SelectorExpr)
		withRecv[0] = dot.X
		copy(withRecv[1:], n.Args)
		n.Args = withRecv

		dot = ir.NewSelectorExpr(dot.Pos(), ir.OXDOT, ir.TypeNode(dot.X.Type()), dot.Selection.Sym)

		n.SetOp(ir.OCALLFUNC)
		n.X = typecheck.Expr(dot)
	}

	args := n.Args
	params := n.X.Type().Params()

	n.X = walkExpr(n.X, init)
	walkExprList(args, init)

	for i, arg := range args {
		// Validate argument and parameter types match.
		param := params.Field(i)
		if !types.Identical(arg.Type(), param.Type) {
			base.FatalfAt(n.Pos(), "assigning %L to parameter %v (type %v)", arg, param.Sym, param.Type)
		}

		// For any argument whose evaluation might require a function call,
		// store that argument into a temporary variable,
		// to prevent that calls from clobbering arguments already on the stack.
		if mayCall(arg) {
			// assignment of arg to Temp
			tmp := typecheck.Temp(param.Type)
			init.Append(convas(typecheck.Stmt(ir.NewAssignStmt(base.Pos, tmp, arg)).(*ir.AssignStmt), init))
			// replace arg with temp
			args[i] = tmp
		}
	}

	n.Args = args
}

// walkDivMod walks an ODIV or OMOD node.
func walkDivMod(n *ir.BinaryExpr, init *ir.Nodes) ir.Node {
	n.X = walkExpr(n.X, init)
	n.Y = walkExpr(n.Y, init)

	// rewrite complex div into function call.
	et := n.X.Type().Kind()

	if types.IsComplex[et] && n.Op() == ir.ODIV {
		t := n.Type()
		call := mkcall("complex128div", types.Types[types.TCOMPLEX128], init, typecheck.Conv(n.X, types.Types[types.TCOMPLEX128]), typecheck.Conv(n.Y, types.Types[types.TCOMPLEX128]))
		return typecheck.Conv(call, t)
	}

	// Nothing to do for float divisions.
	if types.IsFloat[et] {
		return n
	}

	// rewrite 64-bit div and mod on 32-bit architectures.
	// TODO: Remove this code once we can introduce
	// runtime calls late in SSA processing.
	if types.RegSize < 8 && (et == types.TINT64 || et == types.TUINT64) {
		if n.Y.Op() == ir.OLITERAL {
			// Leave div/mod by constant powers of 2 or small 16-bit constants.
			// The SSA backend will handle those.
			switch et {
			case types.TINT64:
				c := ir.Int64Val(n.Y)
				if c < 0 {
					c = -c
				}
				if c != 0 && c&(c-1) == 0 {
					return n
				}
			case types.TUINT64:
				c := ir.Uint64Val(n.Y)
				if c < 1<<16 {
					return n
				}
				if c != 0 && c&(c-1) == 0 {
					return n
				}
			}
		}
		var fn string
		if et == types.TINT64 {
			fn = "int64"
		} else {
			fn = "uint64"
		}
		if n.Op() == ir.ODIV {
			fn += "div"
		} else {
			fn += "mod"
		}
		return mkcall(fn, n.Type(), init, typecheck.Conv(n.X, types.Types[et]), typecheck.Conv(n.Y, types.Types[et]))
	}
	return n
}

// walkDot walks an ODOT or ODOTPTR node.
func walkDot(n *ir.SelectorExpr, init *ir.Nodes) ir.Node {
	usefield(n)
	n.X = walkExpr(n.X, init)
	return n
}

// walkDotType walks an ODOTTYPE or ODOTTYPE2 node.
func walkDotType(n *ir.TypeAssertExpr, init *ir.Nodes) ir.Node {
	n.X = walkExpr(n.X, init)
	// Set up interface type addresses for back end.
	if !n.Type().IsInterface() && !n.X.Type().IsEmptyInterface() {
		n.Itab = reflectdata.ITabAddr(n.Type(), n.X.Type())
	}
	return n
}

// walkIndex walks an OINDEX node.
func walkIndex(n *ir.IndexExpr, init *ir.Nodes) ir.Node {
	n.X = walkExpr(n.X, init)

