memcombine.go

// Copyright 2023 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 ssacompile

import (
	"cmp"
	"slices"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ssa"
	"cmd/compile/internal/ssa/ssaop"
	"cmd/compile/internal/types"
	"cmd/internal/src"
)

// memcombine combines smaller loads and stores into larger ones.
// This produces good code for encoding/binary operations and may help other
// cases too. On architectures that do not allow unaligned accesses, the pass
// uses pointer alignment facts to avoid introducing unaligned wider operations.
func memcombine(f *ssa.Func) {
	var ptrAlignments []int8
	if !f.Config.UnalignedOK {
		ptrAlignments = f.Cache.AllocInt8Slice(f.NumValues())
		defer f.Cache.FreeInt8Slice(ptrAlignments)
		computePtrAlignments(f, ptrAlignments)
	}
	memcombineLoads(f, ptrAlignments)
	memcombineStores(f, ptrAlignments)
}

func memcombineLoads(f *ssa.Func, ptrAlignments []int8) {
	// Find "OR trees" to start with.
	mark := f.NewSparseSet(f.NumValues())
	defer f.RetSparseSet(mark)
	var order []*ssa.Value

	// Mark all values that are the argument of an OR.
	for _, b := range f.Blocks {
		for _, v := range b.Values {
			if v.Op == ssaop.OpOr16 || v.Op == ssaop.OpOr32 || v.Op == ssaop.OpOr64 {
				mark.Add(v.Args[0].ID)
				mark.Add(v.Args[1].ID)
			}
		}
	}
	for _, b := range f.Blocks {
		order = order[:0]
		for _, v := range b.Values {
			if v.Op != ssaop.OpOr16 && v.Op != ssaop.OpOr32 && v.Op != ssaop.OpOr64 {
				continue
			}
			if v.Uses == 1 && mark.Contains(v.ID) {
				// A single-use argument belongs to its parent's OR tree.
				continue
			}
			// Add the OR tree rooted at v to the order.
			// We use BFS here, but any walk that puts roots before leaves would work.
			i := len(order)
			order = append(order, v)
			for ; i < len(order); i++ {
				x := order[i]
				for j := 0; j < 2; j++ {
					a := x.Args[j]
					// combineLoads cannot combine through shared values.
					if a.Uses == 1 && (a.Op == ssaop.OpOr16 || a.Op == ssaop.OpOr32 || a.Op == ssaop.OpOr64) {
						order = append(order, a)
					}
				}
			}
		}
		for _, v := range order {
			max := f.Config.RegSize
			switch v.Op {
			case ssaop.OpOr64:
			case ssaop.OpOr32:
				max = 4
			case ssaop.OpOr16:
				max = 2
			default:
				continue
			}
			for n := max; n > 1; n /= 2 {
				if combineLoads(v, n, ptrAlignments) {
					break
				}
			}
		}
	}
}

// A BaseAddress represents the address ptr+idx, where
// ptr is a pointer type and idx is an integer type.
// idx may be nil, in which case it is treated as 0.
type BaseAddress struct {
	ptr *ssa.Value
	idx Index
}

// Index represents an address index in the form exp<<shift.
//
// The shift is typically introduced by slice indexing (log2(element size)),
// but may also originate from shifts in the source expression.
type Index struct {
	exp   *ssa.Value
	shift int64
}

func getConst(v *ssa.Value) (int64, bool) {
	if v.Op == ssaop.OpConst32 || v.Op == ssaop.OpConst64 {
		return v.AuxInt, true
	}
	return 0, false
}

func peelAdd(v *ssa.Value) (exp *ssa.Value, imm int64) {
	if v == nil {
		return nil, 0
	}

	if v.Op == ssaop.OpAdd32 || v.Op == ssaop.OpAdd64 {
		if imm, ok := getConst(v.Args[0]); ok {
			return v.Args[1], imm
		}

		if imm, ok := getConst(v.Args[1]); ok {
			return v.Args[0], imm
		}
	}

	return v, 0
}

func peelShift(v *ssa.Value) (exp *ssa.Value, shift int64) {
	if v == nil {
		return nil, 0
	}

	if v.Op == ssaop.OpLsh64x64 || v.Op == ssaop.OpLsh32x64 || v.Op == ssaop.OpLsh16x64 {
		if imm, ok := getConst(v.Args[1]); ok {
			return v.Args[0], imm
		}
	}
	return v, 0
}

