go/ssa/emit.go - tools - Git at Google

 // Copyright 2013 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 ssa

 // Helpers for emitting SSA instructions.

 import (
 	"fmt"
 	"go/ast"
 	"go/token"
 	"go/types"
 )

 // emitNew emits to f a new (heap Alloc) instruction allocating an
 // object of type typ.  pos is the optional source location.
 //
 func emitNew(f *Function, typ types.Type, pos token.Pos) *Alloc {
 	v := &Alloc{Heap: true}
 	v.setType(types.NewPointer(typ))
 	v.setPos(pos)
 	f.emit(v)
 	return v
 }

 // emitLoad emits to f an instruction to load the address addr into a
 // new temporary, and returns the value so defined.
 //
 func emitLoad(f *Function, addr Value) *UnOp {
 	v := &UnOp{Op: token.MUL, X: addr}
 	v.setType(deref(addr.Type()))
 	f.emit(v)
 	return v
 }

 // emitDebugRef emits to f a DebugRef pseudo-instruction associating
 // expression e with value v.
 //
 func emitDebugRef(f *Function, e ast.Expr, v Value, isAddr bool) {
 	if !f.debugInfo() {
 		return // debugging not enabled
 	}
 	if v == nil || e == nil {
 		panic("nil")
 	}
 	var obj types.Object
 	e = unparen(e)
 	if id, ok := e.(*ast.Ident); ok {
 		if isBlankIdent(id) {
 			return
 		}
 		obj = f.Pkg.objectOf(id)
 		switch obj.(type) {
 		case *types.Nil, *types.Const, *types.Builtin:
 			return
 		}
 	}
 	f.emit(&DebugRef{
 		X:      v,
 		Expr:   e,
 		IsAddr: isAddr,
 		object: obj,
 	})
 }

 // emitArith emits to f code to compute the binary operation op(x, y)
 // where op is an eager shift, logical or arithmetic operation.
 // (Use emitCompare() for comparisons and Builder.logicalBinop() for
 // non-eager operations.)
 //
 func emitArith(f *Function, op token.Token, x, y Value, t types.Type, pos token.Pos) Value {
 	switch op {
 	case token.SHL, token.SHR:
 		x = emitConv(f, x, t)
 		// y may be signed or an 'untyped' constant.
 		// TODO(adonovan): whence signed values?
 		if b, ok := y.Type().Underlying().(*types.Basic); ok && b.Info()&types.IsUnsigned == 0 {
 			y = emitConv(f, y, types.Typ[types.Uint64])
 		}

 	case token.ADD, token.SUB, token.MUL, token.QUO, token.REM, token.AND, token.OR, token.XOR, token.AND_NOT:
 		x = emitConv(f, x, t)
 		y = emitConv(f, y, t)

 	default:
 		panic("illegal op in emitArith: " + op.String())

 	}
 	v := &BinOp{
 		Op: op,
 		X:  x,
 		Y:  y,
 	}
 	v.setPos(pos)
 	v.setType(t)
 	return f.emit(v)
 }

 // emitCompare emits to f code compute the boolean result of
 // comparison comparison 'x op y'.
 //
 func emitCompare(f *Function, op token.Token, x, y Value, pos token.Pos) Value {
 	xt := x.Type().Underlying()
 	yt := y.Type().Underlying()

 	// Special case to optimise a tagless SwitchStmt so that
 	// these are equivalent
 	//   switch { case e: ...}
 	//   switch true { case e: ... }
 	//   if e==true { ... }
 	// even in the case when e's type is an interface.
 	// TODO(adonovan): opt: generalise to x==true, false!=y, etc.
 	if x == vTrue && op == token.EQL {
 		if yt, ok := yt.(*types.Basic); ok && yt.Info()&types.IsBoolean != 0 {
 			return y
 		}
 	}

 	if types.Identical(xt, yt) {
 		// no conversion necessary
 	} else if _, ok := xt.(*types.Interface); ok {
 		y = emitConv(f, y, x.Type())
 	} else if _, ok := yt.(*types.Interface); ok {
 		x = emitConv(f, x, y.Type())
 	} else if _, ok := x.(*Const); ok {
 		x = emitConv(f, x, y.Type())
 	} else if _, ok := y.(*Const); ok {
 		y = emitConv(f, y, x.Type())
 	} else {
 		// other cases, e.g. channels.  No-op.
 	}

 	v := &BinOp{
 		Op: op,
 		X:  x,
 		Y:  y,
 	}
 	v.setPos(pos)
 	v.setType(tBool)
 	return f.emit(v)
 }

