src/cmd/compile/internal/ssa/decompose.go - go - Git at Google

 // Copyright 2015 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

 import (
 	"cmd/compile/internal/types"
 )

 // decompose converts phi ops on compound builtin types into phi
 // ops on simple types, then invokes rewrite rules to decompose
 // other ops on those types.
 func decomposeBuiltIn(f *Func) {
 	// Decompose phis
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if v.Op != OpPhi {
 				continue
 			}
 			decomposeBuiltInPhi(v)
 		}
 	}

 	// Decompose other values
 	applyRewrite(f, rewriteBlockdec, rewriteValuedec)
 	if f.Config.RegSize == 4 {
 		applyRewrite(f, rewriteBlockdec64, rewriteValuedec64)
 	}

 	// Split up named values into their components.
 	var newNames []LocalSlot
 	for _, name := range f.Names {
 		t := name.Type
 		switch {
 		case t.IsInteger() && t.Size() > f.Config.RegSize:
 			hiName, loName := f.fe.SplitInt64(name)
 			newNames = append(newNames, hiName, loName)
 			for _, v := range f.NamedValues[name] {
 				if v.Op != OpInt64Make {
 					continue
 				}
 				f.NamedValues[hiName] = append(f.NamedValues[hiName], v.Args[0])
 				f.NamedValues[loName] = append(f.NamedValues[loName], v.Args[1])
 			}
 			delete(f.NamedValues, name)
 		case t.IsComplex():
 			rName, iName := f.fe.SplitComplex(name)
 			newNames = append(newNames, rName, iName)
 			for _, v := range f.NamedValues[name] {
 				if v.Op != OpComplexMake {
 					continue
 				}
 				f.NamedValues[rName] = append(f.NamedValues[rName], v.Args[0])
 				f.NamedValues[iName] = append(f.NamedValues[iName], v.Args[1])

 			}
 			delete(f.NamedValues, name)
 		case t.IsString():
 			ptrName, lenName := f.fe.SplitString(name)
 			newNames = append(newNames, ptrName, lenName)
 			for _, v := range f.NamedValues[name] {
 				if v.Op != OpStringMake {
 					continue
 				}
 				f.NamedValues[ptrName] = append(f.NamedValues[ptrName], v.Args[0])
 				f.NamedValues[lenName] = append(f.NamedValues[lenName], v.Args[1])
 			}
 			delete(f.NamedValues, name)
 		case t.IsSlice():
 			ptrName, lenName, capName := f.fe.SplitSlice(name)
 			newNames = append(newNames, ptrName, lenName, capName)
 			for _, v := range f.NamedValues[name] {
 				if v.Op != OpSliceMake {
 					continue
 				}
 				f.NamedValues[ptrName] = append(f.NamedValues[ptrName], v.Args[0])
 				f.NamedValues[lenName] = append(f.NamedValues[lenName], v.Args[1])
 				f.NamedValues[capName] = append(f.NamedValues[capName], v.Args[2])
 			}
 			delete(f.NamedValues, name)
 		case t.IsInterface():
 			typeName, dataName := f.fe.SplitInterface(name)
 			newNames = append(newNames, typeName, dataName)
 			for _, v := range f.NamedValues[name] {
 				if v.Op != OpIMake {
 					continue
 				}
 				f.NamedValues[typeName] = append(f.NamedValues[typeName], v.Args[0])
 				f.NamedValues[dataName] = append(f.NamedValues[dataName], v.Args[1])
 			}
 			delete(f.NamedValues, name)
 		case t.IsFloat():
 			// floats are never decomposed, even ones bigger than RegSize
 			newNames = append(newNames, name)
 		case t.Size() > f.Config.RegSize:
 			f.Fatalf("undecomposed named type %s %v", name, t)
 		default:
 			newNames = append(newNames, name)
 		}
 	}
 	f.Names = newNames
 }