// splitPtr returns the base address of ptr and any
// constant offset from that base.
// BaseAddress{ptr,nil},0 is always a valid result, but splitPtr
// tries to peel away as many constants into off as possible.
func splitPtr(ptr *ssa.Value) (BaseAddress, int64) {
	var idx Index
	var off int64
	for {
		if ptr.Op == ssaop.OpOffPtr {
			off += ptr.AuxInt
			ptr = ptr.Args[0]
			continue
		}

		if ptr.Op == ssaop.OpAddPtr {
			if idx.exp != nil {
				// We have two or more indexing values.
				// Pick the first one we found.
				break
			}

			// Common slice indexing patterns:
			//
			// exp
			// exp + offset
			// (exp << shift) + offset
			// (exp+imm)<<shift + offset
			//
			// where shift is typically log2(element size).

			idx.exp = ptr.Args[1]
			ptr = ptr.Args[0]

			// Peel offset
			var offset int64
			idx.exp, offset = peelAdd(idx.exp)
			off += offset

			// Peel shift
			idx.exp, idx.shift = peelShift(idx.exp)

			// Peel imm
			var imm int64
			idx.exp, imm = peelAdd(idx.exp)

			off += imm << idx.shift
			continue
		}

		break
	}

	return BaseAddress{ptr: ptr, idx: idx}, off
}

// computePtrAlignments computes pointer alignment facts from typed base pointers
// and constant offsets.
func computePtrAlignments(f *ssa.Func, ptrAlignments []int8) {
	for _, b := range slices.Backward(f.Postorder()) {
		for _, v := range b.Values {
			ptrAlignments[v.ID] = int8(valuePtrAlignment(v, ptrAlignments))
		}
	}
}

// ptrAlignment only reads already-computed facts. Zero/not-yet-known means
// alignment 1, avoiding recursive Phi/cycle walks.
func ptrAlignment(ptr *ssa.Value, ptrAlignments []int8) int64 {
	if align := ptrAlignments[ptr.ID]; align > 0 {
		return int64(align)
	}
	return 1
}

// valuePtrAlignment computes one entry in ptrAlignments.
func valuePtrAlignment(v *ssa.Value, ptrAlignments []int8) int64 {
	// computePtrAlignments visits every SSA value, not just pointer values.
	if !v.Type.IsPtr() {
		return 1
	}

	switch v.Op {
	case ssaop.OpOffPtr:
		return offsetAlignment(ptrAlignment(v.Args[0], ptrAlignments), v.AuxInt)
	case ssaop.OpCopy, ssaop.OpNilCheck:
		return ptrAlignment(v.Args[0], ptrAlignments)
	case ssaop.OpAddr, ssaop.OpLocalAddr, ssaop.OpArg, ssaop.OpArgIntReg:
		return typeAlignment(v.Type.Elem())
	case ssaop.OpPhi:
		align := ptrAlignment(v.Args[0], ptrAlignments)
		for _, arg := range v.Args[1:] {
			if argAlign := ptrAlignment(arg, ptrAlignments); argAlign < align {
				align = argAlign
			}
		}
		return align
	}
	return 1
}