 // isValuePreserving returns true if a conversion from ut_src to
 // ut_dst is value-preserving, i.e. just a change of type.
 // Precondition: neither argument is a named type.
 //
 func isValuePreserving(ut_src, ut_dst types.Type) bool {
 	// Identical underlying types?
 	if structTypesIdentical(ut_dst, ut_src) {
 		return true
 	}

 	switch ut_dst.(type) {
 	case *types.Chan:
 		// Conversion between channel types?
 		_, ok := ut_src.(*types.Chan)
 		return ok

 	case *types.Pointer:
 		// Conversion between pointers with identical base types?
 		_, ok := ut_src.(*types.Pointer)
 		return ok
 	}
 	return false
 }

 // emitConv emits to f code to convert Value val to exactly type typ,
 // and returns the converted value.  Implicit conversions are required
 // by language assignability rules in assignments, parameter passing,
 // etc.
 //
 func emitConv(f *Function, val Value, typ types.Type) Value {
 	t_src := val.Type()

 	// Identical types?  Conversion is a no-op.
 	if types.Identical(t_src, typ) {
 		return val
 	}

 	ut_dst := typ.Underlying()
 	ut_src := t_src.Underlying()

 	// Just a change of type, but not value or representation?
 	if isValuePreserving(ut_src, ut_dst) {
 		c := &ChangeType{X: val}
 		c.setType(typ)
 		return f.emit(c)
 	}

 	// Conversion to, or construction of a value of, an interface type?
 	if _, ok := ut_dst.(*types.Interface); ok {
 		// Assignment from one interface type to another?
 		if _, ok := ut_src.(*types.Interface); ok {
 			c := &ChangeInterface{X: val}
 			c.setType(typ)
 			return f.emit(c)
 		}

 		// Untyped nil constant?  Return interface-typed nil constant.
 		if ut_src == tUntypedNil {
 			return nilConst(typ)
 		}

 		// Convert (non-nil) "untyped" literals to their default type.
 		if t, ok := ut_src.(*types.Basic); ok && t.Info()&types.IsUntyped != 0 {
 			val = emitConv(f, val, types.Default(ut_src))
 		}

 		f.Pkg.Prog.needMethodsOf(val.Type())
 		mi := &MakeInterface{X: val}
 		mi.setType(typ)
 		return f.emit(mi)
 	}

 	// Conversion of a compile-time constant value?
 	if c, ok := val.(*Const); ok {
 		if _, ok := ut_dst.(*types.Basic); ok || c.IsNil() {
 			// Conversion of a compile-time constant to
 			// another constant type results in a new
 			// constant of the destination type and
 			// (initially) the same abstract value.
 			// We don't truncate the value yet.
 			return NewConst(c.Value, typ)
 		}

 		// We're converting from constant to non-constant type,
 		// e.g. string -> []byte/[]rune.
 	}

 	// Conversion from slice to array pointer?
 	if slice, ok := ut_src.(*types.Slice); ok {
 		if ptr, ok := ut_dst.(*types.Pointer); ok {
 			if arr, ok := ptr.Elem().Underlying().(*types.Array); ok && types.Identical(slice.Elem(), arr.Elem()) {
 				c := &SliceToArrayPointer{X: val}
 				c.setType(ut_dst)
 				return f.emit(c)
 			}
 		}
 	}
 	// A representation-changing conversion?
 	// At least one of {ut_src,ut_dst} must be *Basic.
 	// (The other may be []byte or []rune.)
 	_, ok1 := ut_src.(*types.Basic)
 	_, ok2 := ut_dst.(*types.Basic)
 	if ok1 || ok2 {
 		c := &Convert{X: val}
 		c.setType(typ)
 		return f.emit(c)
 	}

 	panic(fmt.Sprintf("in %s: cannot convert %s (%s) to %s", f, val, val.Type(), typ))
 }

