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

 // Copyright 2016 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"
 	"cmd/internal/obj"
 	"cmd/internal/src"
 )

 // needwb returns whether we need write barrier for store op v.
 // v must be Store/Move/Zero.
 func needwb(v *Value) bool {
 	t, ok := v.Aux.(*types.Type)
 	if !ok {
 		v.Fatalf("store aux is not a type: %s", v.LongString())
 	}
 	if !t.HasPointer() {
 		return false
 	}
 	if IsStackAddr(v.Args[0]) {
 		return false // write on stack doesn't need write barrier
 	}
 	return true
 }

 // writebarrier pass inserts write barriers for store ops (Store, Move, Zero)
 // when necessary (the condition above). It rewrites store ops to branches
 // and runtime calls, like
 //
 // if writeBarrier.enabled {
 //   writebarrierptr(ptr, val)
 // } else {
 //   *ptr = val
 // }
 //
 // A sequence of WB stores for many pointer fields of a single type will
 // be emitted together, with a single branch.
 func writebarrier(f *Func) {
 	if !f.fe.UseWriteBarrier() {
 		return
 	}

 	var sb, sp, wbaddr, const0 *Value
 	var writebarrierptr, typedmemmove, typedmemclr *obj.LSym
 	var stores, after []*Value
 	var sset *sparseSet
 	var storeNumber []int32

 	for _, b := range f.Blocks { // range loop is safe since the blocks we added contain no stores to expand
 		// first, identify all the stores that need to insert a write barrier.
 		// mark them with WB ops temporarily. record presence of WB ops.
 		hasStore := false
 		for _, v := range b.Values {
 			switch v.Op {
 			case OpStore, OpMove, OpZero:
 				if needwb(v) {
 					switch v.Op {
 					case OpStore:
 						v.Op = OpStoreWB
 					case OpMove:
 						v.Op = OpMoveWB
 					case OpZero:
 						v.Op = OpZeroWB
 					}
 					hasStore = true
 				}
 			}
 		}
 		if !hasStore {
 			continue
 		}

 		if wbaddr == nil {
 			// lazily initialize global values for write barrier test and calls
 			// find SB and SP values in entry block
 			initpos := f.Entry.Pos
 			for _, v := range f.Entry.Values {
 				if v.Op == OpSB {
 					sb = v
 				}
 				if v.Op == OpSP {
 					sp = v
 				}
 				if sb != nil && sp != nil {
 					break
 				}
 			}
 			if sb == nil {
 				sb = f.Entry.NewValue0(initpos, OpSB, f.Config.Types.Uintptr)
 			}
 			if sp == nil {
 				sp = f.Entry.NewValue0(initpos, OpSP, f.Config.Types.Uintptr)
 			}
 			wbsym := &ExternSymbol{Sym: f.fe.Syslook("writeBarrier")}
 			wbaddr = f.Entry.NewValue1A(initpos, OpAddr, f.Config.Types.UInt32Ptr, wbsym, sb)
 			writebarrierptr = f.fe.Syslook("writebarrierptr")
 			typedmemmove = f.fe.Syslook("typedmemmove")
 			typedmemclr = f.fe.Syslook("typedmemclr")
 			const0 = f.ConstInt32(initpos, f.Config.Types.UInt32, 0)

 			// allocate auxiliary data structures for computing store order
 			sset = f.newSparseSet(f.NumValues())
 			defer f.retSparseSet(sset)
 			storeNumber = make([]int32, f.NumValues())
 		}

 		// order values in store order
 		b.Values = storeOrder(b.Values, sset, storeNumber)

 	again:
 		// find the start and end of the last contiguous WB store sequence.
 		// a branch will be inserted there. values after it will be moved
 		// to a new block.
 		var last *Value
 		var start, end int
 		values := b.Values
 	FindSeq:
 		for i := len(values) - 1; i >= 0; i-- {
 			w := values[i]
 			switch w.Op {
 			case OpStoreWB, OpMoveWB, OpZeroWB:
 				start = i
 				if last == nil {
 					last = w
 					end = i + 1
 				}
 			case OpVarDef, OpVarLive, OpVarKill:
 				continue
 			default:
 				if last == nil {
 					continue
 				}
 				break FindSeq
 			}
 		}
 		stores = append(stores[:0], b.Values[start:end]...) // copy to avoid aliasing
 		after = append(after[:0], b.Values[end:]...)
 		b.Values = b.Values[:start]