// typeAlignment returns a conservative alignment for t without calling
// Type.Alignment, which may try to calculate type sizes while the compiler
// back end is running concurrently.
func typeAlignment(t *types.Type) int64 {
	switch t.Kind() {
	case types.TBOOL, types.TINT8, types.TUINT8:
		return 1
	case types.TINT16, types.TUINT16:
		return 2
	case types.TINT32, types.TUINT32, types.TFLOAT32, types.TCOMPLEX64:
		return 4
	case types.TINT64, types.TUINT64, types.TFLOAT64, types.TCOMPLEX128:
		return 8
	case types.TINT, types.TUINT, types.TUINTPTR, types.TPTR, types.TUNSAFEPTR, types.TSTRING, types.TSLICE, types.TFUNC, types.TMAP, types.TCHAN:
		return int64(types.PtrSize)
	case types.TARRAY:
		return typeAlignment(t.Elem())
	case types.TSTRUCT:
		align := int64(1)
		for _, f := range t.Fields() {
			fieldAlign := typeAlignment(f.Type)
			if fieldAlign > align {
				align = fieldAlign
			}
		}
		return align
	}
	return 1
}

func offsetAlignment(align, off int64) int64 {
	off &= align - 1
	if off == 0 {
		return align
	}
	return off & -off
}

func combineLoads(root *ssa.Value, n int64, ptrAlignments []int8) bool {
	orOp := root.Op
	var shiftOp ssaop.Op
	switch orOp {
	case ssaop.OpOr64:
		shiftOp = ssaop.OpLsh64x64
	case ssaop.OpOr32:
		shiftOp = ssaop.OpLsh32x64
	case ssaop.OpOr16:
		shiftOp = ssaop.OpLsh16x64
	default:
		return false
	}

	// Find n values that are ORed together with the above op.
	a := make([]*ssa.Value, 0, 8)
	a = append(a, root)
	for i := 0; i < len(a) && int64(len(a)) < n; i++ {
		v := a[i]
		if v.Uses != 1 && v != root {
			// Something in this subtree is used somewhere else.
			return false
		}
		if v.Op == orOp {
			a[i] = v.Args[0]
			a = append(a, v.Args[1])
			i--
		}
	}
	if int64(len(a)) != n {
		return false
	}

	// Check that the first entry to see what ops we're looking for.
	// All the entries should be of the form shift(extend(load)), maybe with no shift.
	v := a[0]
	if v.Op == shiftOp {
		v = v.Args[0]
	}
	var extOp ssaop.Op
	if orOp == ssaop.OpOr64 && (v.Op == ssaop.OpZeroExt8to64 || v.Op == ssaop.OpZeroExt16to64 || v.Op == ssaop.OpZeroExt32to64) ||
		orOp == ssaop.OpOr32 && (v.Op == ssaop.OpZeroExt8to32 || v.Op == ssaop.OpZeroExt16to32) ||
		orOp == ssaop.OpOr16 && v.Op == ssaop.OpZeroExt8to16 {
		extOp = v.Op
		v = v.Args[0]
	} else {
		return false
	}
	if v.Op != ssaop.OpLoad {
		return false
	}
	base, _ := splitPtr(v.Args[0])
	mem := v.Args[1]
	size := v.Type.Size()

	if root.Block.Func.Config.Arch == "S390X" {
		// s390x can't handle unaligned accesses to global variables.
		if base.ptr.Op == ssaop.OpAddr {
			return false
		}
	}

	// Check all the entries, extract useful info.
	type LoadRecord struct {
		load   *ssa.Value
		offset int64 // offset of load address from base
		shift  int64
	}
	r := make([]LoadRecord, n, 8)
	for i := int64(0); i < n; i++ {
		v := a[i]
		if v.Uses != 1 {
			return false
		}
		shift := int64(0)
		if v.Op == shiftOp {
			v, shift = peelShift(v)
			if v.Uses != 1 {
				return false
			}
		}
		if v.Op != extOp {
			return false
		}
		load := v.Args[0]
		if load.Op != ssaop.OpLoad {
			return false
		}
		if load.Uses != 1 {
			return false
		}
		if load.Args[1] != mem {
			return false
		}
		p, off := splitPtr(load.Args[0])
		if p != base {
			return false
		}
		r[i] = LoadRecord{load: load, offset: off, shift: shift}
	}