 // emitStore emits to f an instruction to store value val at location
 // addr, applying implicit conversions as required by assignability rules.
 //
 func emitStore(f *Function, addr, val Value, pos token.Pos) *Store {
 	s := &Store{
 		Addr: addr,
 		Val:  emitConv(f, val, deref(addr.Type())),
 		pos:  pos,
 	}
 	f.emit(s)
 	return s
 }

 // emitJump emits to f a jump to target, and updates the control-flow graph.
 // Postcondition: f.currentBlock is nil.
 //
 func emitJump(f *Function, target *BasicBlock) {
 	b := f.currentBlock
 	b.emit(new(Jump))
 	addEdge(b, target)
 	f.currentBlock = nil
 }

 // emitIf emits to f a conditional jump to tblock or fblock based on
 // cond, and updates the control-flow graph.
 // Postcondition: f.currentBlock is nil.
 //
 func emitIf(f *Function, cond Value, tblock, fblock *BasicBlock) {
 	b := f.currentBlock
 	b.emit(&If{Cond: cond})
 	addEdge(b, tblock)
 	addEdge(b, fblock)
 	f.currentBlock = nil
 }

 // emitExtract emits to f an instruction to extract the index'th
 // component of tuple.  It returns the extracted value.
 //
 func emitExtract(f *Function, tuple Value, index int) Value {
 	e := &Extract{Tuple: tuple, Index: index}
 	e.setType(tuple.Type().(*types.Tuple).At(index).Type())
 	return f.emit(e)
 }

 // emitTypeAssert emits to f a type assertion value := x.(t) and
 // returns the value.  x.Type() must be an interface.
 //
 func emitTypeAssert(f *Function, x Value, t types.Type, pos token.Pos) Value {
 	a := &TypeAssert{X: x, AssertedType: t}
 	a.setPos(pos)
 	a.setType(t)
 	return f.emit(a)
 }

 // emitTypeTest emits to f a type test value,ok := x.(t) and returns
 // a (value, ok) tuple.  x.Type() must be an interface.
 //
 func emitTypeTest(f *Function, x Value, t types.Type, pos token.Pos) Value {
 	a := &TypeAssert{
 		X:            x,
 		AssertedType: t,
 		CommaOk:      true,
 	}
 	a.setPos(pos)
 	a.setType(types.NewTuple(
 		newVar("value", t),
 		varOk,
 	))
 	return f.emit(a)
 }

 // emitTailCall emits to f a function call in tail position.  The
 // caller is responsible for all fields of 'call' except its type.
 // Intended for wrapper methods.
 // Precondition: f does/will not use deferred procedure calls.
 // Postcondition: f.currentBlock is nil.
 //
 func emitTailCall(f *Function, call *Call) {
 	tresults := f.Signature.Results()
 	nr := tresults.Len()
 	if nr == 1 {
 		call.typ = tresults.At(0).Type()
 	} else {
 		call.typ = tresults
 	}
 	tuple := f.emit(call)
 	var ret Return
 	switch nr {
 	case 0:
 		// no-op
 	case 1:
 		ret.Results = []Value{tuple}
 	default:
 		for i := 0; i < nr; i++ {
 			v := emitExtract(f, tuple, i)
 			// TODO(adonovan): in principle, this is required:
 			//   v = emitConv(f, o.Type, f.Signature.Results[i].Type)
 			// but in practice emitTailCall is only used when
 			// the types exactly match.
 			ret.Results = append(ret.Results, v)
 		}
 	}
 	f.emit(&ret)
 	f.currentBlock = nil
 }

 // emitImplicitSelections emits to f code to apply the sequence of
 // implicit field selections specified by indices to base value v, and
 // returns the selected value.
 //
 // If v is the address of a struct, the result will be the address of
 // a field; if it is the value of a struct, the result will be the
 // value of a field.
 //
 func emitImplicitSelections(f *Function, v Value, indices []int) Value {
 	for _, index := range indices {
 		fld := deref(v.Type()).Underlying().(*types.Struct).Field(index)

 		if isPointer(v.Type()) {
 			instr := &FieldAddr{
 				X:     v,
 				Field: index,
 			}
 			instr.setType(types.NewPointer(fld.Type()))
 			v = f.emit(instr)
 			// Load the field's value iff indirectly embedded.
 			if isPointer(fld.Type()) {
 				v = emitLoad(f, v)
 			}
 		} else {
 			instr := &Field{
 				X:     v,
 				Field: index,
 			}
 			instr.setType(fld.Type())
 			v = f.emit(instr)
 		}
 	}
 	return v
 }