 		// find the memory before the WB stores
 		mem := stores[0].MemoryArg()
 		pos := stores[0].Pos
 		bThen := f.NewBlock(BlockPlain)
 		bElse := f.NewBlock(BlockPlain)
 		bEnd := f.NewBlock(b.Kind)
 		bThen.Pos = pos
 		bElse.Pos = pos
 		bEnd.Pos = b.Pos
 		b.Pos = pos

 		// set up control flow for end block
 		bEnd.SetControl(b.Control)
 		bEnd.Likely = b.Likely
 		for _, e := range b.Succs {
 			bEnd.Succs = append(bEnd.Succs, e)
 			e.b.Preds[e.i].b = bEnd
 		}

 		// set up control flow for write barrier test
 		// load word, test word, avoiding partial register write from load byte.
 		cfgtypes := &f.Config.Types
 		flag := b.NewValue2(pos, OpLoad, cfgtypes.UInt32, wbaddr, mem)
 		flag = b.NewValue2(pos, OpNeq32, cfgtypes.Bool, flag, const0)
 		b.Kind = BlockIf
 		b.SetControl(flag)
 		b.Likely = BranchUnlikely
 		b.Succs = b.Succs[:0]
 		b.AddEdgeTo(bThen)
 		b.AddEdgeTo(bElse)
 		bThen.AddEdgeTo(bEnd)
 		bElse.AddEdgeTo(bEnd)

 		// for each write barrier store, append write barrier version to bThen
 		// and simple store version to bElse
 		memThen := mem
 		memElse := mem
 		for _, w := range stores {
 			ptr := w.Args[0]
 			pos := w.Pos

 			var fn *obj.LSym
 			var typ *ExternSymbol
 			var val *Value
 			switch w.Op {
 			case OpStoreWB:
 				fn = writebarrierptr
 				val = w.Args[1]
 			case OpMoveWB:
 				fn = typedmemmove
 				val = w.Args[1]
 				typ = &ExternSymbol{Sym: w.Aux.(*types.Type).Symbol()}
 			case OpZeroWB:
 				fn = typedmemclr
 				typ = &ExternSymbol{Sym: w.Aux.(*types.Type).Symbol()}
 			case OpVarDef, OpVarLive, OpVarKill:
 			}

 			// then block: emit write barrier call
 			switch w.Op {
 			case OpStoreWB, OpMoveWB, OpZeroWB:
 				volatile := w.Op == OpMoveWB && isVolatile(val)
 				memThen = wbcall(pos, bThen, fn, typ, ptr, val, memThen, sp, sb, volatile)
 			case OpVarDef, OpVarLive, OpVarKill:
 				memThen = bThen.NewValue1A(pos, w.Op, types.TypeMem, w.Aux, memThen)
 			}

 			// else block: normal store
 			switch w.Op {
 			case OpStoreWB:
 				memElse = bElse.NewValue3A(pos, OpStore, types.TypeMem, w.Aux, ptr, val, memElse)
 			case OpMoveWB:
 				memElse = bElse.NewValue3I(pos, OpMove, types.TypeMem, w.AuxInt, ptr, val, memElse)
 				memElse.Aux = w.Aux
 			case OpZeroWB:
 				memElse = bElse.NewValue2I(pos, OpZero, types.TypeMem, w.AuxInt, ptr, memElse)
 				memElse.Aux = w.Aux
 			case OpVarDef, OpVarLive, OpVarKill:
 				memElse = bElse.NewValue1A(pos, w.Op, types.TypeMem, w.Aux, memElse)
 			}

 			if fn != nil {
 				// Note that we set up a writebarrier function call.
 				if !f.WBPos.IsKnown() {
 					f.WBPos = pos
 				}
 				if f.fe.Debug_wb() {
 					f.Warnl(pos, "write barrier")
 				}
 			}
 		}

 		// merge memory
 		// Splice memory Phi into the last memory of the original sequence,
 		// which may be used in subsequent blocks. Other memories in the
 		// sequence must be dead after this block since there can be only
 		// one memory live.
 		bEnd.Values = append(bEnd.Values, last)
 		last.Block = bEnd
 		last.reset(OpPhi)
 		last.Type = types.TypeMem
 		last.AddArg(memThen)
 		last.AddArg(memElse)
 		for _, w := range stores {
 			if w != last {
 				w.resetArgs()
 			}
 		}
 		for _, w := range stores {
 			if w != last {
 				f.freeValue(w)
 			}
 		}