	// Sort in memory address order.
	slices.SortFunc(r, func(a, b LoadRecord) int {
		return cmp.Compare(a.offset, b.offset)
	})

	// Check that we have contiguous offsets.
	for i := int64(0); i < n; i++ {
		if r[i].offset != r[0].offset+i*size {
			return false
		}
	}
	if !root.Block.Func.Config.UnalignedOK && ptrAlignment(r[0].load.Args[0], ptrAlignments) < n*size {
		return false
	}

	// Check for reads in little-endian or big-endian order.
	shift0 := r[0].shift
	isLittleEndian := true
	for i := int64(0); i < n; i++ {
		if r[i].shift != shift0+i*size*8 {
			isLittleEndian = false
			break
		}
	}
	isBigEndian := true
	for i := int64(0); i < n; i++ {
		if r[i].shift != shift0-i*size*8 {
			isBigEndian = false
			break
		}
	}
	if !isLittleEndian && !isBigEndian {
		return false
	}

	// Find a place to put the new load.
	// This is tricky, because it has to be at a point where
	// its memory argument is live. We can't just put it in root.Block.
	// We use the block of the latest load.
	loads := make([]*ssa.Value, n, 8)
	for i := int64(0); i < n; i++ {
		loads[i] = r[i].load
	}
	loadBlock := mergePoint(root.Block, loads...)
	if loadBlock == nil {
		return false
	}
	// Find a source position to use.
	pos := src.NoXPos
	for _, load := range loads {
		if load.Block == loadBlock {
			pos = load.Pos
			break
		}
	}
	if pos == src.NoXPos {
		return false
	}

	// Check to see if we need byte swap before storing.
	needSwap := isLittleEndian && root.Block.Func.Config.BigEndian ||
		isBigEndian && !root.Block.Func.Config.BigEndian
	if needSwap && (size != 1 || !root.Block.Func.Config.HaveByteSwap(n)) {
		return false
	}

	// This is the commit point.

	// First, issue load at lowest address.
	v = loadBlock.NewValue2(pos, ssaop.OpLoad, sizeType(n*size), r[0].load.Args[0], mem)

	// Byte swap if needed,
	if needSwap {
		v = byteSwap(loadBlock, pos, v)
	}

	// Extend if needed.
	if n*size < root.Type.Size() {
		v = zeroExtend(loadBlock, pos, v, n*size, root.Type.Size())
	}

	// Shift if needed.
	if isLittleEndian && shift0 != 0 {
		v = leftShift(loadBlock, pos, v, shift0)
	}
	if isBigEndian && shift0-(n-1)*size*8 != 0 {
		v = leftShift(loadBlock, pos, v, shift0-(n-1)*size*8)
	}

	// Install with (Copy v).
	root.Reset(ssaop.OpCopy)
	root.AddArg(v)

	// Clobber the loads, just to prevent additional work being done on
	// subtrees (which are now unreachable).
	for i := int64(0); i < n; i++ {
		ssa.Clobber(r[i].load)
	}
	return true
}

func memcombineStores(f *ssa.Func, ptrAlignments []int8) {
	mark := f.NewSparseSet(f.NumValues())
	defer f.RetSparseSet(mark)
	var order []*ssa.Value

	for _, b := range f.Blocks {
		// Mark all stores which are not last in a store sequence.
		mark.Clear()
		for _, v := range b.Values {
			if v.Op == ssaop.OpStore {
				mark.Add(v.MemoryArg().ID)
			}
		}

		// pick an order for visiting stores such that
		// later stores come earlier in the ordering.
		order = order[:0]
		for _, v := range b.Values {
			if v.Op != ssaop.OpStore {
				continue
			}
			if mark.Contains(v.ID) {
				continue // not last in a chain of stores
			}
			for {
				order = append(order, v)
				v = v.Args[2]
				if v.Block != b || v.Op != ssaop.OpStore {
					break
				}
			}
		}