 // emitFieldSelection emits to f code to select the index'th field of v.
 //
 // If wantAddr, the input must be a pointer-to-struct and the result
 // will be the field's address; otherwise the result will be the
 // field's value.
 // Ident id is used for position and debug info.
 //
 func emitFieldSelection(f *Function, v Value, index int, wantAddr bool, id *ast.Ident) Value {
 	fld := deref(v.Type()).Underlying().(*types.Struct).Field(index)
 	if isPointer(v.Type()) {
 		instr := &FieldAddr{
 			X:     v,
 			Field: index,
 		}
 		instr.setPos(id.Pos())
 		instr.setType(types.NewPointer(fld.Type()))
 		v = f.emit(instr)
 		// Load the field's value iff we don't want its address.
 		if !wantAddr {
 			v = emitLoad(f, v)
 		}
 	} else {
 		instr := &Field{
 			X:     v,
 			Field: index,
 		}
 		instr.setPos(id.Pos())
 		instr.setType(fld.Type())
 		v = f.emit(instr)
 	}
 	emitDebugRef(f, id, v, wantAddr)
 	return v
 }

 // zeroValue emits to f code to produce a zero value of type t,
 // and returns it.
 //
 func zeroValue(f *Function, t types.Type) Value {
 	switch t.Underlying().(type) {
 	case *types.Struct, *types.Array:
 		return emitLoad(f, f.addLocal(t, token.NoPos))
 	default:
 		return zeroConst(t)
 	}
 }

 // createRecoverBlock emits to f a block of code to return after a
 // recovered panic, and sets f.Recover to it.
 //
 // If f's result parameters are named, the code loads and returns
 // their current values, otherwise it returns the zero values of their
 // type.
 //
 // Idempotent.
 //
 func createRecoverBlock(f *Function) {
 	if f.Recover != nil {
 		return // already created
 	}
 	saved := f.currentBlock

 	f.Recover = f.newBasicBlock("recover")
 	f.currentBlock = f.Recover

 	var results []Value
 	if f.namedResults != nil {
 		// Reload NRPs to form value tuple.
 		for _, r := range f.namedResults {
 			results = append(results, emitLoad(f, r))
 		}
 	} else {
 		R := f.Signature.Results()
 		for i, n := 0, R.Len(); i < n; i++ {
 			T := R.At(i).Type()

 			// Return zero value of each result type.
 			results = append(results, zeroValue(f, T))
 		}
 	}
 	f.emit(&Return{Results: results})

 	f.currentBlock = saved
 }
	// Copyright 2013 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 ssa

	// Helpers for emitting SSA instructions.

	import (
	"fmt"
	"go/ast"
	"go/token"
	"go/types"
	)

	// emitNew emits to f a new (heap Alloc) instruction allocating an
	// object of type typ. pos is the optional source location.
	//
	func emitNew(f Function, typ types.Type, pos token.Pos) Alloc {
	v := &Alloc{Heap: true}
	v.setType(types.NewPointer(typ))
	v.setPos(pos)
	f.emit(v)
	return v
	}

	// emitLoad emits to f an instruction to load the address addr into a
	// new temporary, and returns the value so defined.
	//
	func emitLoad(f Function, addr Value) UnOp {
	v := &UnOp{Op: token.MUL, X: addr}
	v.setType(deref(addr.Type()))
	f.emit(v)
	return v
	}

	// emitDebugRef emits to f a DebugRef pseudo-instruction associating
	// expression e with value v.
	//
	func emitDebugRef(f *Function, e ast.Expr, v Value, isAddr bool) {
	if !f.debugInfo() {
	return // debugging not enabled
	}
	if v == nil \|\| e == nil {
	panic("nil")
	}
	var obj types.Object
	e = unparen(e)
	if id, ok := e.(*ast.Ident); ok {
	if isBlankIdent(id) {
	return
	}
	obj = f.Pkg.objectOf(id)
	switch obj.(type) {
	case types.Nil, types.Const, *types.Builtin:
	return
	}
	}
	f.emit(&DebugRef{
	X: v,
	Expr: e,
	IsAddr: isAddr,
	object: obj,
	})
	}