 		// put values after the store sequence into the end block
 		bEnd.Values = append(bEnd.Values, after...)
 		for _, w := range after {
 			w.Block = bEnd
 		}

 		// if we have more stores in this block, do this block again
 		// check from end to beginning, to avoid quadratic behavior; issue 13554
 		// TODO: track the final value to avoid any looping here at all
 		for i := len(b.Values) - 1; i >= 0; i-- {
 			switch b.Values[i].Op {
 			case OpStoreWB, OpMoveWB, OpZeroWB:
 				goto again
 			}
 		}
 	}
 }

 // wbcall emits write barrier runtime call in b, returns memory.
 // if valIsVolatile, it moves val into temp space before making the call.
 func wbcall(pos src.XPos, b *Block, fn *obj.LSym, typ *ExternSymbol, ptr, val, mem, sp, sb *Value, valIsVolatile bool) *Value {
 	config := b.Func.Config

 	var tmp GCNode
 	if valIsVolatile {
 		// Copy to temp location if the source is volatile (will be clobbered by
 		// a function call). Marshaling the args to typedmemmove might clobber the
 		// value we're trying to move.
 		t := val.Type.ElemType()
 		tmp = b.Func.fe.Auto(val.Pos, t)
 		aux := &AutoSymbol{Node: tmp}
 		mem = b.NewValue1A(pos, OpVarDef, types.TypeMem, tmp, mem)
 		tmpaddr := b.NewValue1A(pos, OpAddr, t.PtrTo(), aux, sp)
 		siz := t.Size()
 		mem = b.NewValue3I(pos, OpMove, types.TypeMem, siz, tmpaddr, val, mem)
 		mem.Aux = t
 		val = tmpaddr
 	}

 	// put arguments on stack
 	off := config.ctxt.FixedFrameSize()

 	if typ != nil { // for typedmemmove
 		taddr := b.NewValue1A(pos, OpAddr, b.Func.Config.Types.Uintptr, typ, sb)
 		off = round(off, taddr.Type.Alignment())
 		arg := b.NewValue1I(pos, OpOffPtr, taddr.Type.PtrTo(), off, sp)
 		mem = b.NewValue3A(pos, OpStore, types.TypeMem, ptr.Type, arg, taddr, mem)
 		off += taddr.Type.Size()
 	}

 	off = round(off, ptr.Type.Alignment())
 	arg := b.NewValue1I(pos, OpOffPtr, ptr.Type.PtrTo(), off, sp)
 	mem = b.NewValue3A(pos, OpStore, types.TypeMem, ptr.Type, arg, ptr, mem)
 	off += ptr.Type.Size()

 	if val != nil {
 		off = round(off, val.Type.Alignment())
 		arg = b.NewValue1I(pos, OpOffPtr, val.Type.PtrTo(), off, sp)
 		mem = b.NewValue3A(pos, OpStore, types.TypeMem, val.Type, arg, val, mem)
 		off += val.Type.Size()
 	}
 	off = round(off, config.PtrSize)

 	// issue call
 	mem = b.NewValue1A(pos, OpStaticCall, types.TypeMem, fn, mem)
 	mem.AuxInt = off - config.ctxt.FixedFrameSize()

 	if valIsVolatile {
 		mem = b.NewValue1A(pos, OpVarKill, types.TypeMem, tmp, mem) // mark temp dead
 	}

 	return mem
 }

 // round to a multiple of r, r is a power of 2
 func round(o int64, r int64) int64 {
 	return (o + r - 1) &^ (r - 1)
 }

 // IsStackAddr returns whether v is known to be an address of a stack slot
 func IsStackAddr(v *Value) bool {
 	for v.Op == OpOffPtr || v.Op == OpAddPtr || v.Op == OpPtrIndex || v.Op == OpCopy {
 		v = v.Args[0]
 	}
 	switch v.Op {
 	case OpSP:
 		return true
 	case OpAddr:
 		return v.Args[0].Op == OpSP
 	}
 	return false
 }