		// Look for combining opportunities at each store in queue order.
		for _, v := range order {
			if v.Op != ssaop.OpStore { // already rewritten
				continue
			}

			size := v.Aux.(*types.Type).Size()
			if size >= f.Config.RegSize || size == 0 {
				continue
			}

			combineStores(v, ptrAlignments)
		}
	}
}

// combineStores tries to combine the stores ending in root.
func combineStores(root *ssa.Value, ptrAlignments []int8) {
	// Helper functions.
	maxRegSize := root.Block.Func.Config.RegSize
	type StoreRecord struct {
		store  *ssa.Value
		offset int64
		size   int64
	}
	getShiftBase := func(a []StoreRecord) *ssa.Value {
		x := a[0].store.Args[1]
		y := a[1].store.Args[1]
		switch x.Op {
		case ssaop.OpTrunc64to8, ssaop.OpTrunc64to16, ssaop.OpTrunc64to32, ssaop.OpTrunc32to8, ssaop.OpTrunc32to16, ssaop.OpTrunc16to8:
			x = x.Args[0]
		default:
			return nil
		}
		switch y.Op {
		case ssaop.OpTrunc64to8, ssaop.OpTrunc64to16, ssaop.OpTrunc64to32, ssaop.OpTrunc32to8, ssaop.OpTrunc32to16, ssaop.OpTrunc16to8:
			y = y.Args[0]
		default:
			return nil
		}
		var x2 *ssa.Value
		switch x.Op {
		case ssaop.OpRsh64Ux64, ssaop.OpRsh32Ux64, ssaop.OpRsh16Ux64:
			x2 = x.Args[0]
		default:
		}
		var y2 *ssa.Value
		switch y.Op {
		case ssaop.OpRsh64Ux64, ssaop.OpRsh32Ux64, ssaop.OpRsh16Ux64:
			y2 = y.Args[0]
		default:
		}
		if y2 == x {
			// a shift of x and x itself.
			return x
		}
		if x2 == y {
			// a shift of y and y itself.
			return y
		}
		if x2 == y2 {
			// 2 shifts both of the same argument.
			return x2
		}
		return nil
	}
	isShiftBase := func(v, base *ssa.Value) bool {
		val := v.Args[1]
		switch val.Op {
		case ssaop.OpTrunc64to8, ssaop.OpTrunc64to16, ssaop.OpTrunc64to32, ssaop.OpTrunc32to8, ssaop.OpTrunc32to16, ssaop.OpTrunc16to8:
			val = val.Args[0]
		default:
			return false
		}
		if val == base {
			return true
		}
		switch val.Op {
		case ssaop.OpRsh64Ux64, ssaop.OpRsh32Ux64, ssaop.OpRsh16Ux64:
			val = val.Args[0]
		default:
			return false
		}
		return val == base
	}
	shift := func(v, base *ssa.Value) int64 {
		val := v.Args[1]
		switch val.Op {
		case ssaop.OpTrunc64to8, ssaop.OpTrunc64to16, ssaop.OpTrunc64to32, ssaop.OpTrunc32to8, ssaop.OpTrunc32to16, ssaop.OpTrunc16to8:
			val = val.Args[0]
		default:
			return -1
		}
		if val == base {
			return 0
		}
		switch val.Op {
		case ssaop.OpRsh64Ux64, ssaop.OpRsh32Ux64, ssaop.OpRsh16Ux64:
			val = val.Args[1]
		default:
			return -1
		}
		if val.Op != ssaop.OpConst64 {
			return -1
		}
		return val.AuxInt
	}