 func decomposeBuiltInPhi(v *Value) {
 	switch {
 	case v.Type.IsInteger() && v.Type.Size() > v.Block.Func.Config.RegSize:
 		decomposeInt64Phi(v)
 	case v.Type.IsComplex():
 		decomposeComplexPhi(v)
 	case v.Type.IsString():
 		decomposeStringPhi(v)
 	case v.Type.IsSlice():
 		decomposeSlicePhi(v)
 	case v.Type.IsInterface():
 		decomposeInterfacePhi(v)
 	case v.Type.IsFloat():
 		// floats are never decomposed, even ones bigger than RegSize
 	case v.Type.Size() > v.Block.Func.Config.RegSize:
 		v.Fatalf("undecomposed type %s", v.Type)
 	}
 }

 func decomposeStringPhi(v *Value) {
 	types := &v.Block.Func.Config.Types
 	ptrType := types.BytePtr
 	lenType := types.Int

 	ptr := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
 	len := v.Block.NewValue0(v.Pos, OpPhi, lenType)
 	for _, a := range v.Args {
 		ptr.AddArg(a.Block.NewValue1(v.Pos, OpStringPtr, ptrType, a))
 		len.AddArg(a.Block.NewValue1(v.Pos, OpStringLen, lenType, a))
 	}
 	v.reset(OpStringMake)
 	v.AddArg(ptr)
 	v.AddArg(len)
 }

 func decomposeSlicePhi(v *Value) {
 	types := &v.Block.Func.Config.Types
 	ptrType := types.BytePtr
 	lenType := types.Int

 	ptr := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
 	len := v.Block.NewValue0(v.Pos, OpPhi, lenType)
 	cap := v.Block.NewValue0(v.Pos, OpPhi, lenType)
 	for _, a := range v.Args {
 		ptr.AddArg(a.Block.NewValue1(v.Pos, OpSlicePtr, ptrType, a))
 		len.AddArg(a.Block.NewValue1(v.Pos, OpSliceLen, lenType, a))
 		cap.AddArg(a.Block.NewValue1(v.Pos, OpSliceCap, lenType, a))
 	}
 	v.reset(OpSliceMake)
 	v.AddArg(ptr)
 	v.AddArg(len)
 	v.AddArg(cap)
 }

 func decomposeInt64Phi(v *Value) {
 	cfgtypes := &v.Block.Func.Config.Types
 	var partType *types.Type
 	if v.Type.IsSigned() {
 		partType = cfgtypes.Int32
 	} else {
 		partType = cfgtypes.UInt32
 	}

 	hi := v.Block.NewValue0(v.Pos, OpPhi, partType)
 	lo := v.Block.NewValue0(v.Pos, OpPhi, cfgtypes.UInt32)
 	for _, a := range v.Args {
 		hi.AddArg(a.Block.NewValue1(v.Pos, OpInt64Hi, partType, a))
 		lo.AddArg(a.Block.NewValue1(v.Pos, OpInt64Lo, cfgtypes.UInt32, a))
 	}
 	v.reset(OpInt64Make)
 	v.AddArg(hi)
 	v.AddArg(lo)
 }

 func decomposeComplexPhi(v *Value) {
 	cfgtypes := &v.Block.Func.Config.Types
 	var partType *types.Type
 	switch z := v.Type.Size(); z {
 	case 8:
 		partType = cfgtypes.Float32
 	case 16:
 		partType = cfgtypes.Float64
 	default:
 		v.Fatalf("decomposeComplexPhi: bad complex size %d", z)
 	}

 	real := v.Block.NewValue0(v.Pos, OpPhi, partType)
 	imag := v.Block.NewValue0(v.Pos, OpPhi, partType)
 	for _, a := range v.Args {
 		real.AddArg(a.Block.NewValue1(v.Pos, OpComplexReal, partType, a))
 		imag.AddArg(a.Block.NewValue1(v.Pos, OpComplexImag, partType, a))
 	}
 	v.reset(OpComplexMake)
 	v.AddArg(real)
 	v.AddArg(imag)
 }

 func decomposeInterfacePhi(v *Value) {
 	uintptrType := v.Block.Func.Config.Types.Uintptr
 	ptrType := v.Block.Func.Config.Types.BytePtr