	// emitArith emits to f code to compute the binary operation op(x, y)
	// where op is an eager shift, logical or arithmetic operation.
	// (Use emitCompare() for comparisons and Builder.logicalBinop() for
	// non-eager operations.)
	//
	func emitArith(f *Function, op token.Token, x, y Value, t types.Type, pos token.Pos) Value {
	switch op {
	case token.SHL, token.SHR:
	x = emitConv(f, x, t)
	// y may be signed or an 'untyped' constant.
	// TODO(adonovan): whence signed values?
	if b, ok := y.Type().Underlying().(*types.Basic); ok && b.Info()&types.IsUnsigned == 0 {
	y = emitConv(f, y, types.Typ[types.Uint64])
	}

	case token.ADD, token.SUB, token.MUL, token.QUO, token.REM, token.AND, token.OR, token.XOR, token.AND_NOT:
	x = emitConv(f, x, t)
	y = emitConv(f, y, t)

	default:
	panic("illegal op in emitArith: " + op.String())

	}
	v := &BinOp{
	Op: op,
	X: x,
	Y: y,
	}
	v.setPos(pos)
	v.setType(t)
	return f.emit(v)
	}

	// emitCompare emits to f code compute the boolean result of
	// comparison comparison 'x op y'.
	//
	func emitCompare(f *Function, op token.Token, x, y Value, pos token.Pos) Value {
	xt := x.Type().Underlying()
	yt := y.Type().Underlying()

	// Special case to optimise a tagless SwitchStmt so that
	// these are equivalent
	// switch { case e: ...}
	// switch true { case e: ... }
	// if e==true { ... }
	// even in the case when e's type is an interface.
	// TODO(adonovan): opt: generalise to x==true, false!=y, etc.
	if x == vTrue && op == token.EQL {
	if yt, ok := yt.(*types.Basic); ok && yt.Info()&types.IsBoolean != 0 {
	return y
	}
	}

	if types.Identical(xt, yt) {
	// no conversion necessary
	} else if _, ok := xt.(*types.Interface); ok {
	y = emitConv(f, y, x.Type())
	} else if _, ok := yt.(*types.Interface); ok {
	x = emitConv(f, x, y.Type())
	} else if _, ok := x.(*Const); ok {
	x = emitConv(f, x, y.Type())
	} else if _, ok := y.(*Const); ok {
	y = emitConv(f, y, x.Type())
	} else {
	// other cases, e.g. channels. No-op.
	}

	v := &BinOp{
	Op: op,
	X: x,
	Y: y,
	}
	v.setPos(pos)
	v.setType(tBool)
	return f.emit(v)
	}

	// isValuePreserving returns true if a conversion from ut_src to
	// ut_dst is value-preserving, i.e. just a change of type.
	// Precondition: neither argument is a named type.
	//
	func isValuePreserving(ut_src, ut_dst types.Type) bool {
	// Identical underlying types?
	if structTypesIdentical(ut_dst, ut_src) {
	return true
	}

	switch ut_dst.(type) {
	case *types.Chan:
	// Conversion between channel types?
	_, ok := ut_src.(*types.Chan)
	return ok

	case *types.Pointer:
	// Conversion between pointers with identical base types?
	_, ok := ut_src.(*types.Pointer)
	return ok
	}
	return false
	}

	// emitConv emits to f code to convert Value val to exactly type typ,
	// and returns the converted value. Implicit conversions are required
	// by language assignability rules in assignments, parameter passing,
	// etc.
	//
	func emitConv(f *Function, val Value, typ types.Type) Value {
	t_src := val.Type()

	// Identical types? Conversion is a no-op.
	if types.Identical(t_src, typ) {
	return val
	}

	ut_dst := typ.Underlying()
	ut_src := t_src.Underlying()

	// Just a change of type, but not value or representation?
	if isValuePreserving(ut_src, ut_dst) {
	c := &ChangeType{X: val}
	c.setType(typ)
	return f.emit(c)
	}

	// Conversion to, or construction of a value of, an interface type?
	if _, ok := ut_dst.(*types.Interface); ok {
	// Assignment from one interface type to another?
	if _, ok := ut_src.(*types.Interface); ok {
	c := &ChangeInterface{X: val}
	c.setType(typ)
	return f.emit(c)
	}

	// Untyped nil constant? Return interface-typed nil constant.
	if ut_src == tUntypedNil {
	return nilConst(typ)
	}