	// Gather n stores to look at. Check easy conditions we require.
	allMergeable := make([]StoreRecord, 0, 8)
	rbase, roff := splitPtr(root.Args[0])
	if root.Block.Func.Config.Arch == "S390X" {
		// s390x can't handle unaligned accesses to global variables.
		if rbase.ptr.Op == ssaop.OpAddr {
			return
		}
	}
	allMergeable = append(allMergeable, StoreRecord{root, roff, root.Aux.(*types.Type).Size()})
	allMergeableSize := root.Aux.(*types.Type).Size()
	// TODO: this loop strictly requires stores to chain together in memory.
	// maybe we can break this constraint and match more patterns.
	for i, x := 1, root.Args[2]; i < 8; i, x = i+1, x.Args[2] {
		if x.Op != ssaop.OpStore {
			break
		}
		if x.Block != root.Block {
			break
		}
		if x.Uses != 1 { // Note: root can have more than one use.
			break
		}
		xSize := x.Aux.(*types.Type).Size()
		if xSize == 0 {
			break
		}
		if xSize > maxRegSize-allMergeableSize {
			break
		}
		base, off := splitPtr(x.Args[0])
		if base != rbase {
			break
		}
		allMergeable = append(allMergeable, StoreRecord{x, off, xSize})
		allMergeableSize += xSize
	}
	if len(allMergeable) <= 1 {
		return
	}
	// Fit the combined total size to be one of the register size.
	mergeableSet := map[int64][]StoreRecord{}
	for i, size := 0, int64(0); i < len(allMergeable); i++ {
		size += allMergeable[i].size
		for _, bucketSize := range []int64{8, 4, 2} {
			if size == bucketSize {
				mergeableSet[size] = slices.Clone(allMergeable[:i+1])
				break
			}
		}
	}
	var a []StoreRecord
	var aTotalSize int64
	var mem *ssa.Value
	var pos src.XPos
	// Pick the largest mergeable set.
	for _, s := range []int64{8, 4, 2} {
		candidate := mergeableSet[s]
		// TODO: a refactoring might be more efficient:
		// Find a bunch of stores that are all adjacent and then decide how big a chunk of
		// those sequential stores to combine.
		if len(candidate) >= 2 {
			// Before we sort, grab the memory arg the result should have.
			mem = candidate[len(candidate)-1].store.Args[2]
			// Also grab position of first store (last in array = first in memory order).
			pos = candidate[len(candidate)-1].store.Pos
			// Sort stores in increasing address order.
			slices.SortFunc(candidate, func(sr1, sr2 StoreRecord) int {
				return cmp.Compare(sr1.offset, sr2.offset)
			})
			// Check that everything is written to sequential locations.
			sequential := true
			for i := 1; i < len(candidate); i++ {
				if candidate[i].offset != candidate[i-1].offset+candidate[i-1].size {
					sequential = false
					break
				}
			}
			if sequential {
				a = candidate
				aTotalSize = s
				break
			}
		}
	}
	if len(a) <= 1 {
		return
	}
	// Memory location we're going to write at (the lowest one).
	ptr := a[0].store.Args[0]
	if !root.Block.Func.Config.UnalignedOK && ptrAlignment(ptr, ptrAlignments) < aTotalSize {
		return
	}

	// Check for constant stores
	isConst := true
	for i := range a {
		switch a[i].store.Args[1].Op {
		case ssaop.OpConst32, ssaop.OpConst16, ssaop.OpConst8, ssaop.OpConstBool:
		default:
			isConst = false
		}
		if !isConst {
			break
		}
	}
	if isConst {
		// Modify root to do all the stores.
		var c int64
		for i := range a {
			mask := int64(1)<<(8*a[i].size) - 1
			s := 8 * (a[i].offset - a[0].offset)
			if root.Block.Func.Config.BigEndian {
				s = (aTotalSize-a[i].size)*8 - s
			}
			c |= (a[i].store.Args[1].AuxInt & mask) << s
		}
		var cv *ssa.Value
		switch aTotalSize {
		case 2:
			cv = root.Block.Func.ConstInt16(types.Types[types.TUINT16], int16(c))
		case 4:
			cv = root.Block.Func.ConstInt32(types.Types[types.TUINT32], int32(c))
		case 8:
			cv = root.Block.Func.ConstInt64(types.Types[types.TUINT64], c)
		}