 	itab := v.Block.NewValue0(v.Pos, OpPhi, uintptrType)
 	data := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
 	for _, a := range v.Args {
 		itab.AddArg(a.Block.NewValue1(v.Pos, OpITab, uintptrType, a))
 		data.AddArg(a.Block.NewValue1(v.Pos, OpIData, ptrType, a))
 	}
 	v.reset(OpIMake)
 	v.AddArg(itab)
 	v.AddArg(data)
 }

 func decomposeArgs(f *Func) {
 	applyRewrite(f, rewriteBlockdecArgs, rewriteValuedecArgs)
 }

 func decomposeUser(f *Func) {
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if v.Op != OpPhi {
 				continue
 			}
 			decomposeUserPhi(v)
 		}
 	}
 	// Split up named values into their components.
 	i := 0
 	var newNames []LocalSlot
 	for _, name := range f.Names {
 		t := name.Type
 		switch {
 		case t.IsStruct():
 			newNames = decomposeUserStructInto(f, name, newNames)
 		case t.IsArray():
 			newNames = decomposeUserArrayInto(f, name, newNames)
 		default:
 			f.Names[i] = name
 			i++
 		}
 	}
 	f.Names = f.Names[:i]
 	f.Names = append(f.Names, newNames...)
 }

 // decomposeUserArrayInto creates names for the element(s) of arrays referenced
 // by name where possible, and appends those new names to slots, which is then
 // returned.
 func decomposeUserArrayInto(f *Func, name LocalSlot, slots []LocalSlot) []LocalSlot {
 	t := name.Type
 	if t.NumElem() == 0 {
 		// TODO(khr): Not sure what to do here.  Probably nothing.
 		// Names for empty arrays aren't important.
 		return slots
 	}
 	if t.NumElem() != 1 {
 		// shouldn't get here due to CanSSA
 		f.Fatalf("array not of size 1")
 	}
 	elemName := f.fe.SplitArray(name)
 	for _, v := range f.NamedValues[name] {
 		if v.Op != OpArrayMake1 {
 			continue
 		}
 		f.NamedValues[elemName] = append(f.NamedValues[elemName], v.Args[0])
 	}
 	// delete the name for the array as a whole
 	delete(f.NamedValues, name)

 	if t.Elem().IsArray() {
 		return decomposeUserArrayInto(f, elemName, slots)
 	} else if t.Elem().IsStruct() {
 		return decomposeUserStructInto(f, elemName, slots)
 	}

 	return append(slots, elemName)
 }

 // decomposeUserStructInto creates names for the fields(s) of structs referenced
 // by name where possible, and appends those new names to slots, which is then
 // returned.
 func decomposeUserStructInto(f *Func, name LocalSlot, slots []LocalSlot) []LocalSlot {
 	fnames := []LocalSlot{} // slots for struct in name
 	t := name.Type
 	n := t.NumFields()

 	for i := 0; i < n; i++ {
 		fs := f.fe.SplitStruct(name, i)
 		fnames = append(fnames, fs)
 		// arrays and structs will be decomposed further, so
 		// there's no need to record a name
 		if !fs.Type.IsArray() && !fs.Type.IsStruct() {
 			slots = append(slots, fs)
 		}
 	}

 	makeOp := StructMakeOp(n)
 	// create named values for each struct field
 	for _, v := range f.NamedValues[name] {
 		if v.Op != makeOp {
 			continue
 		}
 		for i := 0; i < len(fnames); i++ {
 			f.NamedValues[fnames[i]] = append(f.NamedValues[fnames[i]], v.Args[i])
 		}
 	}
 	// remove the name of the struct as a whole
 	delete(f.NamedValues, name)

 	// now that this f.NamedValues contains values for the struct
 	// fields, recurse into nested structs
 	for i := 0; i < n; i++ {
 		if name.Type.FieldType(i).IsStruct() {
 			slots = decomposeUserStructInto(f, fnames[i], slots)
 			delete(f.NamedValues, fnames[i])
 		} else if name.Type.FieldType(i).IsArray() {
 			slots = decomposeUserArrayInto(f, fnames[i], slots)
 			delete(f.NamedValues, fnames[i])
 		}
 	}
 	return slots
 }
 func decomposeUserPhi(v *Value) {
 	switch {
 	case v.Type.IsStruct():
 		decomposeStructPhi(v)
 	case v.Type.IsArray():
 		decomposeArrayPhi(v)
 	}
 }