 // isVolatile returns whether v is a pointer to argument region on stack which
 // will be clobbered by a function call.
 func isVolatile(v *Value) bool {
 	for v.Op == OpOffPtr || v.Op == OpAddPtr || v.Op == OpPtrIndex || v.Op == OpCopy {
 		v = v.Args[0]
 	}
 	return v.Op == OpSP
 }
	// Copyright 2016 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"
	"cmd/internal/obj"
	"cmd/internal/src"
	)

	// needwb returns whether we need write barrier for store op v.
	// v must be Store/Move/Zero.
	func needwb(v *Value) bool {
	t, ok := v.Aux.(*types.Type)
	if !ok {
	v.Fatalf("store aux is not a type: %s", v.LongString())
	}
	if !t.HasPointer() {
	return false
	}
	if IsStackAddr(v.Args[0]) {
	return false // write on stack doesn't need write barrier
	}
	return true
	}

	// writebarrier pass inserts write barriers for store ops (Store, Move, Zero)
	// when necessary (the condition above). It rewrites store ops to branches
	// and runtime calls, like
	//
	// if writeBarrier.enabled {
	// writebarrierptr(ptr, val)
	// } else {
	// *ptr = val
	// }
	//
	// A sequence of WB stores for many pointer fields of a single type will
	// be emitted together, with a single branch.
	func writebarrier(f *Func) {
	if !f.fe.UseWriteBarrier() {
	return
	}

	var sb, sp, wbaddr, const0 *Value
	var writebarrierptr, typedmemmove, typedmemclr *obj.LSym
	var stores, after []*Value
	var sset *sparseSet
	var storeNumber []int32

	for _, b := range f.Blocks { // range loop is safe since the blocks we added contain no stores to expand
	// first, identify all the stores that need to insert a write barrier.
	// mark them with WB ops temporarily. record presence of WB ops.
	hasStore := false
	for _, v := range b.Values {
	switch v.Op {
	case OpStore, OpMove, OpZero:
	if needwb(v) {
	switch v.Op {
	case OpStore:
	v.Op = OpStoreWB
	case OpMove:
	v.Op = OpMoveWB
	case OpZero:
	v.Op = OpZeroWB
	}
	hasStore = true
	}
	}
	}
	if !hasStore {
	continue
	}

	if wbaddr == nil {
	// lazily initialize global values for write barrier test and calls
	// find SB and SP values in entry block
	initpos := f.Entry.Pos
	for _, v := range f.Entry.Values {
	if v.Op == OpSB {
	sb = v
	}
	if v.Op == OpSP {
	sp = v
	}
	if sb != nil && sp != nil {
	break
	}
	}
	if sb == nil {
	sb = f.Entry.NewValue0(initpos, OpSB, f.Config.Types.Uintptr)
	}
	if sp == nil {
	sp = f.Entry.NewValue0(initpos, OpSP, f.Config.Types.Uintptr)
	}
	wbsym := &ExternSymbol{Sym: f.fe.Syslook("writeBarrier")}
	wbaddr = f.Entry.NewValue1A(initpos, OpAddr, f.Config.Types.UInt32Ptr, wbsym, sb)
	writebarrierptr = f.fe.Syslook("writebarrierptr")
	typedmemmove = f.fe.Syslook("typedmemmove")
	typedmemclr = f.fe.Syslook("typedmemclr")
	const0 = f.ConstInt32(initpos, f.Config.Types.UInt32, 0)

	// allocate auxiliary data structures for computing store order
	sset = f.newSparseSet(f.NumValues())
	defer f.retSparseSet(sset)
	storeNumber = make([]int32, f.NumValues())
	}

	// order values in store order
	b.Values = storeOrder(b.Values, sset, storeNumber)

	again:
	// find the start and end of the last contiguous WB store sequence.
	// a branch will be inserted there. values after it will be moved
	// to a new block.
	var last *Value
	var start, end int
	values := b.Values
	FindSeq:
	for i := len(values) - 1; i >= 0; i-- {
	w := values[i]
	switch w.Op {
	case OpStoreWB, OpMoveWB, OpZeroWB:
	start = i
	if last == nil {
	last = w
	end = i + 1
	}
	case OpVarDef, OpVarLive, OpVarKill:
	continue
	default:
	if last == nil {
	continue
	}
	break FindSeq
	}
	}
	stores = append(stores[:0], b.Values[start:end]...) // copy to avoid aliasing
	after = append(after[:0], b.Values[end:]...)
	b.Values = b.Values[:start]