		// Move all the stores to the root.
		for i := range a {
			v := a[i].store
			if v == root {
				v.Aux = cv.Type // widen store type
				v.Pos = pos
				v.SetArg(0, ptr)
				v.SetArg(1, cv)
				v.SetArg(2, mem)
			} else {
				ssa.Clobber(v)
				v.Type = types.Types[types.TBOOL] // erase memory type
			}
		}
		return
	}

	// Check for consecutive loads as the source of the stores.
	var loadMem *ssa.Value
	var loadBase BaseAddress
	var loadIdx int64
	for i := range a {
		load := a[i].store.Args[1]
		if load.Op != ssaop.OpLoad {
			loadMem = nil
			break
		}
		if load.Uses != 1 {
			loadMem = nil
			break
		}
		if load.Type.HasPointers() {
			// Don't combine stores containing a pointer, as we need
			// a write barrier for those. This can happen on an
			// 8-byte-reg/4-byte-ptr architecture like wasm32.
			loadMem = nil
			break
		}
		mem := load.Args[1]
		base, idx := splitPtr(load.Args[0])
		if loadMem == nil {
			// First one we found
			loadMem = mem
			loadBase = base
			loadIdx = idx
			continue
		}
		if base != loadBase || mem != loadMem {
			loadMem = nil
			break
		}
		if idx != loadIdx+(a[i].offset-a[0].offset) {
			loadMem = nil
			break
		}
	}
	if loadMem != nil {
		// Modify the first load to do a larger load instead.
		load := a[0].store.Args[1]
		if !root.Block.Func.Config.UnalignedOK && ptrAlignment(load.Args[0], ptrAlignments) < aTotalSize {
			return
		}
		switch aTotalSize {
		case 2:
			load.Type = types.Types[types.TUINT16]
		case 4:
			load.Type = types.Types[types.TUINT32]
		case 8:
			load.Type = types.Types[types.TUINT64]
		}

		// Modify root to do the store.
		for i := range a {
			v := a[i].store
			if v == root {
				v.Aux = load.Type // widen store type
				v.Pos = pos
				v.SetArg(0, ptr)
				v.SetArg(1, load)
				v.SetArg(2, mem)
			} else {
				ssa.Clobber(v)
				v.Type = types.Types[types.TBOOL] // erase memory type
			}
		}
		return
	}

	// Check that all the shift/trunc are of the same base value.
	shiftBase := getShiftBase(a)
	if shiftBase == nil {
		return
	}
	for i := range a {
		if !isShiftBase(a[i].store, shiftBase) {
			return
		}
	}

	// Check for writes in little-endian or big-endian order.
	isLittleEndian := true
	shift0 := shift(a[0].store, shiftBase)
	for i := 1; i < len(a); i++ {
		if shift(a[i].store, shiftBase) != shift0+(a[i].offset-a[0].offset)*8 {
			isLittleEndian = false
			break
		}
	}
	isBigEndian := true
	shiftedSize := int64(0)
	for i := 1; i < len(a); i++ {
		shiftedSize += a[i].size
		if shift(a[i].store, shiftBase) != shift0-shiftedSize*8 {
			isBigEndian = false
			break
		}
	}
	if !isLittleEndian && !isBigEndian {
		return
	}

	// Check to see if we need byte swap before storing.
	needSwap := isLittleEndian && root.Block.Func.Config.BigEndian ||
		isBigEndian && !root.Block.Func.Config.BigEndian
	if needSwap && (int64(len(a)) != aTotalSize || !root.Block.Func.Config.HaveByteSwap(aTotalSize)) {
		return
	}

	// This is the commit point.