 // decomposeStructPhi replaces phi-of-struct with structmake(phi-for-each-field),
 // and then recursively decomposes the phis for each field.
 func decomposeStructPhi(v *Value) {
 	t := v.Type
 	n := t.NumFields()
 	var fields [MaxStruct]*Value
 	for i := 0; i < n; i++ {
 		fields[i] = v.Block.NewValue0(v.Pos, OpPhi, t.FieldType(i))
 	}
 	for _, a := range v.Args {
 		for i := 0; i < n; i++ {
 			fields[i].AddArg(a.Block.NewValue1I(v.Pos, OpStructSelect, t.FieldType(i), int64(i), a))
 		}
 	}
 	v.reset(StructMakeOp(n))
 	v.AddArgs(fields[:n]...)

 	// Recursively decompose phis for each field.
 	for _, f := range fields[:n] {
 		decomposeUserPhi(f)
 	}
 }

 // decomposeArrayPhi replaces phi-of-array with arraymake(phi-of-array-element),
 // and then recursively decomposes the element phi.
 func decomposeArrayPhi(v *Value) {
 	t := v.Type
 	if t.NumElem() == 0 {
 		v.reset(OpArrayMake0)
 		return
 	}
 	if t.NumElem() != 1 {
 		v.Fatalf("SSAable array must have no more than 1 element")
 	}
 	elem := v.Block.NewValue0(v.Pos, OpPhi, t.Elem())
 	for _, a := range v.Args {
 		elem.AddArg(a.Block.NewValue1I(v.Pos, OpArraySelect, t.Elem(), 0, a))
 	}
 	v.reset(OpArrayMake1)
 	v.AddArg(elem)

 	// Recursively decompose elem phi.
 	decomposeUserPhi(elem)
 }

 // MaxStruct is the maximum number of fields a struct
 // can have and still be SSAable.
 const MaxStruct = 4

 // StructMakeOp returns the opcode to construct a struct with the
 // given number of fields.
 func StructMakeOp(nf int) Op {
 	switch nf {
 	case 0:
 		return OpStructMake0
 	case 1:
 		return OpStructMake1
 	case 2:
 		return OpStructMake2
 	case 3:
 		return OpStructMake3
 	case 4:
 		return OpStructMake4
 	}
 	panic("too many fields in an SSAable struct")
 }
	// Copyright 2015 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

	import (
	"cmd/compile/internal/types"
	)

	// decompose converts phi ops on compound builtin types into phi
	// ops on simple types, then invokes rewrite rules to decompose
	// other ops on those types.
	func decomposeBuiltIn(f *Func) {
	// Decompose phis
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	if v.Op != OpPhi {
	continue
	}
	decomposeBuiltInPhi(v)
	}
	}

	// Decompose other values
	applyRewrite(f, rewriteBlockdec, rewriteValuedec)
	if f.Config.RegSize == 4 {
	applyRewrite(f, rewriteBlockdec64, rewriteValuedec64)
	}

	// Split up named values into their components.
	var newNames []LocalSlot
	for _, name := range f.Names {
	t := name.Type
	switch {
	case t.IsInteger() && t.Size() > f.Config.RegSize:
	hiName, loName := f.fe.SplitInt64(name)
	newNames = append(newNames, hiName, loName)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpInt64Make {
	continue
	}
	f.NamedValues[hiName] = append(f.NamedValues[hiName], v.Args[0])
	f.NamedValues[loName] = append(f.NamedValues[loName], v.Args[1])
	}
	delete(f.NamedValues, name)
	case t.IsComplex():
	rName, iName := f.fe.SplitComplex(name)
	newNames = append(newNames, rName, iName)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpComplexMake {
	continue
	}
	f.NamedValues[rName] = append(f.NamedValues[rName], v.Args[0])
	f.NamedValues[iName] = append(f.NamedValues[iName], v.Args[1])