	// find the memory before the WB stores
	mem := stores[0].MemoryArg()
	pos := stores[0].Pos
	bThen := f.NewBlock(BlockPlain)
	bElse := f.NewBlock(BlockPlain)
	bEnd := f.NewBlock(b.Kind)
	bThen.Pos = pos
	bElse.Pos = pos
	bEnd.Pos = b.Pos
	b.Pos = pos

	// set up control flow for end block
	bEnd.SetControl(b.Control)
	bEnd.Likely = b.Likely
	for _, e := range b.Succs {
	bEnd.Succs = append(bEnd.Succs, e)
	e.b.Preds[e.i].b = bEnd
	}

	// set up control flow for write barrier test
	// load word, test word, avoiding partial register write from load byte.
	cfgtypes := &f.Config.Types
	flag := b.NewValue2(pos, OpLoad, cfgtypes.UInt32, wbaddr, mem)
	flag = b.NewValue2(pos, OpNeq32, cfgtypes.Bool, flag, const0)
	b.Kind = BlockIf
	b.SetControl(flag)
	b.Likely = BranchUnlikely
	b.Succs = b.Succs[:0]
	b.AddEdgeTo(bThen)
	b.AddEdgeTo(bElse)
	bThen.AddEdgeTo(bEnd)
	bElse.AddEdgeTo(bEnd)

	// for each write barrier store, append write barrier version to bThen
	// and simple store version to bElse
	memThen := mem
	memElse := mem
	for _, w := range stores {
	ptr := w.Args[0]
	pos := w.Pos

	var fn *obj.LSym
	var typ *ExternSymbol
	var val *Value
	switch w.Op {
	case OpStoreWB:
	fn = writebarrierptr
	val = w.Args[1]
	case OpMoveWB:
	fn = typedmemmove
	val = w.Args[1]
	typ = &ExternSymbol{Sym: w.Aux.(*types.Type).Symbol()}
	case OpZeroWB:
	fn = typedmemclr
	typ = &ExternSymbol{Sym: w.Aux.(*types.Type).Symbol()}
	case OpVarDef, OpVarLive, OpVarKill:
	}

	// then block: emit write barrier call
	switch w.Op {
	case OpStoreWB, OpMoveWB, OpZeroWB:
	volatile := w.Op == OpMoveWB && isVolatile(val)
	memThen = wbcall(pos, bThen, fn, typ, ptr, val, memThen, sp, sb, volatile)
	case OpVarDef, OpVarLive, OpVarKill:
	memThen = bThen.NewValue1A(pos, w.Op, types.TypeMem, w.Aux, memThen)
	}

	// else block: normal store
	switch w.Op {
	case OpStoreWB:
	memElse = bElse.NewValue3A(pos, OpStore, types.TypeMem, w.Aux, ptr, val, memElse)
	case OpMoveWB:
	memElse = bElse.NewValue3I(pos, OpMove, types.TypeMem, w.AuxInt, ptr, val, memElse)
	memElse.Aux = w.Aux
	case OpZeroWB:
	memElse = bElse.NewValue2I(pos, OpZero, types.TypeMem, w.AuxInt, ptr, memElse)
	memElse.Aux = w.Aux
	case OpVarDef, OpVarLive, OpVarKill:
	memElse = bElse.NewValue1A(pos, w.Op, types.TypeMem, w.Aux, memElse)
	}

	if fn != nil {
	// Note that we set up a writebarrier function call.
	if !f.WBPos.IsKnown() {
	f.WBPos = pos
	}
	if f.fe.Debug_wb() {
	f.Warnl(pos, "write barrier")
	}
	}
	}

	// merge memory
	// Splice memory Phi into the last memory of the original sequence,
	// which may be used in subsequent blocks. Other memories in the
	// sequence must be dead after this block since there can be only
	// one memory live.
	bEnd.Values = append(bEnd.Values, last)
	last.Block = bEnd
	last.reset(OpPhi)
	last.Type = types.TypeMem
	last.AddArg(memThen)
	last.AddArg(memElse)
	for _, w := range stores {
	if w != last {
	w.resetArgs()
	}
	}
	for _, w := range stores {
	if w != last {
	f.freeValue(w)
	}
	}

