cmd/compile: fix defer desugar keepalive arg handling buglet

[gostls13.git] / src / cmd / compile / internal / walk / order.go

// Copyright 2012 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"

        "cmd/compile/internal/base"
        "cmd/compile/internal/escape"
        "cmd/compile/internal/ir"
        "cmd/compile/internal/reflectdata"
        "cmd/compile/internal/staticinit"
        "cmd/compile/internal/typecheck"
        "cmd/compile/internal/types"
        "cmd/internal/objabi"
        "cmd/internal/src"
)

// Rewrite tree to use separate statements to enforce
// order of evaluation. Makes walk easier, because it
// can (after this runs) reorder at will within an expression.
//
// Rewrite m[k] op= r into m[k] = m[k] op r if op is / or %.
//
// Introduce temporaries as needed by runtime routines.
// For example, the map runtime routines take the map key
// by reference, so make sure all map keys are addressable
// by copying them to temporaries as needed.
// The same is true for channel operations.
//
// Arrange that map index expressions only appear in direct
// assignments x = m[k] or m[k] = x, never in larger expressions.
//
// Arrange that receive expressions only appear in direct assignments
// x = <-c or as standalone statements <-c, never in larger expressions.

// TODO(rsc): The temporary introduction during multiple assignments
// should be moved into this file, so that the temporaries can be cleaned
// and so that conversions implicit in the OAS2FUNC and OAS2RECV
// nodes can be made explicit and then have their temporaries cleaned.

// TODO(rsc): Goto and multilevel break/continue can jump over
// inserted VARKILL annotations. Work out a way to handle these.
// The current implementation is safe, in that it will execute correctly.
// But it won't reuse temporaries as aggressively as it might, and
// it can result in unnecessary zeroing of those variables in the function
// prologue.

// orderState holds state during the ordering process.
type orderState struct {
        out  []ir.Node             // list of generated statements
        temp []*ir.Name            // stack of temporary variables
        free map[string][]*ir.Name // free list of unused temporaries, by type.LongString().
        edit func(ir.Node) ir.Node // cached closure of o.exprNoLHS
}

// Order rewrites fn.Nbody to apply the ordering constraints
// described in the comment at the top of the file.
func order(fn *ir.Func) {
        if base.Flag.W > 1 {
                s := fmt.Sprintf("\nbefore order %v", fn.Sym())
                ir.DumpList(s, fn.Body)
        }

        orderBlock(&fn.Body, map[string][]*ir.Name{})
}

// append typechecks stmt and appends it to out.
func (o *orderState) append(stmt ir.Node) {
        o.out = append(o.out, typecheck.Stmt(stmt))
}

// newTemp allocates a new temporary with the given type,
// pushes it onto the temp stack, and returns it.
// If clear is true, newTemp emits code to zero the temporary.
func (o *orderState) newTemp(t *types.Type, clear bool) *ir.Name {
        var v *ir.Name
        // Note: LongString is close to the type equality we want,
        // but not exactly. We still need to double-check with types.Identical.
        key := t.LongString()
        a := o.free[key]
        for i, n := range a {
                if types.Identical(t, n.Type()) {
                        v = a[i]
                        a[i] = a[len(a)-1]
                        a = a[:len(a)-1]
                        o.free[key] = a
                        break
                }
        }
        if v == nil {
                v = typecheck.Temp(t)
        }
        if clear {
                o.append(ir.NewAssignStmt(base.Pos, v, nil))
        }

        o.temp = append(o.temp, v)
        return v
}

// copyExpr behaves like newTemp but also emits
// code to initialize the temporary to the value n.
func (o *orderState) copyExpr(n ir.Node) *ir.Name {
        return o.copyExpr1(n, false)
}

// copyExprClear is like copyExpr but clears the temp before assignment.
// It is provided for use when the evaluation of tmp = n turns into
// a function call that is passed a pointer to the temporary as the output space.
// If the call blocks before tmp has been written,
// the garbage collector will still treat the temporary as live,
// so we must zero it before entering that call.
// Today, this only happens for channel receive operations.
// (The other candidate would be map access, but map access
// returns a pointer to the result data instead of taking a pointer
// to be filled in.)
func (o *orderState) copyExprClear(n ir.Node) *ir.Name {
        return o.copyExpr1(n, true)
}

func (o *orderState) copyExpr1(n ir.Node, clear bool) *ir.Name {
        t := n.Type()
        v := o.newTemp(t, clear)
        o.append(ir.NewAssignStmt(base.Pos, v, n))
        return v
}

// cheapExpr returns a cheap version of n.
// The definition of cheap is that n is a variable or constant.
// If not, cheapExpr allocates a new tmp, emits tmp = n,
// and then returns tmp.
func (o *orderState) cheapExpr(n ir.Node) ir.Node {
        if n == nil {
                return nil
        }

        switch n.Op() {
        case ir.ONAME, ir.OLITERAL, ir.ONIL:
                return n
        case ir.OLEN, ir.OCAP:
                n := n.(*ir.UnaryExpr)
                l := o.cheapExpr(n.X)
                if l == n.X {
                        return n
                }
                a := ir.SepCopy(n).(*ir.UnaryExpr)
                a.X = l
                return typecheck.Expr(a)
        }

        return o.copyExpr(n)
}

// safeExpr returns a safe version of n.
// The definition of safe is that n can appear multiple times
// without violating the semantics of the original program,
// and that assigning to the safe version has the same effect
// as assigning to the original n.
//
// The intended use is to apply to x when rewriting x += y into x = x + y.
func (o *orderState) safeExpr(n ir.Node) ir.Node {
        switch n.Op() {
        case ir.ONAME, ir.OLITERAL, ir.ONIL:
                return n

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

        case ir.ODOT:
                n := n.(*ir.SelectorExpr)
                l := o.safeExpr(n.X)
                if l == n.X {
                        return n
                }
                a := ir.SepCopy(n).(*ir.SelectorExpr)
                a.X = l
                return typecheck.Expr(a)

        case ir.ODOTPTR:
                n := n.(*ir.SelectorExpr)
                l := o.cheapExpr(n.X)
                if l == n.X {
                        return n
                }
                a := ir.SepCopy(n).(*ir.SelectorExpr)
                a.X = l
                return typecheck.Expr(a)

        case ir.ODEREF:
                n := n.(*ir.StarExpr)
                l := o.cheapExpr(n.X)
                if l == n.X {
                        return n
                }
                a := ir.SepCopy(n).(*ir.StarExpr)
                a.X = l
                return typecheck.Expr(a)

        case ir.OINDEX, ir.OINDEXMAP:
                n := n.(*ir.IndexExpr)
                var l ir.Node
                if n.X.Type().IsArray() {
                        l = o.safeExpr(n.X)
                } else {
                        l = o.cheapExpr(n.X)
                }
                r := o.cheapExpr(n.Index)
                if l == n.X && r == n.Index {
                        return n
                }
                a := ir.SepCopy(n).(*ir.IndexExpr)
                a.X = l
                a.Index = r
                return typecheck.Expr(a)

        default:
                base.Fatalf("order.safeExpr %v", n.Op())
                return nil // not reached
        }
}

// isaddrokay reports whether it is okay to pass n's address to runtime routines.
// Taking the address of a variable makes the liveness and optimization analyses
// lose track of where the variable's lifetime ends. To avoid hurting the analyses
// of ordinary stack variables, those are not 'isaddrokay'. Temporaries are okay,
// because we emit explicit VARKILL instructions marking the end of those
// temporaries' lifetimes.
func isaddrokay(n ir.Node) bool {
        return ir.IsAddressable(n) && (n.Op() != ir.ONAME || n.(*ir.Name).Class == ir.PEXTERN || ir.IsAutoTmp(n))
}