	// Convert (non-nil) "untyped" literals to their default type.
	if t, ok := ut_src.(*types.Basic); ok && t.Info()&types.IsUntyped != 0 {
	val = emitConv(f, val, types.Default(ut_src))
	}

	f.Pkg.Prog.needMethodsOf(val.Type())
	mi := &MakeInterface{X: val}
	mi.setType(typ)
	return f.emit(mi)
	}

	// Conversion of a compile-time constant value?
	if c, ok := val.(*Const); ok {
	if _, ok := ut_dst.(*types.Basic); ok \|\| c.IsNil() {
	// Conversion of a compile-time constant to
	// another constant type results in a new
	// constant of the destination type and
	// (initially) the same abstract value.
	// We don't truncate the value yet.
	return NewConst(c.Value, typ)
	}

	// We're converting from constant to non-constant type,
	// e.g. string -> []byte/[]rune.
	}

	// Conversion from slice to array pointer?
	if slice, ok := ut_src.(*types.Slice); ok {
	if ptr, ok := ut_dst.(*types.Pointer); ok {
	if arr, ok := ptr.Elem().Underlying().(*types.Array); ok && types.Identical(slice.Elem(), arr.Elem()) {
	c := &SliceToArrayPointer{X: val}
	c.setType(ut_dst)
	return f.emit(c)
	}
	}
	}
	// A representation-changing conversion?
	// At least one of {ut_src,ut_dst} must be *Basic.
	// (The other may be []byte or []rune.)
	_, ok1 := ut_src.(*types.Basic)
	_, ok2 := ut_dst.(*types.Basic)
	if ok1 \|\| ok2 {
	c := &Convert{X: val}
	c.setType(typ)
	return f.emit(c)
	}

	panic(fmt.Sprintf("in %s: cannot convert %s (%s) to %s", f, val, val.Type(), typ))
	}

	// emitStore emits to f an instruction to store value val at location
	// addr, applying implicit conversions as required by assignability rules.
	//
	func emitStore(f Function, addr, val Value, pos token.Pos) Store {
	s := &Store{
	Addr: addr,
	Val: emitConv(f, val, deref(addr.Type())),
	pos: pos,
	}
	f.emit(s)
	return s
	}

	// emitJump emits to f a jump to target, and updates the control-flow graph.
	// Postcondition: f.currentBlock is nil.
	//
	func emitJump(f Function, target BasicBlock) {
	b := f.currentBlock
	b.emit(new(Jump))
	addEdge(b, target)
	f.currentBlock = nil
	}

	// emitIf emits to f a conditional jump to tblock or fblock based on
	// cond, and updates the control-flow graph.
	// Postcondition: f.currentBlock is nil.
	//
	func emitIf(f Function, cond Value, tblock, fblock BasicBlock) {
	b := f.currentBlock
	b.emit(&If{Cond: cond})
	addEdge(b, tblock)
	addEdge(b, fblock)
	f.currentBlock = nil
	}

	// emitExtract emits to f an instruction to extract the index'th
	// component of tuple. It returns the extracted value.
	//
	func emitExtract(f *Function, tuple Value, index int) Value {
	e := &Extract{Tuple: tuple, Index: index}
	e.setType(tuple.Type().(*types.Tuple).At(index).Type())
	return f.emit(e)
	}

	// emitTypeAssert emits to f a type assertion value := x.(t) and
	// returns the value. x.Type() must be an interface.
	//
	func emitTypeAssert(f *Function, x Value, t types.Type, pos token.Pos) Value {
	a := &TypeAssert{X: x, AssertedType: t}
	a.setPos(pos)
	a.setType(t)
	return f.emit(a)
	}

	// emitTypeTest emits to f a type test value,ok := x.(t) and returns
	// a (value, ok) tuple. x.Type() must be an interface.
	//
	func emitTypeTest(f *Function, x Value, t types.Type, pos token.Pos) Value {
	a := &TypeAssert{
	X: x,
	AssertedType: t,
	CommaOk: true,
	}
	a.setPos(pos)
	a.setType(types.NewTuple(
	newVar("value", t),
	varOk,
	))
	return f.emit(a)
	}