	// put values after the store sequence into the end block
	bEnd.Values = append(bEnd.Values, after...)
	for _, w := range after {
	w.Block = bEnd
	}

	// if we have more stores in this block, do this block again
	// check from end to beginning, to avoid quadratic behavior; issue 13554
	// TODO: track the final value to avoid any looping here at all
	for i := len(b.Values) - 1; i >= 0; i-- {
	switch b.Values[i].Op {
	case OpStoreWB, OpMoveWB, OpZeroWB:
	goto again
	}
	}
	}
	}

	// wbcall emits write barrier runtime call in b, returns memory.
	// if valIsVolatile, it moves val into temp space before making the call.
	func wbcall(pos src.XPos, b Block, fn obj.LSym, typ ExternSymbol, ptr, val, mem, sp, sb Value, valIsVolatile bool) *Value {
	config := b.Func.Config

	var tmp GCNode
	if valIsVolatile {
	// Copy to temp location if the source is volatile (will be clobbered by
	// a function call). Marshaling the args to typedmemmove might clobber the
	// value we're trying to move.
	t := val.Type.ElemType()
	tmp = b.Func.fe.Auto(val.Pos, t)
	aux := &AutoSymbol{Node: tmp}
	mem = b.NewValue1A(pos, OpVarDef, types.TypeMem, tmp, mem)
	tmpaddr := b.NewValue1A(pos, OpAddr, t.PtrTo(), aux, sp)
	siz := t.Size()
	mem = b.NewValue3I(pos, OpMove, types.TypeMem, siz, tmpaddr, val, mem)
	mem.Aux = t
	val = tmpaddr
	}

	// put arguments on stack
	off := config.ctxt.FixedFrameSize()

	if typ != nil { // for typedmemmove
	taddr := b.NewValue1A(pos, OpAddr, b.Func.Config.Types.Uintptr, typ, sb)
	off = round(off, taddr.Type.Alignment())
	arg := b.NewValue1I(pos, OpOffPtr, taddr.Type.PtrTo(), off, sp)
	mem = b.NewValue3A(pos, OpStore, types.TypeMem, ptr.Type, arg, taddr, mem)
	off += taddr.Type.Size()
	}

	off = round(off, ptr.Type.Alignment())
	arg := b.NewValue1I(pos, OpOffPtr, ptr.Type.PtrTo(), off, sp)
	mem = b.NewValue3A(pos, OpStore, types.TypeMem, ptr.Type, arg, ptr, mem)
	off += ptr.Type.Size()

	if val != nil {
	off = round(off, val.Type.Alignment())
	arg = b.NewValue1I(pos, OpOffPtr, val.Type.PtrTo(), off, sp)
	mem = b.NewValue3A(pos, OpStore, types.TypeMem, val.Type, arg, val, mem)
	off += val.Type.Size()
	}
	off = round(off, config.PtrSize)

	// issue call
	mem = b.NewValue1A(pos, OpStaticCall, types.TypeMem, fn, mem)
	mem.AuxInt = off - config.ctxt.FixedFrameSize()

	if valIsVolatile {
	mem = b.NewValue1A(pos, OpVarKill, types.TypeMem, tmp, mem) // mark temp dead
	}

	return mem
	}

	// round to a multiple of r, r is a power of 2
	func round(o int64, r int64) int64 {
	return (o + r - 1) &^ (r - 1)
	}

	// IsStackAddr returns whether v is known to be an address of a stack slot
	func IsStackAddr(v *Value) bool {
	for v.Op == OpOffPtr \|\| v.Op == OpAddPtr \|\| v.Op == OpPtrIndex \|\| v.Op == OpCopy {
	v = v.Args[0]
	}
	switch v.Op {
	case OpSP:
	return true
	case OpAddr:
	return v.Args[0].Op == OpSP
	}
	return false
	}

	// isVolatile returns whether v is a pointer to argument region on stack which
	// will be clobbered by a function call.
	func isVolatile(v *Value) bool {
	for v.Op == OpOffPtr \|\| v.Op == OpAddPtr \|\| v.Op == OpPtrIndex \|\| v.Op == OpCopy {
	v = v.Args[0]
	}
	return v.Op == OpSP
	}