	}
	delete(f.NamedValues, name)
	case t.IsString():
	ptrName, lenName := f.fe.SplitString(name)
	newNames = append(newNames, ptrName, lenName)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpStringMake {
	continue
	}
	f.NamedValues[ptrName] = append(f.NamedValues[ptrName], v.Args[0])
	f.NamedValues[lenName] = append(f.NamedValues[lenName], v.Args[1])
	}
	delete(f.NamedValues, name)
	case t.IsSlice():
	ptrName, lenName, capName := f.fe.SplitSlice(name)
	newNames = append(newNames, ptrName, lenName, capName)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpSliceMake {
	continue
	}
	f.NamedValues[ptrName] = append(f.NamedValues[ptrName], v.Args[0])
	f.NamedValues[lenName] = append(f.NamedValues[lenName], v.Args[1])
	f.NamedValues[capName] = append(f.NamedValues[capName], v.Args[2])
	}
	delete(f.NamedValues, name)
	case t.IsInterface():
	typeName, dataName := f.fe.SplitInterface(name)
	newNames = append(newNames, typeName, dataName)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpIMake {
	continue
	}
	f.NamedValues[typeName] = append(f.NamedValues[typeName], v.Args[0])
	f.NamedValues[dataName] = append(f.NamedValues[dataName], v.Args[1])
	}
	delete(f.NamedValues, name)
	case t.IsFloat():
	// floats are never decomposed, even ones bigger than RegSize
	newNames = append(newNames, name)
	case t.Size() > f.Config.RegSize:
	f.Fatalf("undecomposed named type %s %v", name, t)
	default:
	newNames = append(newNames, name)
	}
	}
	f.Names = newNames
	}

	func decomposeBuiltInPhi(v *Value) {
	switch {
	case v.Type.IsInteger() && v.Type.Size() > v.Block.Func.Config.RegSize:
	decomposeInt64Phi(v)
	case v.Type.IsComplex():
	decomposeComplexPhi(v)
	case v.Type.IsString():
	decomposeStringPhi(v)
	case v.Type.IsSlice():
	decomposeSlicePhi(v)
	case v.Type.IsInterface():
	decomposeInterfacePhi(v)
	case v.Type.IsFloat():
	// floats are never decomposed, even ones bigger than RegSize
	case v.Type.Size() > v.Block.Func.Config.RegSize:
	v.Fatalf("undecomposed type %s", v.Type)
	}
	}

	func decomposeStringPhi(v *Value) {
	types := &v.Block.Func.Config.Types
	ptrType := types.BytePtr
	lenType := types.Int

	ptr := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
	len := v.Block.NewValue0(v.Pos, OpPhi, lenType)
	for _, a := range v.Args {
	ptr.AddArg(a.Block.NewValue1(v.Pos, OpStringPtr, ptrType, a))
	len.AddArg(a.Block.NewValue1(v.Pos, OpStringLen, lenType, a))
	}
	v.reset(OpStringMake)
	v.AddArg(ptr)
	v.AddArg(len)
	}

	func decomposeSlicePhi(v *Value) {
	types := &v.Block.Func.Config.Types
	ptrType := types.BytePtr
	lenType := types.Int

	ptr := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
	len := v.Block.NewValue0(v.Pos, OpPhi, lenType)
	cap := v.Block.NewValue0(v.Pos, OpPhi, lenType)
	for _, a := range v.Args {
	ptr.AddArg(a.Block.NewValue1(v.Pos, OpSlicePtr, ptrType, a))
	len.AddArg(a.Block.NewValue1(v.Pos, OpSliceLen, lenType, a))
	cap.AddArg(a.Block.NewValue1(v.Pos, OpSliceCap, lenType, a))
	}
	v.reset(OpSliceMake)
	v.AddArg(ptr)
	v.AddArg(len)
	v.AddArg(cap)
	}

	func decomposeInt64Phi(v *Value) {
	cfgtypes := &v.Block.Func.Config.Types
	var partType *types.Type
	if v.Type.IsSigned() {
	partType = cfgtypes.Int32
	} else {
	partType = cfgtypes.UInt32
	}