	// emitTailCall emits to f a function call in tail position. The
	// caller is responsible for all fields of 'call' except its type.
	// Intended for wrapper methods.
	// Precondition: f does/will not use deferred procedure calls.
	// Postcondition: f.currentBlock is nil.
	//
	func emitTailCall(f Function, call Call) {
	tresults := f.Signature.Results()
	nr := tresults.Len()
	if nr == 1 {
	call.typ = tresults.At(0).Type()
	} else {
	call.typ = tresults
	}
	tuple := f.emit(call)
	var ret Return
	switch nr {
	case 0:
	// no-op
	case 1:
	ret.Results = []Value{tuple}
	default:
	for i := 0; i < nr; i++ {
	v := emitExtract(f, tuple, i)
	// TODO(adonovan): in principle, this is required:
	// v = emitConv(f, o.Type, f.Signature.Results[i].Type)
	// but in practice emitTailCall is only used when
	// the types exactly match.
	ret.Results = append(ret.Results, v)
	}
	}
	f.emit(&ret)
	f.currentBlock = nil
	}

	// emitImplicitSelections emits to f code to apply the sequence of
	// implicit field selections specified by indices to base value v, and
	// returns the selected value.
	//
	// If v is the address of a struct, the result will be the address of
	// a field; if it is the value of a struct, the result will be the
	// value of a field.
	//
	func emitImplicitSelections(f *Function, v Value, indices []int) Value {
	for _, index := range indices {
	fld := deref(v.Type()).Underlying().(*types.Struct).Field(index)

	if isPointer(v.Type()) {
	instr := &FieldAddr{
	X: v,
	Field: index,
	}
	instr.setType(types.NewPointer(fld.Type()))
	v = f.emit(instr)
	// Load the field's value iff indirectly embedded.
	if isPointer(fld.Type()) {
	v = emitLoad(f, v)
	}
	} else {
	instr := &Field{
	X: v,
	Field: index,
	}
	instr.setType(fld.Type())
	v = f.emit(instr)
	}
	}
	return v
	}

	// emitFieldSelection emits to f code to select the index'th field of v.
	//
	// If wantAddr, the input must be a pointer-to-struct and the result
	// will be the field's address; otherwise the result will be the
	// field's value.
	// Ident id is used for position and debug info.
	//
	func emitFieldSelection(f Function, v Value, index int, wantAddr bool, id ast.Ident) Value {
	fld := deref(v.Type()).Underlying().(*types.Struct).Field(index)
	if isPointer(v.Type()) {
	instr := &FieldAddr{
	X: v,
	Field: index,
	}
	instr.setPos(id.Pos())
	instr.setType(types.NewPointer(fld.Type()))
	v = f.emit(instr)
	// Load the field's value iff we don't want its address.
	if !wantAddr {
	v = emitLoad(f, v)
	}
	} else {
	instr := &Field{
	X: v,
	Field: index,
	}
	instr.setPos(id.Pos())
	instr.setType(fld.Type())
	v = f.emit(instr)
	}
	emitDebugRef(f, id, v, wantAddr)
	return v
	}

	// zeroValue emits to f code to produce a zero value of type t,
	// and returns it.
	//
	func zeroValue(f *Function, t types.Type) Value {
	switch t.Underlying().(type) {
	case types.Struct, types.Array:
	return emitLoad(f, f.addLocal(t, token.NoPos))
	default:
	return zeroConst(t)
	}
	}

	// createRecoverBlock emits to f a block of code to return after a
	// recovered panic, and sets f.Recover to it.
	//
	// If f's result parameters are named, the code loads and returns
	// their current values, otherwise it returns the zero values of their
	// type.
	//
	// Idempotent.
	//
	func createRecoverBlock(f *Function) {
	if f.Recover != nil {
	return // already created
	}
	saved := f.currentBlock

	f.Recover = f.newBasicBlock("recover")
	f.currentBlock = f.Recover

	var results []Value
	if f.namedResults != nil {
	// Reload NRPs to form value tuple.
	for _, r := range f.namedResults {
	results = append(results, emitLoad(f, r))
	}
	} else {
	R := f.Signature.Results()
	for i, n := 0, R.Len(); i < n; i++ {
	T := R.At(i).Type()

	// Return zero value of each result type.
	results = append(results, zeroValue(f, T))
	}
	}
	f.emit(&Return{Results: results})

	f.currentBlock = saved
	}