	// save the original node for bounds checking elision.
	// If it was a ODIV/OMOD walk might rewrite it.
	r := n.Index

	n.Index = walkExpr(n.Index, init)

	// if range of type cannot exceed static array bound,
	// disable bounds check.
	if n.Bounded() {
		return n
	}
	t := n.X.Type()
	if t != nil && t.IsPtr() {
		t = t.Elem()
	}
	if t.IsArray() {
		n.SetBounded(bounded(r, t.NumElem()))
		if base.Flag.LowerM != 0 && n.Bounded() && !ir.IsConst(n.Index, constant.Int) {
			base.Warn("index bounds check elided")
		}
		if ir.IsSmallIntConst(n.Index) && !n.Bounded() {
			base.Errorf("index out of bounds")
		}
	} else if ir.IsConst(n.X, constant.String) {
		n.SetBounded(bounded(r, int64(len(ir.StringVal(n.X)))))
		if base.Flag.LowerM != 0 && n.Bounded() && !ir.IsConst(n.Index, constant.Int) {
			base.Warn("index bounds check elided")
		}
		if ir.IsSmallIntConst(n.Index) && !n.Bounded() {
			base.Errorf("index out of bounds")
		}
	}

	if ir.IsConst(n.Index, constant.Int) {
		if v := n.Index.Val(); constant.Sign(v) < 0 || ir.ConstOverflow(v, types.Types[types.TINT]) {
			base.Errorf("index out of bounds")
		}
	}
	return n
}

// walkIndexMap walks an OINDEXMAP node.
func walkIndexMap(n *ir.IndexExpr, init *ir.Nodes) ir.Node {
	// Replace m[k] with *map{access1,assign}(maptype, m, &k)
	n.X = walkExpr(n.X, init)
	n.Index = walkExpr(n.Index, init)
	map_ := n.X
	key := n.Index
	t := map_.Type()
	var call *ir.CallExpr
	if n.Assigned {
		// This m[k] expression is on the left-hand side of an assignment.
		fast := mapfast(t)
		switch fast {
		case mapslow:
			// standard version takes key by reference.
			// order.expr made sure key is addressable.
			key = typecheck.NodAddr(key)
		case mapfast32ptr, mapfast64ptr:
			// pointer version takes pointer key.
			key = ir.NewConvExpr(n.Pos(), ir.OCONVNOP, types.Types[types.TUNSAFEPTR], key)
		}
		call = mkcall1(mapfn(mapassign[fast], t, false), nil, init, reflectdata.TypePtr(t), map_, key)
	} else {
		// m[k] is not the target of an assignment.
		fast := mapfast(t)
		if fast == mapslow {
			// standard version takes key by reference.
			// order.expr made sure key is addressable.
			key = typecheck.NodAddr(key)
		}

		if w := t.Elem().Width; w <= zeroValSize {
			call = mkcall1(mapfn(mapaccess1[fast], t, false), types.NewPtr(t.Elem()), init, reflectdata.TypePtr(t), map_, key)
		} else {
			z := reflectdata.ZeroAddr(w)
			call = mkcall1(mapfn("mapaccess1_fat", t, true), types.NewPtr(t.Elem()), init, reflectdata.TypePtr(t), map_, key, z)
		}
	}
	call.SetType(types.NewPtr(t.Elem()))
	call.MarkNonNil() // mapaccess1* and mapassign always return non-nil pointers.
	star := ir.NewStarExpr(base.Pos, call)
	star.SetType(t.Elem())
	star.SetTypecheck(1)
	return star
}

// walkLogical walks an OANDAND or OOROR node.
func walkLogical(n *ir.LogicalExpr, init *ir.Nodes) ir.Node {
	n.X = walkExpr(n.X, init)