// addrTemp ensures that n is okay to pass by address to runtime routines.
// If the original argument n is not okay, addrTemp creates a tmp, emits
// tmp = n, and then returns tmp.
// The result of addrTemp MUST be assigned back to n, e.g.
//      n.Left = o.addrTemp(n.Left)
func (o *orderState) addrTemp(n ir.Node) ir.Node {
        if n.Op() == ir.OLITERAL || n.Op() == ir.ONIL {
                // TODO: expand this to all static composite literal nodes?
                n = typecheck.DefaultLit(n, nil)
                types.CalcSize(n.Type())
                vstat := readonlystaticname(n.Type())
                var s staticinit.Schedule
                s.StaticAssign(vstat, 0, n, n.Type())
                if s.Out != nil {
                        base.Fatalf("staticassign of const generated code: %+v", n)
                }
                vstat = typecheck.Expr(vstat).(*ir.Name)
                return vstat
        }
        if isaddrokay(n) {
                return n
        }
        return o.copyExpr(n)
}

// mapKeyTemp prepares n to be a key in a map runtime call and returns n.
// It should only be used for map runtime calls which have *_fast* versions.
func (o *orderState) mapKeyTemp(t *types.Type, n ir.Node) ir.Node {
        // Most map calls need to take the address of the key.
        // Exception: map*_fast* calls. See golang.org/issue/19015.
        if mapfast(t) == mapslow {
                return o.addrTemp(n)
        }
        return n
}

// mapKeyReplaceStrConv replaces OBYTES2STR by OBYTES2STRTMP
// in n to avoid string allocations for keys in map lookups.
// Returns a bool that signals if a modification was made.
//
// For:
//  x = m[string(k)]
//  x = m[T1{... Tn{..., string(k), ...}]
// where k is []byte, T1 to Tn is a nesting of struct and array literals,
// the allocation of backing bytes for the string can be avoided
// by reusing the []byte backing array. These are special cases
// for avoiding allocations when converting byte slices to strings.
// It would be nice to handle these generally, but because
// []byte keys are not allowed in maps, the use of string(k)
// comes up in important cases in practice. See issue 3512.
func mapKeyReplaceStrConv(n ir.Node) bool {
        var replaced bool
        switch n.Op() {
        case ir.OBYTES2STR:
                n := n.(*ir.ConvExpr)
                n.SetOp(ir.OBYTES2STRTMP)
                replaced = true
        case ir.OSTRUCTLIT:
                n := n.(*ir.CompLitExpr)
                for _, elem := range n.List {
                        elem := elem.(*ir.StructKeyExpr)
                        if mapKeyReplaceStrConv(elem.Value) {
                                replaced = true
                        }
                }
        case ir.OARRAYLIT:
                n := n.(*ir.CompLitExpr)
                for _, elem := range n.List {
                        if elem.Op() == ir.OKEY {
                                elem = elem.(*ir.KeyExpr).Value
                        }
                        if mapKeyReplaceStrConv(elem) {
                                replaced = true
                        }
                }
        }
        return replaced
}

type ordermarker int

// markTemp returns the top of the temporary variable stack.
func (o *orderState) markTemp() ordermarker {
        return ordermarker(len(o.temp))
}

// popTemp pops temporaries off the stack until reaching the mark,
// which must have been returned by markTemp.
func (o *orderState) popTemp(mark ordermarker) {
        for _, n := range o.temp[mark:] {
                key := n.Type().LongString()
                o.free[key] = append(o.free[key], n)
        }
        o.temp = o.temp[:mark]
}

// cleanTempNoPop emits VARKILL instructions to *out
// for each temporary above the mark on the temporary stack.
// It does not pop the temporaries from the stack.
func (o *orderState) cleanTempNoPop(mark ordermarker) []ir.Node {
        var out []ir.Node
        for i := len(o.temp) - 1; i >= int(mark); i-- {
                n := o.temp[i]
                out = append(out, typecheck.Stmt(ir.NewUnaryExpr(base.Pos, ir.OVARKILL, n)))
        }
        return out
}

// cleanTemp emits VARKILL instructions for each temporary above the
// mark on the temporary stack and removes them from the stack.
func (o *orderState) cleanTemp(top ordermarker) {
        o.out = append(o.out, o.cleanTempNoPop(top)...)
        o.popTemp(top)
}

// stmtList orders each of the statements in the list.
func (o *orderState) stmtList(l ir.Nodes) {
        s := l
        for i := range s {
                orderMakeSliceCopy(s[i:])
                o.stmt(s[i])
        }
}

// orderMakeSliceCopy matches the pattern:
//  m = OMAKESLICE([]T, x); OCOPY(m, s)
// and rewrites it to:
//  m = OMAKESLICECOPY([]T, x, s); nil
func orderMakeSliceCopy(s []ir.Node) {
        if base.Flag.N != 0 || base.Flag.Cfg.Instrumenting {
                return
        }
        if len(s) < 2 || s[0] == nil || s[0].Op() != ir.OAS || s[1] == nil || s[1].Op() != ir.OCOPY {
                return
        }

        as := s[0].(*ir.AssignStmt)
        cp := s[1].(*ir.BinaryExpr)
        if as.Y == nil || as.Y.Op() != ir.OMAKESLICE || ir.IsBlank(as.X) ||
                as.X.Op() != ir.ONAME || cp.X.Op() != ir.ONAME || cp.Y.Op() != ir.ONAME ||
                as.X.Name() != cp.X.Name() || cp.X.Name() == cp.Y.Name() {
                // The line above this one is correct with the differing equality operators:
                // we want as.X and cp.X to be the same name,
                // but we want the initial data to be coming from a different name.
                return
        }

        mk := as.Y.(*ir.MakeExpr)
        if mk.Esc() == ir.EscNone || mk.Len == nil || mk.Cap != nil {
                return
        }
        mk.SetOp(ir.OMAKESLICECOPY)
        mk.Cap = cp.Y
        // Set bounded when m = OMAKESLICE([]T, len(s)); OCOPY(m, s)
        mk.SetBounded(mk.Len.Op() == ir.OLEN && ir.SameSafeExpr(mk.Len.(*ir.UnaryExpr).X, cp.Y))
        as.Y = typecheck.Expr(mk)
        s[1] = nil // remove separate copy call
}

// edge inserts coverage instrumentation for libfuzzer.
func (o *orderState) edge() {
        if base.Debug.Libfuzzer == 0 {
                return
        }

        // Create a new uint8 counter to be allocated in section
        // __libfuzzer_extra_counters.
        counter := staticinit.StaticName(types.Types[types.TUINT8])
        counter.SetLibfuzzerExtraCounter(true)

        // counter += 1
        incr := ir.NewAssignOpStmt(base.Pos, ir.OADD, counter, ir.NewInt(1))
        o.append(incr)
}

// orderBlock orders the block of statements in n into a new slice,
// and then replaces the old slice in n with the new slice.
// free is a map that can be used to obtain temporary variables by type.
func orderBlock(n *ir.Nodes, free map[string][]*ir.Name) {
        var order orderState
        order.free = free
        mark := order.markTemp()
        order.edge()
        order.stmtList(*n)
        order.cleanTemp(mark)
        *n = order.out
}

// exprInPlace orders the side effects in *np and
// leaves them as the init list of the final *np.
// The result of exprInPlace MUST be assigned back to n, e.g.
//      n.Left = o.exprInPlace(n.Left)
func (o *orderState) exprInPlace(n ir.Node) ir.Node {
        var order orderState
        order.free = o.free
        n = order.expr(n, nil)
        n = ir.InitExpr(order.out, n)

