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/objabi"
 	"cmd/internal/src"
 	"fmt"
 )

 // A ZeroRegion records parts of an object which are known to be zero.
 // A ZeroRegion only applies to a single memory state.
 // Each bit in mask is set if the corresponding pointer-sized word of
 // the base object is known to be zero.
 // In other words, if mask & (1<<i) != 0, then [base+i*ptrSize, base+(i+1)*ptrSize)
 // is known to be zero.
 type ZeroRegion struct {
 	base *Value
 	mask uint64
 }

 // needwb reports whether we need write barrier for store op v.
 // v must be Store/Move/Zero.
 // zeroes provides known zero information (keyed by ID of memory-type values).
 func needwb(v *Value, zeroes map[ID]ZeroRegion) bool {
 	t, ok := v.Aux.(*types.Type)
 	if !ok {
 		v.Fatalf("store aux is not a type: %s", v.LongString())
 	}
 	if !t.HasPointers() {
 		return false
 	}
 	if IsStackAddr(v.Args[0]) {
 		return false // write on stack doesn't need write barrier
 	}
 	if v.Op == OpMove && IsReadOnlyGlobalAddr(v.Args[1]) && IsNewObject(v.Args[0], v.MemoryArg()) {
 		// Copying data from readonly memory into a fresh object doesn't need a write barrier.
 		return false
 	}
 	if v.Op == OpStore && IsGlobalAddr(v.Args[1]) {
 		// Storing pointers to non-heap locations into zeroed memory doesn't need a write barrier.
 		ptr := v.Args[0]
 		var off int64
 		size := v.Aux.(*types.Type).Size()
 		for ptr.Op == OpOffPtr {
 			off += ptr.AuxInt
 			ptr = ptr.Args[0]
 		}
 		ptrSize := v.Block.Func.Config.PtrSize
 		if off%ptrSize != 0 || size%ptrSize != 0 {
 			v.Fatalf("unaligned pointer write")
 		}
 		if off < 0 || off+size > 64*ptrSize {
 			// write goes off end of tracked offsets
 			return true
 		}
 		z := zeroes[v.MemoryArg().ID]
 		if ptr != z.base {
 			return true
 		}
 		for i := off; i < off+size; i += ptrSize {
 			if z.mask>>uint(i/ptrSize)&1 == 0 {
 				return true // not known to be zero
 			}
 		}
 		// All written locations are known to be zero - write barrier not needed.
 		return false
 	}
 	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 {
 //   gcWriteBarrier(ptr, val)	// Not a regular Go call
 // } 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 typedmemmove, typedmemclr, gcWriteBarrier *obj.LSym
 	var stores, after []*Value
 	var sset *sparseSet
 	var storeNumber []int32

 	zeroes := f.computeZeroMap()
 	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.
 		nWBops := 0 // count of temporarily created WB ops remaining to be rewritten in the current block
 		for _, v := range b.Values {
 			switch v.Op {
 			case OpStore, OpMove, OpZero:
 				if needwb(v, zeroes) {
 					switch v.Op {
 					case OpStore:
 						v.Op = OpStoreWB
 					case OpMove:
 						v.Op = OpMoveWB
 					case OpZero:
 						v.Op = OpZeroWB
 					}
 					nWBops++
 				}
 			}
 		}
 		if nWBops == 0 {
 			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
 			sp, sb = f.spSb()
 			wbsym := f.fe.Syslook("writeBarrier")
 			wbaddr = f.Entry.NewValue1A(initpos, OpAddr, f.Config.Types.UInt32Ptr, wbsym, sb)
 			gcWriteBarrier = f.fe.Syslook("gcWriteBarrier")
 			typedmemmove = f.fe.Syslook("typedmemmove")
 			typedmemclr = f.fe.Syslook("typedmemclr")
 			const0 = f.ConstInt32(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)

 		firstSplit := true
 	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.CopyControls(b)
 		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)
 		// TODO: For OpStoreWB and the buffered write barrier,
 		// we could move the write out of the write barrier,
 		// which would lead to fewer branches. We could do
 		// something similar to OpZeroWB, since the runtime
 		// could provide just the barrier half and then we
 		// could unconditionally do an OpZero (which could
 		// also generate better zeroing code). OpMoveWB is
 		// trickier and would require changing how
 		// cgoCheckMemmove works.
 		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

 		// If the source of a MoveWB is volatile (will be clobbered by a
 		// function call), we need to copy it to a temporary location, as
 		// marshaling the args of typedmemmove might clobber the value we're
 		// trying to move.
 		// Look for volatile source, copy it to temporary before we emit any
 		// call.
 		// It is unlikely to have more than one of them. Just do a linear
 		// search instead of using a map.
 		type volatileCopy struct {
 			src *Value // address of original volatile value
 			tmp *Value // address of temporary we've copied the volatile value into
 		}
 		var volatiles []volatileCopy
 	copyLoop:
 		for _, w := range stores {
 			if w.Op == OpMoveWB {
 				val := w.Args[1]
 				if isVolatile(val) {
 					for _, c := range volatiles {
 						if val == c.src {
 							continue copyLoop // already copied
 						}
 					}