	// cannot put side effects from n.Right on init,
	// because they cannot run before n.Left is checked.
	// save elsewhere and store on the eventual n.Right.
	var ll ir.Nodes

	n.Y = walkExpr(n.Y, &ll)
	n.Y = ir.InitExpr(ll, n.Y)
	return n
}

// walkSend walks an OSEND node.
func walkSend(n *ir.SendStmt, init *ir.Nodes) ir.Node {
	n1 := n.Value
	n1 = typecheck.AssignConv(n1, n.Chan.Type().Elem(), "chan send")
	n1 = walkExpr(n1, init)
	n1 = typecheck.NodAddr(n1)
	return mkcall1(chanfn("chansend1", 2, n.Chan.Type()), nil, init, n.Chan, n1)
}

// walkSlice walks an OSLICE, OSLICEARR, OSLICESTR, OSLICE3, or OSLICE3ARR node.
func walkSlice(n *ir.SliceExpr, init *ir.Nodes) ir.Node {

	checkSlice := ir.ShouldCheckPtr(ir.CurFunc, 1) && n.Op() == ir.OSLICE3ARR && n.X.Op() == ir.OCONVNOP && n.X.(*ir.ConvExpr).X.Type().IsUnsafePtr()
	if checkSlice {
		conv := n.X.(*ir.ConvExpr)
		conv.X = walkExpr(conv.X, init)
	} else {
		n.X = walkExpr(n.X, init)
	}

	n.Low = walkExpr(n.Low, init)
	if n.Low != nil && ir.IsZero(n.Low) {
		// Reduce x[0:j] to x[:j] and x[0:j:k] to x[:j:k].
		n.Low = nil
	}
	n.High = walkExpr(n.High, init)
	n.Max = walkExpr(n.Max, init)
	if checkSlice {
		n.X = walkCheckPtrAlignment(n.X.(*ir.ConvExpr), init, n.Max)
	}

	if n.Op().IsSlice3() {
		if n.Max != nil && n.Max.Op() == ir.OCAP && ir.SameSafeExpr(n.X, n.Max.(*ir.UnaryExpr).X) {
			// Reduce x[i:j:cap(x)] to x[i:j].
			if n.Op() == ir.OSLICE3 {
				n.SetOp(ir.OSLICE)
			} else {
				n.SetOp(ir.OSLICEARR)
			}
			return reduceSlice(n)
		}
		return n
	}
	return reduceSlice(n)
}

// walkSliceHeader walks an OSLICEHEADER node.
func walkSliceHeader(n *ir.SliceHeaderExpr, init *ir.Nodes) ir.Node {
	n.Ptr = walkExpr(n.Ptr, init)
	n.Len = walkExpr(n.Len, init)
	n.Cap = walkExpr(n.Cap, init)
	return n
}

// TODO(josharian): combine this with its caller and simplify
func reduceSlice(n *ir.SliceExpr) ir.Node {
	if n.High != nil && n.High.Op() == ir.OLEN && ir.SameSafeExpr(n.X, n.High.(*ir.UnaryExpr).X) {
		// Reduce x[i:len(x)] to x[i:].
		n.High = nil
	}
	if (n.Op() == ir.OSLICE || n.Op() == ir.OSLICESTR) && n.Low == nil && n.High == nil {
		// Reduce x[:] to x.
		if base.Debug.Slice > 0 {
			base.Warn("slice: omit slice operation")
		}
		return n.X
	}
	return n
}

// return 1 if integer n must be in range [0, max), 0 otherwise
func bounded(n ir.Node, max int64) bool {
	if n.Type() == nil || !n.Type().IsInteger() {
		return false
	}

	sign := n.Type().IsSigned()
	bits := int32(8 * n.Type().Width)

	if ir.IsSmallIntConst(n) {
		v := ir.Int64Val(n)
		return 0 <= v && v < max
	}

	switch n.Op() {
	case ir.OAND, ir.OANDNOT:
		n := n.(*ir.BinaryExpr)
		v := int64(-1)
		switch {
		case ir.IsSmallIntConst(n.X):
			v = ir.Int64Val(n.X)
		case ir.IsSmallIntConst(n.Y):
			v = ir.Int64Val(n.Y)
			if n.Op() == ir.OANDNOT {
				v = ^v
				if !sign {
					v &= 1<<uint(bits) - 1
				}
			}
		}
		if 0 <= v && v < max {
			return true
		}

