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"
	"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"
	"cmd/internal/objabi"
)

// 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
	}

	// Eagerly checkwidth all expressions for the back end.
	if n.Type() != nil && !n.Type().WidthCalculated() {
		switch n.Type().Kind() {
		case types.TBLANK, types.TNIL, types.TIDEAL:
		default:
			types.CheckSize(n.Type())
		}
	}

	if 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(n.PtrInit().Take()...)
	}

	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)
	}

	if n.Op() == ir.ONAME && n.(*ir.Name).Class_ == ir.PAUTOHEAP {
		n := n.(*ir.Name)
		nn := ir.NewStarExpr(base.Pos, n.Name().Heapaddr)
		nn.X.MarkNonNil()
		return walkExpr(typecheck.Expr(nn), init)
	}

	n = walkExpr1(n, init)

	// Expressions that are constant at run time but not
	// considered const by the language spec are not turned into
	// constants until walk. For example, if n is y%1 == 0, the
	// walk of y%1 may have replaced it by 0.
	// Check whether n with its updated args is itself now a constant.
	t := n.Type()
	n = typecheck.EvalConst(n)
	if n.Type() != t {
		base.Fatalf("evconst changed Type: %v had type %v, now %v", n, t, n.Type())
	}
	if n.Op() == ir.OLITERAL {
		n = typecheck.Expr(n)
		// Emit string symbol now to avoid emitting
		// any concurrently during the backend.
		if v := n.Val(); v.Kind() == constant.String {
			_ = staticdata.StringSym(n.Pos(), constant.StringVal(v))
		}
	}

	updateHasCall(n)

	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, ir.ONEWOBJ, ir.OMETHEXPR:
		return n

	case ir.OTYPE, ir.ONAME, ir.OLITERAL, ir.ONIL, ir.ONAMEOFFSET:
		// 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.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:
		n := n.(*ir.CallExpr)
		return mkcall("gorecover", n.Type(), init, typecheck.NodAddr(ir.RegFP))

	case ir.OCLOSUREREAD, 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.CallPartExpr), 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(n.PtrInit().Take()...)
	}

	switch n.Op() {
	case ir.ONAME, ir.OLITERAL, ir.ONIL, ir.ONAMEOFFSET:
		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.IsAssignable(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("addstr 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.
			t := types.NewArray(types.Types[types.TUINT8], tmpstringbufsize)
			buf = typecheck.NodAddr(typecheck.Temp(t))
		}
	}

	// 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.Set(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 {
		usemethod(n)
		reflectdata.MarkUsedIfaceMethod(n)
	}

	if n.Op() == ir.OCALLFUNC && n.X.Op() == ir.OCLOSURE {
		// Transform direct call of a closure to call of a normal function.
		// transformclosure already did all preparation work.

		// Prepend captured variables to argument list.
		clo := n.X.(*ir.ClosureExpr)
		n.Args.Prepend(clo.Func.ClosureEnter...)
		clo.Func.ClosureEnter.Set(nil)

		// Replace OCLOSURE with ONAME/PFUNC.
		n.X = clo.Func.Nname

		// Update type of OCALLFUNC node.
		// Output arguments had not changed, but their offsets could.
		if n.X.Type().NumResults() == 1 {
			n.SetType(n.X.Type().Results().Field(0).Type)
		} else {
			n.SetType(n.X.Type().Results())
		}
	}

	walkCall1(n, init)
	return n
}

func walkCall1(n *ir.CallExpr, init *ir.Nodes) {
	if len(n.Rargs) != 0 {
		return // already walked
	}

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

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

	// If this is a method call, add the receiver at the beginning of the args.
	if n.Op() == ir.OCALLMETH {
		withRecv := make([]ir.Node, len(args)+1)
		dot := n.X.(*ir.SelectorExpr)
		withRecv[0] = dot.X
		dot.X = nil
		copy(withRecv[1:], args)
		args = withRecv
	}

	// 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.
	// When instrumenting, all arguments might require function calls.
	var tempAssigns []ir.Node
	for i, arg := range args {
		updateHasCall(arg)
		// Determine param type.
		var t *types.Type
		if n.Op() == ir.OCALLMETH {
			if i == 0 {
				t = n.X.Type().Recv().Type
			} else {
				t = params.Field(i - 1).Type
			}
		} else {
			t = params.Field(i).Type
		}
		if base.Flag.Cfg.Instrumenting || fncall(arg, t) {
			// make assignment of fncall to tempAt
			tmp := typecheck.Temp(t)
			a := convas(ir.NewAssignStmt(base.Pos, tmp, arg), init)
			tempAssigns = append(tempAssigns, a)
			// replace arg with temp
			args[i] = tmp
		}
	}

	n.Args.Set(tempAssigns)
	n.Rargs.Set(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.
	n.Ntype = reflectdata.TypePtr(n.Type())
	if n.Op() == ir.ODOTTYPE {
		n.Ntype.(*ir.AddrExpr).Alloc = reflectdata.TypePtr(n.X.Type())
	}
	if !n.Type().IsInterface() && !n.X.Type().IsEmptyInterface() {
		n.Itab = []ir.Node{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)
		if fast == mapslow {
			// standard version takes key by reference.
			// order.expr made sure key is addressable.
			key = typecheck.NodAddr(key)
		}
		call = mkcall1(mapfn(mapassign[fast], t), 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), types.NewPtr(t.Elem()), init, reflectdata.TypePtr(t), map_, key)
		} else {
			z := reflectdata.ZeroAddr(w)
			call = mkcall1(mapfn("mapaccess1_fat", t), 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)
	}

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

	if n.Op().IsSlice3() {
		if max != nil && max.Op() == ir.OCAP && ir.SameSafeExpr(n.X, 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.LenCap[0] = walkExpr(n.LenCap[0], init)
	n.LenCap[1] = walkExpr(n.LenCap[1], init)
	return n
}

// TODO(josharian): combine this with its caller and simplify
func reduceSlice(n *ir.SliceExpr) ir.Node {
	low, high, max := n.SliceBounds()
	if high != nil && high.Op() == ir.OLEN && ir.SameSafeExpr(n.X, high.(*ir.UnaryExpr).X) {
		// Reduce x[i:len(x)] to x[i:].
		high = nil
	}
	n.SetSliceBounds(low, high, max)
	if (n.Op() == ir.OSLICE || n.Op() == ir.OSLICESTR) && low == nil && 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
		}
	}

	// 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 objabi.Fieldtrack_enabled == 0 {
		return
	}

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

	case ir.ODOT, ir.ODOTPTR:
		break
	}
	if n.Sel == nil {
		// No field name.  This DOTPTR was built by the compiler for access
		// to runtime data structures.  Ignore.
		return
	}

	t := n.X.Type()
	if t.IsPtr() {
		t = t.Elem()
	}
	field := n.Selection
	if field == nil {
		base.Fatalf("usefield %v %v without paramfld", n.X.Type(), n.Sel)
	}
	if field.Sym != n.Sel || field.Offset != n.Offset {
		base.Fatalf("field inconsistency: %v,%v != %v,%v", field.Sym, field.Offset, n.Sel, n.Offset)
	}
	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[*types.Sym]struct{})
	}
	ir.CurFunc.FieldTrack[sym] = struct{}{}
}