        // insert new temporaries from order
        // at head of outer list.
        o.temp = append(o.temp, order.temp...)
        return n
}

// orderStmtInPlace orders the side effects of the single statement *np
// and replaces it with the resulting statement list.
// The result of orderStmtInPlace MUST be assigned back to n, e.g.
//      n.Left = orderStmtInPlace(n.Left)
// free is a map that can be used to obtain temporary variables by type.
func orderStmtInPlace(n ir.Node, free map[string][]*ir.Name) ir.Node {
        var order orderState
        order.free = free
        mark := order.markTemp()
        order.stmt(n)
        order.cleanTemp(mark)
        return ir.NewBlockStmt(src.NoXPos, order.out)
}

// init moves n's init list to o.out.
func (o *orderState) init(n ir.Node) {
        if ir.MayBeShared(n) {
                // For concurrency safety, don't mutate potentially shared nodes.
                // First, ensure that no work is required here.
                if len(n.Init()) > 0 {
                        base.Fatalf("order.init shared node with ninit")
                }
                return
        }
        o.stmtList(ir.TakeInit(n))
}

// call orders the call expression n.
// n.Op is OCALLMETH/OCALLFUNC/OCALLINTER or a builtin like OCOPY.
func (o *orderState) call(nn ir.Node) {
        if len(nn.Init()) > 0 {
                // Caller should have already called o.init(nn).
                base.Fatalf("%v with unexpected ninit", nn.Op())
        }

        // Builtin functions.
        if nn.Op() != ir.OCALLFUNC && nn.Op() != ir.OCALLMETH && nn.Op() != ir.OCALLINTER {
                switch n := nn.(type) {
                default:
                        base.Fatalf("unexpected call: %+v", n)
                case *ir.UnaryExpr:
                        n.X = o.expr(n.X, nil)
                case *ir.ConvExpr:
                        n.X = o.expr(n.X, nil)
                case *ir.BinaryExpr:
                        n.X = o.expr(n.X, nil)
                        n.Y = o.expr(n.Y, nil)
                case *ir.MakeExpr:
                        n.Len = o.expr(n.Len, nil)
                        n.Cap = o.expr(n.Cap, nil)
                case *ir.CallExpr:
                        o.exprList(n.Args)
                }
                return
        }

        n := nn.(*ir.CallExpr)
        typecheck.FixVariadicCall(n)
        n.X = o.expr(n.X, nil)
        o.exprList(n.Args)

        if n.Op() == ir.OCALLINTER {
                return
        }
        keepAlive := func(arg ir.Node) {
                // If the argument is really a pointer being converted to uintptr,
                // arrange for the pointer to be kept alive until the call returns,
                // by copying it into a temp and marking that temp
                // still alive when we pop the temp stack.
                if arg.Op() == ir.OCONVNOP {
                        arg := arg.(*ir.ConvExpr)
                        if arg.X.Type().IsUnsafePtr() {
                                x := o.copyExpr(arg.X)
                                arg.X = x
                                x.SetAddrtaken(true) // ensure SSA keeps the x variable
                                n.KeepAlive = append(n.KeepAlive, x)
                        }
                }
        }

        // Check for "unsafe-uintptr" tag provided by escape analysis.
        for i, param := range n.X.Type().Params().FieldSlice() {
                if param.Note == escape.UnsafeUintptrNote || param.Note == escape.UintptrEscapesNote {
                        if arg := n.Args[i]; arg.Op() == ir.OSLICELIT {
                                arg := arg.(*ir.CompLitExpr)
                                for _, elt := range arg.List {
                                        keepAlive(elt)
                                }
                        } else {
                                keepAlive(arg)
                        }
                }
        }
}

// mapAssign appends n to o.out.
func (o *orderState) mapAssign(n ir.Node) {
        switch n.Op() {
        default:
                base.Fatalf("order.mapAssign %v", n.Op())

        case ir.OAS:
                n := n.(*ir.AssignStmt)
                if n.X.Op() == ir.OINDEXMAP {
                        n.Y = o.safeMapRHS(n.Y)
                }
                o.out = append(o.out, n)
        case ir.OASOP:
                n := n.(*ir.AssignOpStmt)
                if n.X.Op() == ir.OINDEXMAP {
                        n.Y = o.safeMapRHS(n.Y)
                }
                o.out = append(o.out, n)
        }
}

func (o *orderState) safeMapRHS(r ir.Node) ir.Node {
        // Make sure we evaluate the RHS before starting the map insert.
        // We need to make sure the RHS won't panic.  See issue 22881.
        if r.Op() == ir.OAPPEND {
                r := r.(*ir.CallExpr)
                s := r.Args[1:]
                for i, n := range s {
                        s[i] = o.cheapExpr(n)
                }
                return r
        }
        return o.cheapExpr(r)
}

// stmt orders the statement n, appending to o.out.
// Temporaries created during the statement are cleaned
// up using VARKILL instructions as possible.
func (o *orderState) stmt(n ir.Node) {
        if n == nil {
                return
        }

        lno := ir.SetPos(n)
        o.init(n)

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

        case ir.OVARKILL, ir.OVARLIVE, ir.OINLMARK:
                o.out = append(o.out, n)

        case ir.OAS:
                n := n.(*ir.AssignStmt)
                t := o.markTemp()
                n.X = o.expr(n.X, nil)
                n.Y = o.expr(n.Y, n.X)
                o.mapAssign(n)
                o.cleanTemp(t)

        case ir.OASOP:
                n := n.(*ir.AssignOpStmt)
                t := o.markTemp()
                n.X = o.expr(n.X, nil)
                n.Y = o.expr(n.Y, nil)

                if base.Flag.Cfg.Instrumenting || n.X.Op() == ir.OINDEXMAP && (n.AsOp == ir.ODIV || n.AsOp == ir.OMOD) {
                        // Rewrite m[k] op= r into m[k] = m[k] op r so
                        // that we can ensure that if op panics
                        // because r is zero, the panic happens before
                        // the map assignment.
                        // DeepCopy is a big hammer here, but safeExpr
                        // makes sure there is nothing too deep being copied.
                        l1 := o.safeExpr(n.X)
                        l2 := ir.DeepCopy(src.NoXPos, l1)
                        if l2.Op() == ir.OINDEXMAP {
                                l2 := l2.(*ir.IndexExpr)
                                l2.Assigned = false
                        }
                        l2 = o.copyExpr(l2)
                        r := o.expr(typecheck.Expr(ir.NewBinaryExpr(n.Pos(), n.AsOp, l2, n.Y)), nil)
                        as := typecheck.Stmt(ir.NewAssignStmt(n.Pos(), l1, r))
                        o.mapAssign(as)
                        o.cleanTemp(t)
                        return
                }

                o.mapAssign(n)
                o.cleanTemp(t)

        case ir.OAS2:
                n := n.(*ir.AssignListStmt)
                t := o.markTemp()
                o.exprList(n.Lhs)
                o.exprList(n.Rhs)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        // Special: avoid copy of func call n.Right
        case ir.OAS2FUNC:
                n := n.(*ir.AssignListStmt)
                t := o.markTemp()
                o.exprList(n.Lhs)
                o.init(n.Rhs[0])
                o.call(n.Rhs[0])
                o.as2func(n)
                o.cleanTemp(t)