	case ir.OMOD:
		n := n.(*ir.BinaryExpr)
		if !sign && ir.IsSmallIntConst(n.Y) {
			v := ir.Int64Val(n.Y)
			if 0 <= v && v <= max {
				return true
			}
		}

	case ir.ODIV:
		n := n.(*ir.BinaryExpr)
		if !sign && ir.IsSmallIntConst(n.Y) {
			v := ir.Int64Val(n.Y)
			for bits > 0 && v >= 2 {
				bits--
				v >>= 1
			}
		}

	case ir.ORSH:
		n := n.(*ir.BinaryExpr)
		if !sign && ir.IsSmallIntConst(n.Y) {
			v := ir.Int64Val(n.Y)
			if v > int64(bits) {
				return true
			}
			bits -= int32(v)
		}
	}

	if !sign && bits <= 62 && 1<<uint(bits) <= max {
		return true
	}

	return false
}

// usemethod checks interface method calls for uses of reflect.Type.Method.
func usemethod(n *ir.CallExpr) {
	t := n.X.Type()

	// Looking for either of:
	//	Method(int) reflect.Method
	//	MethodByName(string) (reflect.Method, bool)
	//
	// TODO(crawshaw): improve precision of match by working out
	//                 how to check the method name.
	if n := t.NumParams(); n != 1 {
		return
	}
	if n := t.NumResults(); n != 1 && n != 2 {
		return
	}
	p0 := t.Params().Field(0)
	res0 := t.Results().Field(0)
	var res1 *types.Field
	if t.NumResults() == 2 {
		res1 = t.Results().Field(1)
	}

	if res1 == nil {
		if p0.Type.Kind() != types.TINT {
			return
		}
	} else {
		if !p0.Type.IsString() {
			return
		}
		if !res1.Type.IsBoolean() {
			return
		}
	}

	// Don't mark reflect.(*rtype).Method, etc. themselves in the reflect package.
	// Those functions may be alive via the itab, which should not cause all methods
	// alive. We only want to mark their callers.
	if base.Ctxt.Pkgpath == "reflect" {
		switch ir.CurFunc.Nname.Sym().Name { // TODO: is there a better way than hardcoding the names?
		case "(*rtype).Method", "(*rtype).MethodByName", "(*interfaceType).Method", "(*interfaceType).MethodByName":
			return
		}
	}

	// Note: Don't rely on res0.Type.String() since its formatting depends on multiple factors
	//       (including global variables such as numImports - was issue #19028).
	// Also need to check for reflect package itself (see Issue #38515).
	if s := res0.Type.Sym(); s != nil && s.Name == "Method" && types.IsReflectPkg(s.Pkg) {
		ir.CurFunc.SetReflectMethod(true)
		// The LSym is initialized at this point. We need to set the attribute on the LSym.
		ir.CurFunc.LSym.Set(obj.AttrReflectMethod, true)
	}
}

func usefield(n *ir.SelectorExpr) {
	if !buildcfg.Experiment.FieldTrack {
		return
	}

	switch n.Op() {
	default:
		base.Fatalf("usefield %v", n.Op())

	case ir.ODOT, ir.ODOTPTR:
		break
	}

	field := n.Selection
	if field == nil {
		base.Fatalf("usefield %v %v without paramfld", n.X.Type(), n.Sel)
	}
	if field.Sym != n.Sel {
		base.Fatalf("field inconsistency: %v != %v", field.Sym, n.Sel)
	}
	if !strings.Contains(field.Note, "go:\"track\"") {
		return
	}

	outer := n.X.Type()
	if outer.IsPtr() {
		outer = outer.Elem()
	}
	if outer.Sym() == nil {
		base.Errorf("tracked field must be in named struct type")
	}
	if !types.IsExported(field.Sym.Name) {
		base.Errorf("tracked field must be exported (upper case)")
	}

	sym := reflectdata.TrackSym(outer, field)
	if ir.CurFunc.FieldTrack == nil {
		ir.CurFunc.FieldTrack = make(map[*obj.LSym]struct{})
	}
	ir.CurFunc.FieldTrack[sym] = struct{}{}
}