	hi := v.Block.NewValue0(v.Pos, OpPhi, partType)
	lo := v.Block.NewValue0(v.Pos, OpPhi, cfgtypes.UInt32)
	for _, a := range v.Args {
	hi.AddArg(a.Block.NewValue1(v.Pos, OpInt64Hi, partType, a))
	lo.AddArg(a.Block.NewValue1(v.Pos, OpInt64Lo, cfgtypes.UInt32, a))
	}
	v.reset(OpInt64Make)
	v.AddArg(hi)
	v.AddArg(lo)
	}

	func decomposeComplexPhi(v *Value) {
	cfgtypes := &v.Block.Func.Config.Types
	var partType *types.Type
	switch z := v.Type.Size(); z {
	case 8:
	partType = cfgtypes.Float32
	case 16:
	partType = cfgtypes.Float64
	default:
	v.Fatalf("decomposeComplexPhi: bad complex size %d", z)
	}

	real := v.Block.NewValue0(v.Pos, OpPhi, partType)
	imag := v.Block.NewValue0(v.Pos, OpPhi, partType)
	for _, a := range v.Args {
	real.AddArg(a.Block.NewValue1(v.Pos, OpComplexReal, partType, a))
	imag.AddArg(a.Block.NewValue1(v.Pos, OpComplexImag, partType, a))
	}
	v.reset(OpComplexMake)
	v.AddArg(real)
	v.AddArg(imag)
	}

	func decomposeInterfacePhi(v *Value) {
	uintptrType := v.Block.Func.Config.Types.Uintptr
	ptrType := v.Block.Func.Config.Types.BytePtr

	itab := v.Block.NewValue0(v.Pos, OpPhi, uintptrType)
	data := v.Block.NewValue0(v.Pos, OpPhi, ptrType)
	for _, a := range v.Args {
	itab.AddArg(a.Block.NewValue1(v.Pos, OpITab, uintptrType, a))
	data.AddArg(a.Block.NewValue1(v.Pos, OpIData, ptrType, a))
	}
	v.reset(OpIMake)
	v.AddArg(itab)
	v.AddArg(data)
	}

	func decomposeArgs(f *Func) {
	applyRewrite(f, rewriteBlockdecArgs, rewriteValuedecArgs)
	}

	func decomposeUser(f *Func) {
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	if v.Op != OpPhi {
	continue
	}
	decomposeUserPhi(v)
	}
	}
	// Split up named values into their components.
	i := 0
	var newNames []LocalSlot
	for _, name := range f.Names {
	t := name.Type
	switch {
	case t.IsStruct():
	newNames = decomposeUserStructInto(f, name, newNames)
	case t.IsArray():
	newNames = decomposeUserArrayInto(f, name, newNames)
	default:
	f.Names[i] = name
	i++
	}
	}
	f.Names = f.Names[:i]
	f.Names = append(f.Names, newNames...)
	}

	// decomposeUserArrayInto creates names for the element(s) of arrays referenced
	// by name where possible, and appends those new names to slots, which is then
	// returned.
	func decomposeUserArrayInto(f *Func, name LocalSlot, slots []LocalSlot) []LocalSlot {
	t := name.Type
	if t.NumElem() == 0 {
	// TODO(khr): Not sure what to do here. Probably nothing.
	// Names for empty arrays aren't important.
	return slots
	}
	if t.NumElem() != 1 {
	// shouldn't get here due to CanSSA
	f.Fatalf("array not of size 1")
	}
	elemName := f.fe.SplitArray(name)
	for _, v := range f.NamedValues[name] {
	if v.Op != OpArrayMake1 {
	continue
	}
	f.NamedValues[elemName] = append(f.NamedValues[elemName], v.Args[0])
	}
	// delete the name for the array as a whole
	delete(f.NamedValues, name)

	if t.Elem().IsArray() {
	return decomposeUserArrayInto(f, elemName, slots)
	} else if t.Elem().IsStruct() {
	return decomposeUserStructInto(f, elemName, slots)
	}