        // Special: use temporary variables to hold result,
        // so that runtime can take address of temporary.
        // No temporary for blank assignment.
        //
        // OAS2MAPR: make sure key is addressable if needed,
        //           and make sure OINDEXMAP is not copied out.
        case ir.OAS2DOTTYPE, ir.OAS2RECV, ir.OAS2MAPR:
                n := n.(*ir.AssignListStmt)
                t := o.markTemp()
                o.exprList(n.Lhs)

                switch r := n.Rhs[0]; r.Op() {
                case ir.ODOTTYPE2:
                        r := r.(*ir.TypeAssertExpr)
                        r.X = o.expr(r.X, nil)
                case ir.ORECV:
                        r := r.(*ir.UnaryExpr)
                        r.X = o.expr(r.X, nil)
                case ir.OINDEXMAP:
                        r := r.(*ir.IndexExpr)
                        r.X = o.expr(r.X, nil)
                        r.Index = o.expr(r.Index, nil)
                        // See similar conversion for OINDEXMAP below.
                        _ = mapKeyReplaceStrConv(r.Index)
                        r.Index = o.mapKeyTemp(r.X.Type(), r.Index)
                default:
                        base.Fatalf("order.stmt: %v", r.Op())
                }

                o.as2ok(n)
                o.cleanTemp(t)

        // Special: does not save n onto out.
        case ir.OBLOCK:
                n := n.(*ir.BlockStmt)
                o.stmtList(n.List)

        // Special: n->left is not an expression; save as is.
        case ir.OBREAK,
                ir.OCONTINUE,
                ir.ODCL,
                ir.ODCLCONST,
                ir.ODCLTYPE,
                ir.OFALL,
                ir.OGOTO,
                ir.OLABEL,
                ir.OTAILCALL:
                o.out = append(o.out, n)

        // Special: handle call arguments.
        case ir.OCALLFUNC, ir.OCALLINTER, ir.OCALLMETH:
                n := n.(*ir.CallExpr)
                t := o.markTemp()
                o.call(n)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.OCLOSE, ir.ORECV:
                n := n.(*ir.UnaryExpr)
                t := o.markTemp()
                n.X = o.expr(n.X, nil)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.OCOPY:
                n := n.(*ir.BinaryExpr)
                t := o.markTemp()
                n.X = o.expr(n.X, nil)
                n.Y = o.expr(n.Y, nil)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.OPRINT, ir.OPRINTN, ir.ORECOVER:
                n := n.(*ir.CallExpr)
                t := o.markTemp()
                o.exprList(n.Args)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        // Special: order arguments to inner call but not call itself.
        case ir.ODEFER, ir.OGO:
                n := n.(*ir.GoDeferStmt)
                t := o.markTemp()
                o.init(n.Call)
                o.call(n.Call)
                if objabi.Experiment.RegabiDefer {
                        o.wrapGoDefer(n)
                }
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.ODELETE:
                n := n.(*ir.CallExpr)
                t := o.markTemp()
                n.Args[0] = o.expr(n.Args[0], nil)
                n.Args[1] = o.expr(n.Args[1], nil)
                n.Args[1] = o.mapKeyTemp(n.Args[0].Type(), n.Args[1])
                o.out = append(o.out, n)
                o.cleanTemp(t)

        // Clean temporaries from condition evaluation at
        // beginning of loop body and after for statement.
        case ir.OFOR:
                n := n.(*ir.ForStmt)
                t := o.markTemp()
                n.Cond = o.exprInPlace(n.Cond)
                n.Body.Prepend(o.cleanTempNoPop(t)...)
                orderBlock(&n.Body, o.free)
                n.Post = orderStmtInPlace(n.Post, o.free)
                o.out = append(o.out, n)
                o.cleanTemp(t)

        // Clean temporaries from condition at
        // beginning of both branches.
        case ir.OIF:
                n := n.(*ir.IfStmt)
                t := o.markTemp()
                n.Cond = o.exprInPlace(n.Cond)
                n.Body.Prepend(o.cleanTempNoPop(t)...)
                n.Else.Prepend(o.cleanTempNoPop(t)...)
                o.popTemp(t)
                orderBlock(&n.Body, o.free)
                orderBlock(&n.Else, o.free)
                o.out = append(o.out, n)

        case ir.OPANIC:
                n := n.(*ir.UnaryExpr)
                t := o.markTemp()
                n.X = o.expr(n.X, nil)
                if !n.X.Type().IsEmptyInterface() {
                        base.FatalfAt(n.Pos(), "bad argument to panic: %L", n.X)
                }
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.ORANGE:
                // n.Right is the expression being ranged over.
                // order it, and then make a copy if we need one.
                // We almost always do, to ensure that we don't
                // see any value changes made during the loop.
                // Usually the copy is cheap (e.g., array pointer,
                // chan, slice, string are all tiny).
                // The exception is ranging over an array value
                // (not a slice, not a pointer to array),
                // which must make a copy to avoid seeing updates made during
                // the range body. Ranging over an array value is uncommon though.

                // Mark []byte(str) range expression to reuse string backing storage.
                // It is safe because the storage cannot be mutated.
                n := n.(*ir.RangeStmt)
                if n.X.Op() == ir.OSTR2BYTES {
                        n.X.(*ir.ConvExpr).SetOp(ir.OSTR2BYTESTMP)
                }

                t := o.markTemp()
                n.X = o.expr(n.X, nil)

                orderBody := true
                xt := typecheck.RangeExprType(n.X.Type())
                switch xt.Kind() {
                default:
                        base.Fatalf("order.stmt range %v", n.Type())

                case types.TARRAY, types.TSLICE:
                        if n.Value == nil || ir.IsBlank(n.Value) {
                                // for i := range x will only use x once, to compute len(x).
                                // No need to copy it.
                                break
                        }
                        fallthrough

                case types.TCHAN, types.TSTRING:
                        // chan, string, slice, array ranges use value multiple times.
                        // make copy.
                        r := n.X

                        if r.Type().IsString() && r.Type() != types.Types[types.TSTRING] {
                                r = ir.NewConvExpr(base.Pos, ir.OCONV, nil, r)
                                r.SetType(types.Types[types.TSTRING])
                                r = typecheck.Expr(r)
                        }

                        n.X = o.copyExpr(r)

                case types.TMAP:
                        if isMapClear(n) {
                                // Preserve the body of the map clear pattern so it can
                                // be detected during walk. The loop body will not be used
                                // when optimizing away the range loop to a runtime call.
                                orderBody = false
                                break
                        }

                        // copy the map value in case it is a map literal.
                        // TODO(rsc): Make tmp = literal expressions reuse tmp.
                        // For maps tmp is just one word so it hardly matters.
                        r := n.X
                        n.X = o.copyExpr(r)

                        // n.Prealloc is the temp for the iterator.
                        // MapIterType contains pointers and needs to be zeroed.
                        n.Prealloc = o.newTemp(reflectdata.MapIterType(xt), true)
                }
                n.Key = o.exprInPlace(n.Key)
                n.Value = o.exprInPlace(n.Value)
                if orderBody {
                        orderBlock(&n.Body, o.free)
                }
                o.out = append(o.out, n)
                o.cleanTemp(t)

        case ir.ORETURN:
                n := n.(*ir.ReturnStmt)
                o.exprList(n.Results)
                o.out = append(o.out, n)

        // Special: clean case temporaries in each block entry.
        // Select must enter one of its blocks, so there is no
        // need for a cleaning at the end.
        // Doubly special: evaluation order for select is stricter
        // than ordinary expressions. Even something like p.c
        // has to be hoisted into a temporary, so that it cannot be
        // reordered after the channel evaluation for a different
        // case (if p were nil, then the timing of the fault would
        // give this away).
        case ir.OSELECT:
                n := n.(*ir.SelectStmt)
                t := o.markTemp()
                for _, ncas := range n.Cases {
                        r := ncas.Comm
                        ir.SetPos(ncas)