	// Modify root to do all the stores.
	sv := shiftBase
	if isLittleEndian && shift0 != 0 {
		sv = rightShift(root.Block, root.Pos, sv, shift0)
	}
	shiftedSize = aTotalSize - a[0].size
	if isBigEndian && shift0-shiftedSize*8 != 0 {
		sv = rightShift(root.Block, root.Pos, sv, shift0-shiftedSize*8)
	}
	if sv.Type.Size() > aTotalSize {
		sv = truncate(root.Block, root.Pos, sv, sv.Type.Size(), aTotalSize)
	}
	if needSwap {
		sv = byteSwap(root.Block, root.Pos, sv)
	}

	// Move all the stores to the root.
	for i := range a {
		v := a[i].store
		if v == root {
			v.Aux = sv.Type // widen store type
			v.Pos = pos
			v.SetArg(0, ptr)
			v.SetArg(1, sv)
			v.SetArg(2, mem)
		} else {
			ssa.Clobber(v)
			v.Type = types.Types[types.TBOOL] // erase memory type
		}
	}
}

func sizeType(size int64) *types.Type {
	switch size {
	case 8:
		return types.Types[types.TUINT64]
	case 4:
		return types.Types[types.TUINT32]
	case 2:
		return types.Types[types.TUINT16]
	default:
		base.Fatalf("bad size %d\n", size)
		return nil
	}
}

func truncate(b *ssa.Block, pos src.XPos, v *ssa.Value, from, to int64) *ssa.Value {
	switch from*10 + to {
	case 82:
		return b.NewValue1(pos, ssaop.OpTrunc64to16, types.Types[types.TUINT16], v)
	case 84:
		return b.NewValue1(pos, ssaop.OpTrunc64to32, types.Types[types.TUINT32], v)
	case 42:
		return b.NewValue1(pos, ssaop.OpTrunc32to16, types.Types[types.TUINT16], v)
	default:
		base.Fatalf("bad sizes %d %d\n", from, to)
		return nil
	}
}
func zeroExtend(b *ssa.Block, pos src.XPos, v *ssa.Value, from, to int64) *ssa.Value {
	switch from*10 + to {
	case 24:
		return b.NewValue1(pos, ssaop.OpZeroExt16to32, types.Types[types.TUINT32], v)
	case 28:
		return b.NewValue1(pos, ssaop.OpZeroExt16to64, types.Types[types.TUINT64], v)
	case 48:
		return b.NewValue1(pos, ssaop.OpZeroExt32to64, types.Types[types.TUINT64], v)
	default:
		base.Fatalf("bad sizes %d %d\n", from, to)
		return nil
	}
}

func leftShift(b *ssa.Block, pos src.XPos, v *ssa.Value, shift int64) *ssa.Value {
	s := b.Func.ConstInt64(types.Types[types.TUINT64], shift)
	size := v.Type.Size()
	switch size {
	case 8:
		return b.NewValue2(pos, ssaop.OpLsh64x64, v.Type, v, s)
	case 4:
		return b.NewValue2(pos, ssaop.OpLsh32x64, v.Type, v, s)
	case 2:
		return b.NewValue2(pos, ssaop.OpLsh16x64, v.Type, v, s)
	default:
		base.Fatalf("bad size %d\n", size)
		return nil
	}
}
func rightShift(b *ssa.Block, pos src.XPos, v *ssa.Value, shift int64) *ssa.Value {
	s := b.Func.ConstInt64(types.Types[types.TUINT64], shift)
	size := v.Type.Size()
	switch size {
	case 8:
		return b.NewValue2(pos, ssaop.OpRsh64Ux64, v.Type, v, s)
	case 4:
		return b.NewValue2(pos, ssaop.OpRsh32Ux64, v.Type, v, s)
	case 2:
		return b.NewValue2(pos, ssaop.OpRsh16Ux64, v.Type, v, s)
	default:
		base.Fatalf("bad size %d\n", size)
		return nil
	}
}
func byteSwap(b *ssa.Block, pos src.XPos, v *ssa.Value) *ssa.Value {
	switch v.Type.Size() {
	case 8:
		return b.NewValue1(pos, ssaop.OpBswap64, v.Type, v)
	case 4:
		return b.NewValue1(pos, ssaop.OpBswap32, v.Type, v)
	case 2:
		return b.NewValue1(pos, ssaop.OpBswap16, v.Type, v)

	default:
		v.Fatalf("bad size %d\n", v.Type.Size())
		return nil
	}
}