	return append(slots, elemName)
	}

	// decomposeUserStructInto creates names for the fields(s) of structs referenced
	// by name where possible, and appends those new names to slots, which is then
	// returned.
	func decomposeUserStructInto(f *Func, name LocalSlot, slots []LocalSlot) []LocalSlot {
	fnames := []LocalSlot{} // slots for struct in name
	t := name.Type
	n := t.NumFields()

	for i := 0; i < n; i++ {
	fs := f.fe.SplitStruct(name, i)
	fnames = append(fnames, fs)
	// arrays and structs will be decomposed further, so
	// there's no need to record a name
	if !fs.Type.IsArray() && !fs.Type.IsStruct() {
	slots = append(slots, fs)
	}
	}

	makeOp := StructMakeOp(n)
	// create named values for each struct field
	for _, v := range f.NamedValues[name] {
	if v.Op != makeOp {
	continue
	}
	for i := 0; i < len(fnames); i++ {
	f.NamedValues[fnames[i]] = append(f.NamedValues[fnames[i]], v.Args[i])
	}
	}
	// remove the name of the struct as a whole
	delete(f.NamedValues, name)

	// now that this f.NamedValues contains values for the struct
	// fields, recurse into nested structs
	for i := 0; i < n; i++ {
	if name.Type.FieldType(i).IsStruct() {
	slots = decomposeUserStructInto(f, fnames[i], slots)
	delete(f.NamedValues, fnames[i])
	} else if name.Type.FieldType(i).IsArray() {
	slots = decomposeUserArrayInto(f, fnames[i], slots)
	delete(f.NamedValues, fnames[i])
	}
	}
	return slots
	}
	func decomposeUserPhi(v *Value) {
	switch {
	case v.Type.IsStruct():
	decomposeStructPhi(v)
	case v.Type.IsArray():
	decomposeArrayPhi(v)
	}
	}

	// decomposeStructPhi replaces phi-of-struct with structmake(phi-for-each-field),
	// and then recursively decomposes the phis for each field.
	func decomposeStructPhi(v *Value) {
	t := v.Type
	n := t.NumFields()
	var fields [MaxStruct]*Value
	for i := 0; i < n; i++ {
	fields[i] = v.Block.NewValue0(v.Pos, OpPhi, t.FieldType(i))
	}
	for _, a := range v.Args {
	for i := 0; i < n; i++ {
	fields[i].AddArg(a.Block.NewValue1I(v.Pos, OpStructSelect, t.FieldType(i), int64(i), a))
	}
	}
	v.reset(StructMakeOp(n))
	v.AddArgs(fields[:n]...)

	// Recursively decompose phis for each field.
	for _, f := range fields[:n] {
	decomposeUserPhi(f)
	}
	}

	// decomposeArrayPhi replaces phi-of-array with arraymake(phi-of-array-element),
	// and then recursively decomposes the element phi.
	func decomposeArrayPhi(v *Value) {
	t := v.Type
	if t.NumElem() == 0 {
	v.reset(OpArrayMake0)
	return
	}
	if t.NumElem() != 1 {
	v.Fatalf("SSAable array must have no more than 1 element")
	}
	elem := v.Block.NewValue0(v.Pos, OpPhi, t.Elem())
	for _, a := range v.Args {
	elem.AddArg(a.Block.NewValue1I(v.Pos, OpArraySelect, t.Elem(), 0, a))
	}
	v.reset(OpArrayMake1)
	v.AddArg(elem)

	// Recursively decompose elem phi.
	decomposeUserPhi(elem)
	}

	// MaxStruct is the maximum number of fields a struct
	// can have and still be SSAable.
	const MaxStruct = 4

	// StructMakeOp returns the opcode to construct a struct with the
	// given number of fields.
	func StructMakeOp(nf int) Op {
	switch nf {
	case 0:
	return OpStructMake0
	case 1:
	return OpStructMake1
	case 2:
	return OpStructMake2
	case 3:
	return OpStructMake3
	case 4:
	return OpStructMake4
	}
	panic("too many fields in an SSAable struct")
	}