                        // Append any new body prologue to ninit.
                        // The next loop will insert ninit into nbody.
                        if len(ncas.Init()) != 0 {
                                base.Fatalf("order select ninit")
                        }
                        if r == nil {
                                continue
                        }
                        switch r.Op() {
                        default:
                                ir.Dump("select case", r)
                                base.Fatalf("unknown op in select %v", r.Op())

                        case ir.OSELRECV2:
                                // case x, ok = <-c
                                r := r.(*ir.AssignListStmt)
                                recv := r.Rhs[0].(*ir.UnaryExpr)
                                recv.X = o.expr(recv.X, nil)
                                if !ir.IsAutoTmp(recv.X) {
                                        recv.X = o.copyExpr(recv.X)
                                }
                                init := ir.TakeInit(r)

                                colas := r.Def
                                do := func(i int, t *types.Type) {
                                        n := r.Lhs[i]
                                        if ir.IsBlank(n) {
                                                return
                                        }
                                        // If this is case x := <-ch or case x, y := <-ch, the case has
                                        // the ODCL nodes to declare x and y. We want to delay that
                                        // declaration (and possible allocation) until inside the case body.
                                        // Delete the ODCL nodes here and recreate them inside the body below.
                                        if colas {
                                                if len(init) > 0 && init[0].Op() == ir.ODCL && init[0].(*ir.Decl).X == n {
                                                        init = init[1:]
                                                }
                                                dcl := typecheck.Stmt(ir.NewDecl(base.Pos, ir.ODCL, n.(*ir.Name)))
                                                ncas.PtrInit().Append(dcl)
                                        }
                                        tmp := o.newTemp(t, t.HasPointers())
                                        as := typecheck.Stmt(ir.NewAssignStmt(base.Pos, n, typecheck.Conv(tmp, n.Type())))
                                        ncas.PtrInit().Append(as)
                                        r.Lhs[i] = tmp
                                }
                                do(0, recv.X.Type().Elem())
                                do(1, types.Types[types.TBOOL])
                                if len(init) != 0 {
                                        ir.DumpList("ninit", r.Init())
                                        base.Fatalf("ninit on select recv")
                                }
                                orderBlock(ncas.PtrInit(), o.free)

                        case ir.OSEND:
                                r := r.(*ir.SendStmt)
                                if len(r.Init()) != 0 {
                                        ir.DumpList("ninit", r.Init())
                                        base.Fatalf("ninit on select send")
                                }

                                // case c <- x
                                // r->left is c, r->right is x, both are always evaluated.
                                r.Chan = o.expr(r.Chan, nil)

                                if !ir.IsAutoTmp(r.Chan) {
                                        r.Chan = o.copyExpr(r.Chan)
                                }
                                r.Value = o.expr(r.Value, nil)
                                if !ir.IsAutoTmp(r.Value) {
                                        r.Value = o.copyExpr(r.Value)
                                }
                        }
                }
                // Now that we have accumulated all the temporaries, clean them.
                // Also insert any ninit queued during the previous loop.
                // (The temporary cleaning must follow that ninit work.)
                for _, cas := range n.Cases {
                        orderBlock(&cas.Body, o.free)
                        cas.Body.Prepend(o.cleanTempNoPop(t)...)

                        // TODO(mdempsky): Is this actually necessary?
                        // walkSelect appears to walk Ninit.
                        cas.Body.Prepend(ir.TakeInit(cas)...)
                }

                o.out = append(o.out, n)
                o.popTemp(t)

        // Special: value being sent is passed as a pointer; make it addressable.
        case ir.OSEND:
                n := n.(*ir.SendStmt)
                t := o.markTemp()
                n.Chan = o.expr(n.Chan, nil)
                n.Value = o.expr(n.Value, nil)
                if base.Flag.Cfg.Instrumenting {
                        // Force copying to the stack so that (chan T)(nil) <- x
                        // is still instrumented as a read of x.
                        n.Value = o.copyExpr(n.Value)
                } else {
                        n.Value = o.addrTemp(n.Value)
                }
                o.out = append(o.out, n)
                o.cleanTemp(t)

        // TODO(rsc): Clean temporaries more aggressively.
        // Note that because walkSwitch will rewrite some of the
        // switch into a binary search, this is not as easy as it looks.
        // (If we ran that code here we could invoke order.stmt on
        // the if-else chain instead.)
        // For now just clean all the temporaries at the end.
        // In practice that's fine.
        case ir.OSWITCH:
                n := n.(*ir.SwitchStmt)
                if base.Debug.Libfuzzer != 0 && !hasDefaultCase(n) {
                        // Add empty "default:" case for instrumentation.
                        n.Cases = append(n.Cases, ir.NewCaseStmt(base.Pos, nil, nil))
                }

                t := o.markTemp()
                n.Tag = o.expr(n.Tag, nil)
                for _, ncas := range n.Cases {
                        o.exprListInPlace(ncas.List)
                        orderBlock(&ncas.Body, o.free)
                }

                o.out = append(o.out, n)
                o.cleanTemp(t)
        }

        base.Pos = lno
}

func hasDefaultCase(n *ir.SwitchStmt) bool {
        for _, ncas := range n.Cases {
                if len(ncas.List) == 0 {
                        return true
                }
        }
        return false
}

// exprList orders the expression list l into o.
func (o *orderState) exprList(l ir.Nodes) {
        s := l
        for i := range s {
                s[i] = o.expr(s[i], nil)
        }
}

// exprListInPlace orders the expression list l but saves
// the side effects on the individual expression ninit lists.
func (o *orderState) exprListInPlace(l ir.Nodes) {
        s := l
        for i := range s {
                s[i] = o.exprInPlace(s[i])
        }
}

func (o *orderState) exprNoLHS(n ir.Node) ir.Node {
        return o.expr(n, nil)
}

// expr orders a single expression, appending side
// effects to o.out as needed.
// If this is part of an assignment lhs = *np, lhs is given.
// Otherwise lhs == nil. (When lhs != nil it may be possible
// to avoid copying the result of the expression to a temporary.)
// The result of expr MUST be assigned back to n, e.g.
//      n.Left = o.expr(n.Left, lhs)
func (o *orderState) expr(n, lhs ir.Node) ir.Node {
        if n == nil {
                return n
        }
        lno := ir.SetPos(n)
        n = o.expr1(n, lhs)
        base.Pos = lno
        return n
}

func (o *orderState) expr1(n, lhs ir.Node) ir.Node {
        o.init(n)

        switch n.Op() {
        default:
                if o.edit == nil {
                        o.edit = o.exprNoLHS // create closure once
                }
                ir.EditChildren(n, o.edit)
                return n

        // Addition of strings turns into a function call.
        // Allocate a temporary to hold the strings.
        // Fewer than 5 strings use direct runtime helpers.
        case ir.OADDSTR:
                n := n.(*ir.AddStringExpr)
                o.exprList(n.List)

                if len(n.List) > 5 {
                        t := types.NewArray(types.Types[types.TSTRING], int64(len(n.List)))
                        n.Prealloc = o.newTemp(t, false)
                }

                // Mark string(byteSlice) arguments to reuse byteSlice backing
                // buffer during conversion. String concatenation does not
                // memorize the strings for later use, so it is safe.
                // However, we can do it only if there is at least one non-empty string literal.
                // Otherwise if all other arguments are empty strings,
                // concatstrings will return the reference to the temp string
                // to the caller.
                hasbyte := false

                haslit := false
                for _, n1 := range n.List {
                        hasbyte = hasbyte || n1.Op() == ir.OBYTES2STR
                        haslit = haslit || n1.Op() == ir.OLITERAL && len(ir.StringVal(n1)) != 0
                }

                if haslit && hasbyte {
                        for _, n2 := range n.List {
                                if n2.Op() == ir.OBYTES2STR {
                                        n2 := n2.(*ir.ConvExpr)
                                        n2.SetOp(ir.OBYTES2STRTMP)
                                }
                        }
                }
                return n

        case ir.OINDEXMAP:
                n := n.(*ir.IndexExpr)
                n.X = o.expr(n.X, nil)
                n.Index = o.expr(n.Index, nil)
                needCopy := false

                if !n.Assigned {
                        // Enforce that any []byte slices we are not copying
                        // can not be changed before the map index by forcing
                        // the map index to happen immediately following the
                        // conversions. See copyExpr a few lines below.
                        needCopy = mapKeyReplaceStrConv(n.Index)

                        if base.Flag.Cfg.Instrumenting {
                                // Race detector needs the copy.
                                needCopy = true
                        }
                }

                // key must be addressable
                n.Index = o.mapKeyTemp(n.X.Type(), n.Index)
                if needCopy {
                        return o.copyExpr(n)
                }
                return n

        // concrete type (not interface) argument might need an addressable
        // temporary to pass to the runtime conversion routine.
        case ir.OCONVIFACE:
                n := n.(*ir.ConvExpr)
                n.X = o.expr(n.X, nil)
                if n.X.Type().IsInterface() {
                        return n
                }
                if _, needsaddr := convFuncName(n.X.Type(), n.Type()); needsaddr || isStaticCompositeLiteral(n.X) {
                        // Need a temp if we need to pass the address to the conversion function.
                        // We also process static composite literal node here, making a named static global
                        // whose address we can put directly in an interface (see OCONVIFACE case in walk).
                        n.X = o.addrTemp(n.X)
                }
                return n

        case ir.OCONVNOP:
                n := n.(*ir.ConvExpr)
                if n.Type().IsKind(types.TUNSAFEPTR) && n.X.Type().IsKind(types.TUINTPTR) && (n.X.Op() == ir.OCALLFUNC || n.X.Op() == ir.OCALLINTER || n.X.Op() == ir.OCALLMETH) {
                        call := n.X.(*ir.CallExpr)
                        // When reordering unsafe.Pointer(f()) into a separate
                        // statement, the conversion and function call must stay
                        // together. See golang.org/issue/15329.
                        o.init(call)
                        o.call(call)
                        if lhs == nil || lhs.Op() != ir.ONAME || base.Flag.Cfg.Instrumenting {
                                return o.copyExpr(n)
                        }
                } else {
                        n.X = o.expr(n.X, nil)
                }
                return n

        case ir.OANDAND, ir.OOROR:
                // ... = LHS && RHS
                //
                // var r bool
                // r = LHS
                // if r {       // or !r, for OROR
                //     r = RHS
                // }
                // ... = r

                n := n.(*ir.LogicalExpr)
                r := o.newTemp(n.Type(), false)

                // Evaluate left-hand side.
                lhs := o.expr(n.X, nil)
                o.out = append(o.out, typecheck.Stmt(ir.NewAssignStmt(base.Pos, r, lhs)))

                // Evaluate right-hand side, save generated code.
                saveout := o.out
                o.out = nil
                t := o.markTemp()
                o.edge()
                rhs := o.expr(n.Y, nil)
                o.out = append(o.out, typecheck.Stmt(ir.NewAssignStmt(base.Pos, r, rhs)))
                o.cleanTemp(t)
                gen := o.out
                o.out = saveout

                // If left-hand side doesn't cause a short-circuit, issue right-hand side.
                nif := ir.NewIfStmt(base.Pos, r, nil, nil)
                if n.Op() == ir.OANDAND {
                        nif.Body = gen
                } else {
                        nif.Else = gen
                }
                o.out = append(o.out, nif)
                return r

        case ir.OCALLFUNC,
                ir.OCALLINTER,
                ir.OCALLMETH,
                ir.OCAP,
                ir.OCOMPLEX,
                ir.OCOPY,
                ir.OIMAG,
                ir.OLEN,
                ir.OMAKECHAN,
                ir.OMAKEMAP,
                ir.OMAKESLICE,
                ir.OMAKESLICECOPY,
                ir.ONEW,
                ir.OREAL,
                ir.ORECOVER,
                ir.OSTR2BYTES,
                ir.OSTR2BYTESTMP,
                ir.OSTR2RUNES:

                if isRuneCount(n) {
                        // len([]rune(s)) is rewritten to runtime.countrunes(s) later.
                        conv := n.(*ir.UnaryExpr).X.(*ir.ConvExpr)
                        conv.X = o.expr(conv.X, nil)
                } else {
                        o.call(n)
                }

                if lhs == nil || lhs.Op() != ir.ONAME || base.Flag.Cfg.Instrumenting {
                        return o.copyExpr(n)
                }
                return n

        case ir.OAPPEND:
                // Check for append(x, make([]T, y)...) .
                n := n.(*ir.CallExpr)
                if isAppendOfMake(n) {
                        n.Args[0] = o.expr(n.Args[0], nil) // order x
                        mk := n.Args[1].(*ir.MakeExpr)
                        mk.Len = o.expr(mk.Len, nil) // order y
                } else {
                        o.exprList(n.Args)
                }

                if lhs == nil || lhs.Op() != ir.ONAME && !ir.SameSafeExpr(lhs, n.Args[0]) {
                        return o.copyExpr(n)
                }
                return n

        case ir.OSLICE, ir.OSLICEARR, ir.OSLICESTR, ir.OSLICE3, ir.OSLICE3ARR:
                n := n.(*ir.SliceExpr)
                n.X = o.expr(n.X, nil)
                n.Low = o.cheapExpr(o.expr(n.Low, nil))
                n.High = o.cheapExpr(o.expr(n.High, nil))
                n.Max = o.cheapExpr(o.expr(n.Max, nil))
                if lhs == nil || lhs.Op() != ir.ONAME && !ir.SameSafeExpr(lhs, n.X) {
                        return o.copyExpr(n)
                }
                return n

        case ir.OCLOSURE:
                n := n.(*ir.ClosureExpr)
                if n.Transient() && len(n.Func.ClosureVars) > 0 {
                        n.Prealloc = o.newTemp(typecheck.ClosureType(n), false)
                }
                return n

        case ir.OCALLPART:
                n := n.(*ir.SelectorExpr)
                n.X = o.expr(n.X, nil)
                if n.Transient() {
                        t := typecheck.PartialCallType(n)
                        n.Prealloc = o.newTemp(t, false)
                }
                return n

        case ir.OSLICELIT:
                n := n.(*ir.CompLitExpr)
                o.exprList(n.List)
                if n.Transient() {
                        t := types.NewArray(n.Type().Elem(), n.Len)
                        n.Prealloc = o.newTemp(t, false)
                }
                return n

        case ir.ODOTTYPE, ir.ODOTTYPE2:
                n := n.(*ir.TypeAssertExpr)
                n.X = o.expr(n.X, nil)
                if !types.IsDirectIface(n.Type()) || base.Flag.Cfg.Instrumenting {
                        return o.copyExprClear(n)
                }
                return n

        case ir.ORECV:
                n := n.(*ir.UnaryExpr)
                n.X = o.expr(n.X, nil)
                return o.copyExprClear(n)

        case ir.OEQ, ir.ONE, ir.OLT, ir.OLE, ir.OGT, ir.OGE:
                n := n.(*ir.BinaryExpr)
                n.X = o.expr(n.X, nil)
                n.Y = o.expr(n.Y, nil)

                t := n.X.Type()
                switch {
                case t.IsString():
                        // Mark string(byteSlice) arguments to reuse byteSlice backing
                        // buffer during conversion. String comparison does not
                        // memorize the strings for later use, so it is safe.
                        if n.X.Op() == ir.OBYTES2STR {
                                n.X.(*ir.ConvExpr).SetOp(ir.OBYTES2STRTMP)
                        }
                        if n.Y.Op() == ir.OBYTES2STR {
                                n.Y.(*ir.ConvExpr).SetOp(ir.OBYTES2STRTMP)
                        }

                case t.IsStruct() || t.IsArray():
                        // for complex comparisons, we need both args to be
                        // addressable so we can pass them to the runtime.
                        n.X = o.addrTemp(n.X)
                        n.Y = o.addrTemp(n.Y)
                }
                return n

        case ir.OMAPLIT:
                // Order map by converting:
                //   map[int]int{
                //     a(): b(),
                //     c(): d(),
                //     e(): f(),
                //   }
                // to
                //   m := map[int]int{}
                //   m[a()] = b()
                //   m[c()] = d()
                //   m[e()] = f()
                // Then order the result.
                // Without this special case, order would otherwise compute all
                // the keys and values before storing any of them to the map.
                // See issue 26552.
                n := n.(*ir.CompLitExpr)
                entries := n.List
                statics := entries[:0]
                var dynamics []*ir.KeyExpr
                for _, r := range entries {
                        r := r.(*ir.KeyExpr)

                        if !isStaticCompositeLiteral(r.Key) || !isStaticCompositeLiteral(r.Value) {
                                dynamics = append(dynamics, r)
                                continue
                        }

                        // Recursively ordering some static entries can change them to dynamic;
                        // e.g., OCONVIFACE nodes. See #31777.
                        r = o.expr(r, nil).(*ir.KeyExpr)
                        if !isStaticCompositeLiteral(r.Key) || !isStaticCompositeLiteral(r.Value) {
                                dynamics = append(dynamics, r)
                                continue
                        }

                        statics = append(statics, r)
                }
                n.List = statics

                if len(dynamics) == 0 {
                        return n
                }

                // Emit the creation of the map (with all its static entries).
                m := o.newTemp(n.Type(), false)
                as := ir.NewAssignStmt(base.Pos, m, n)
                typecheck.Stmt(as)
                o.stmt(as)

                // Emit eval+insert of dynamic entries, one at a time.
                for _, r := range dynamics {
                        as := ir.NewAssignStmt(base.Pos, ir.NewIndexExpr(base.Pos, m, r.Key), r.Value)
                        typecheck.Stmt(as) // Note: this converts the OINDEX to an OINDEXMAP
                        o.stmt(as)
                }
                return m
        }

        // No return - type-assertions above. Each case must return for itself.
}

// as2func orders OAS2FUNC nodes. It creates temporaries to ensure left-to-right assignment.
// The caller should order the right-hand side of the assignment before calling order.as2func.
// It rewrites,
//      a, b, a = ...
// as
//      tmp1, tmp2, tmp3 = ...
//      a, b, a = tmp1, tmp2, tmp3
// This is necessary to ensure left to right assignment order.
func (o *orderState) as2func(n *ir.AssignListStmt) {
        results := n.Rhs[0].Type()
        as := ir.NewAssignListStmt(n.Pos(), ir.OAS2, nil, nil)
        for i, nl := range n.Lhs {
                if !ir.IsBlank(nl) {
                        typ := results.Field(i).Type
                        tmp := o.newTemp(typ, typ.HasPointers())
                        n.Lhs[i] = tmp
                        as.Lhs = append(as.Lhs, nl)
                        as.Rhs = append(as.Rhs, tmp)
                }
        }

        o.out = append(o.out, n)
        o.stmt(typecheck.Stmt(as))
}

// as2ok orders OAS2XXX with ok.
// Just like as2func, this also adds temporaries to ensure left-to-right assignment.
func (o *orderState) as2ok(n *ir.AssignListStmt) {
        as := ir.NewAssignListStmt(n.Pos(), ir.OAS2, nil, nil)

        do := func(i int, typ *types.Type) {
                if nl := n.Lhs[i]; !ir.IsBlank(nl) {
                        var tmp ir.Node = o.newTemp(typ, typ.HasPointers())
                        n.Lhs[i] = tmp
                        as.Lhs = append(as.Lhs, nl)
                        if i == 1 {
                                // The "ok" result is an untyped boolean according to the Go
                                // spec. We need to explicitly convert it to the LHS type in
                                // case the latter is a defined boolean type (#8475).
                                tmp = typecheck.Conv(tmp, nl.Type())
                        }
                        as.Rhs = append(as.Rhs, tmp)
                }
        }

        do(0, n.Rhs[0].Type())
        do(1, types.Types[types.TBOOL])

        o.out = append(o.out, n)
        o.stmt(typecheck.Stmt(as))
}

var wrapGoDefer_prgen int

// wrapGoDefer wraps the target of a "go" or "defer" statement with a
// new "function with no arguments" closure. Specifically, it converts
//
//   defer f(x, y)
//
// to
//
//   x1, y1 := x, y
//   defer func() { f(x1, y1) }()
//
// This is primarily to enable a quicker bringup of defers under the
// new register ABI; by doing this conversion, we can simplify the
// code in the runtime that invokes defers on the panic path.
func (o *orderState) wrapGoDefer(n *ir.GoDeferStmt) {
        call := n.Call

        var callX ir.Node        // thing being called
        var callArgs []ir.Node   // call arguments
        var keepAlive []*ir.Name // KeepAlive list from call, if present

        // A helper to recreate the call within the closure.
        var mkNewCall func(pos src.XPos, op ir.Op, fun ir.Node, args []ir.Node) ir.Node

        // Defer calls come in many shapes and sizes; not all of them
        // are ir.CallExpr's. Examine the type to see what we're dealing with.
        switch x := call.(type) {
        case *ir.CallExpr:
                callX = x.X
                callArgs = x.Args
                keepAlive = x.KeepAlive
                mkNewCall = func(pos src.XPos, op ir.Op, fun ir.Node, args []ir.Node) ir.Node {
                        newcall := ir.NewCallExpr(pos, op, fun, args)
                        newcall.IsDDD = x.IsDDD
                        return ir.Node(newcall)
                }
        case *ir.UnaryExpr: // ex: OCLOSE
                callArgs = []ir.Node{x.X}
                mkNewCall = func(pos src.XPos, op ir.Op, fun ir.Node, args []ir.Node) ir.Node {
                        if len(args) != 1 {
                                panic("internal error, expecting single arg to close")
                        }
                        return ir.Node(ir.NewUnaryExpr(pos, op, args[0]))
                }
        case *ir.BinaryExpr: // ex: OCOPY
                callArgs = []ir.Node{x.X, x.Y}
                mkNewCall = func(pos src.XPos, op ir.Op, fun ir.Node, args []ir.Node) ir.Node {
                        if len(args) != 2 {
                                panic("internal error, expecting two args")
                        }
                        return ir.Node(ir.NewBinaryExpr(pos, op, args[0], args[1]))
                }
        default:
                panic("unhandled op")
        }

        // No need to wrap if called func has no args. However in the case
        // of "defer func() { ... }()" we need to protect against the
        // possibility of directClosureCall rewriting things so that the
        // call does have arguments.
        if len(callArgs) == 0 {
                if c, ok := call.(*ir.CallExpr); ok && callX != nil && callX.Op() == ir.OCLOSURE {
                        cloFunc := callX.(*ir.ClosureExpr).Func
                        cloFunc.SetClosureCalled(false)
                        c.PreserveClosure = true
                }
                return
        }

        if c, ok := call.(*ir.CallExpr); ok {
                // To simplify things, turn f(a, b, []T{c, d, e}...) back
                // into f(a, b, c, d, e) -- when the final call is run through the
                // type checker below, it will rebuild the proper slice literal.
                undoVariadic(c)
                callX = c.X
                callArgs = c.Args
        }

        // This is set to true if the closure we're generating escapes
        // (needs heap allocation).
        cloEscapes := func() bool {
                if n.Op() == ir.OGO {
                        // For "go", assume that all closures escape (with an
                        // exception for the runtime, which doesn't permit
                        // heap-allocated closures).
                        return base.Ctxt.Pkgpath != "runtime"
                }
                // For defer, just use whatever result escape analysis
                // has determined for the defer.
                return n.Esc() != ir.EscNever
        }()

        // A helper for making a copy of an argument.
        mkArgCopy := func(arg ir.Node) *ir.Name {
                argCopy := o.copyExpr(arg)
                // The value of 128 below is meant to be consistent with code
                // in escape analysis that picks byval/byaddr based on size.
                argCopy.SetByval(argCopy.Type().Size() <= 128 || cloEscapes)
                return argCopy
        }

        // getUnsafeArg looks for an unsafe.Pointer arg that has been
        // previously captured into the call's keepalive list, returning
        // the name node for it if found.
        getUnsafeArg := func(arg ir.Node) *ir.Name {
                // Look for uintptr(unsafe.Pointer(name))
                if arg.Op() != ir.OCONVNOP {
                        return nil
                }
                if !arg.Type().IsUintptr() {
                        return nil
                }
                if !arg.(*ir.ConvExpr).X.Type().IsUnsafePtr() {
                        return nil
                }
                arg = arg.(*ir.ConvExpr).X
                argname, ok := arg.(*ir.Name)
                if !ok {
                        return nil
                }
                for i := range keepAlive {
                        if argname == keepAlive[i] {
                                return argname
                        }
                }
                return nil
        }

        // Copy the arguments to the function into temps.
        //
        // For calls with uintptr(unsafe.Pointer(...)) args that are being
        // kept alive (see code in (*orderState).call that does this), use
        // the existing arg copy instead of creating a new copy.
        unsafeArgs := make([]*ir.Name, len(callArgs))
        origArgs := callArgs
        var newNames []*ir.Name
        for i := range callArgs {
                arg := callArgs[i]
                var argname *ir.Name
                unsafeArgName := getUnsafeArg(arg)
                if unsafeArgName != nil {
                        // arg has been copied already, use keepalive copy
                        argname = unsafeArgName
                        unsafeArgs[i] = unsafeArgName
                } else {
                        argname = mkArgCopy(arg)
                }
                newNames = append(newNames, argname)
        }

        // Deal with cases where the function expression (what we're
        // calling) is not a simple function symbol.
        var fnExpr *ir.Name
        var methSelectorExpr *ir.SelectorExpr
        if callX != nil {
                switch {
                case callX.Op() == ir.ODOTMETH || callX.Op() == ir.ODOTINTER:
                        // Handle defer of a method call, e.g. "defer v.MyMethod(x, y)"
                        n := callX.(*ir.SelectorExpr)
                        n.X = mkArgCopy(n.X)
                        methSelectorExpr = n
                case !(callX.Op() == ir.ONAME && callX.(*ir.Name).Class == ir.PFUNC):
                        // Deal with "defer returnsafunc()(x, y)" (for
                        // example) by copying the callee expression.
                        fnExpr = mkArgCopy(callX)
                        if callX.Op() == ir.OCLOSURE {
                                // For "defer func(...)", in addition to copying the
                                // closure into a temp, mark it as no longer directly
                                // called.
                                callX.(*ir.ClosureExpr).Func.SetClosureCalled(false)
                        }
                }
        }

        // Create a new no-argument function that we'll hand off to defer.
        var noFuncArgs []*ir.Field
        noargst := ir.NewFuncType(base.Pos, nil, noFuncArgs, nil)
        wrapGoDefer_prgen++
        outerfn := ir.CurFunc
        wrapname := fmt.Sprintf("%v·dwrap·%d", outerfn, wrapGoDefer_prgen)
        sym := types.LocalPkg.Lookup(wrapname)
        fn := typecheck.DeclFunc(sym, noargst)
        fn.SetIsHiddenClosure(true)
        fn.SetWrapper(true)

        // helper for capturing reference to a var declared in an outer scope.
        capName := func(pos src.XPos, fn *ir.Func, n *ir.Name) *ir.Name {
                t := n.Type()
                cv := ir.CaptureName(pos, fn, n)
                cv.SetType(t)
                return typecheck.Expr(cv).(*ir.Name)
        }

        // Call args (x1, y1) need to be captured as part of the newly
        // created closure.
        newCallArgs := []ir.Node{}
        for i := range newNames {
                var arg ir.Node
                arg = capName(callArgs[i].Pos(), fn, newNames[i])
                if unsafeArgs[i] != nil {
                        arg = ir.NewConvExpr(arg.Pos(), origArgs[i].Op(), origArgs[i].Type(), arg)
                }
                newCallArgs = append(newCallArgs, arg)
        }
        // Also capture the function or method expression (if needed) into
        // the closure.
        if fnExpr != nil {
                callX = capName(callX.Pos(), fn, fnExpr)
        }
        if methSelectorExpr != nil {
                methSelectorExpr.X = capName(callX.Pos(), fn, methSelectorExpr.X.(*ir.Name))
        }
        ir.FinishCaptureNames(n.Pos(), outerfn, fn)

        // This flags a builtin as opposed to a regular call.
        irregular := (call.Op() != ir.OCALLFUNC &&
                call.Op() != ir.OCALLMETH &&
                call.Op() != ir.OCALLINTER)

        // Construct new function body:  f(x1, y1)
        op := ir.OCALL
        if irregular {
                op = call.Op()
        }
        newcall := mkNewCall(call.Pos(), op, callX, newCallArgs)

        // Type-check the result.
        if !irregular {
                typecheck.Call(newcall.(*ir.CallExpr))
        } else {
                typecheck.Stmt(newcall)
        }

        // Finalize body, register function on the main decls list.
        fn.Body = []ir.Node{newcall}
        typecheck.FinishFuncBody()
        typecheck.Func(fn)
        typecheck.Target.Decls = append(typecheck.Target.Decls, fn)

        // Create closure expr
        clo := ir.NewClosureExpr(n.Pos(), fn)
        fn.OClosure = clo
        clo.SetType(fn.Type())

        // Set escape properties for closure.
        if n.Op() == ir.OGO {
                // For "go", assume that the closure is going to escape
                // (with an exception for the runtime, which doesn't
                // permit heap-allocated closures).
                if base.Ctxt.Pkgpath != "runtime" {
                        clo.SetEsc(ir.EscHeap)
                }
        } else {
                // For defer, just use whatever result escape analysis
                // has determined for the defer.
                if n.Esc() == ir.EscNever {
                        clo.SetTransient(true)
                        clo.SetEsc(ir.EscNone)
                }
        }

        // Create new top level call to closure over argless function.
        topcall := ir.NewCallExpr(n.Pos(), ir.OCALL, clo, []ir.Node{})
        typecheck.Call(topcall)

        // Tag the call to insure that directClosureCall doesn't undo our work.
        topcall.PreserveClosure = true

        fn.SetClosureCalled(false)

        // Finally, point the defer statement at the newly generated call.
        n.Call = topcall
}