 					t := val.Type.Elem()
 					tmp := f.fe.Auto(w.Pos, t)
 					memThen = bThen.NewValue1A(w.Pos, OpVarDef, types.TypeMem, tmp, memThen)
 					tmpaddr := bThen.NewValue2A(w.Pos, OpLocalAddr, t.PtrTo(), tmp, sp, memThen)
 					siz := t.Size()
 					memThen = bThen.NewValue3I(w.Pos, OpMove, types.TypeMem, siz, tmpaddr, val, memThen)
 					memThen.Aux = t
 					volatiles = append(volatiles, volatileCopy{val, tmpaddr})
 				}
 			}
 		}

 		for _, w := range stores {
 			ptr := w.Args[0]
 			pos := w.Pos

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

 			// then block: emit write barrier call
 			switch w.Op {
 			case OpStoreWB, OpMoveWB, OpZeroWB:
 				if w.Op == OpStoreWB {
 					memThen = bThen.NewValue3A(pos, OpWB, types.TypeMem, gcWriteBarrier, ptr, val, memThen)
 				} else {
 					srcval := val
 					if w.Op == OpMoveWB && isVolatile(srcval) {
 						for _, c := range volatiles {
 							if srcval == c.src {
 								srcval = c.tmp
 								break
 							}
 						}
 					}
 					memThen = wbcall(pos, bThen, fn, typ, ptr, srcval, memThen, sp, sb)
 				}
 				// Note that we set up a writebarrier function call.
 				f.fe.SetWBPos(pos)
 			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)
 			}
 		}

 		// mark volatile temps dead
 		for _, c := range volatiles {
 			tmpNode := c.tmp.Aux
 			memThen = bThen.NewValue1A(memThen.Pos, OpVarKill, types.TypeMem, tmpNode, memThen)
 		}

 		// 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.Pos = last.Pos.WithNotStmt()
 		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
 		}

 		// Preemption is unsafe between loading the write
 		// barrier-enabled flag and performing the write
 		// because that would allow a GC phase transition,
 		// which would invalidate the flag. Remember the
 		// conditional block so liveness analysis can disable
 		// safe-points. This is somewhat subtle because we're
 		// splitting b bottom-up.
 		if firstSplit {
 			// Add b itself.
 			b.Func.WBLoads = append(b.Func.WBLoads, b)
 			firstSplit = false
 		} else {
 			// We've already split b, so we just pushed a
 			// write barrier test into bEnd.
 			b.Func.WBLoads = append(b.Func.WBLoads, bEnd)
 		}

 		// if we have more stores in this block, do this block again
 		if nWBops > 0 {
 			goto again
 		}
 	}
 }

 // computeZeroMap returns a map from an ID of a memory value to
 // a set of locations that are known to be zeroed at that memory value.
 func (f *Func) computeZeroMap() map[ID]ZeroRegion {
 	ptrSize := f.Config.PtrSize
 	// Keep track of which parts of memory are known to be zero.
 	// This helps with removing write barriers for various initialization patterns.
 	// This analysis is conservative. We only keep track, for each memory state, of
 	// which of the first 64 words of a single object are known to be zero.
 	zeroes := map[ID]ZeroRegion{}
 	// Find new objects.
 	for _, b := range f.Blocks {
 		for _, v := range b.Values {
 			if v.Op != OpLoad {
 				continue
 			}
 			mem := v.MemoryArg()
 			if IsNewObject(v, mem) {
 				nptr := v.Type.Elem().Size() / ptrSize
 				if nptr > 64 {
 					nptr = 64
 				}
 				zeroes[mem.ID] = ZeroRegion{base: v, mask: 1<<uint(nptr) - 1}
 			}
 		}
 	}
 	// Find stores to those new objects.
 	for {
 		changed := false
 		for _, b := range f.Blocks {
 			// Note: iterating forwards helps convergence, as values are
 			// typically (but not always!) in store order.
 			for _, v := range b.Values {
 				if v.Op != OpStore {
 					continue
 				}
 				z, ok := zeroes[v.MemoryArg().ID]
 				if !ok {
 					continue
 				}
 				ptr := v.Args[0]
 				var off int64
 				size := v.Aux.(*types.Type).Size()
 				for ptr.Op == OpOffPtr {
 					off += ptr.AuxInt
 					ptr = ptr.Args[0]
 				}
 				if ptr != z.base {
 					// Different base object - we don't know anything.
 					// We could even be writing to the base object we know
 					// about, but through an aliased but offset pointer.
 					// So we have to throw all the zero information we have away.
 					continue
 				}
 				// Round to cover any partially written pointer slots.
 				// Pointer writes should never be unaligned like this, but non-pointer
 				// writes to pointer-containing types will do this.
 				if d := off % ptrSize; d != 0 {
 					off -= d
 					size += d
 				}
 				if d := size % ptrSize; d != 0 {
 					size += ptrSize - d
 				}
 				// Clip to the 64 words that we track.
 				min := off
 				max := off + size
 				if min < 0 {
 					min = 0
 				}
 				if max > 64*ptrSize {
 					max = 64 * ptrSize
 				}
 				// Clear bits for parts that we are writing (and hence
 				// will no longer necessarily be zero).
 				for i := min; i < max; i += ptrSize {
 					bit := i / ptrSize
 					z.mask &^= 1 << uint(bit)
 				}
 				if z.mask == 0 {
 					// No more known zeros - don't bother keeping.
 					continue
 				}
 				// Save updated known zero contents for new store.
 				if zeroes[v.ID] != z {
 					zeroes[v.ID] = z
 					changed = true
 				}
 			}
 		}
 		if !changed {
 			break
 		}
 	}
 	if f.pass.debug > 0 {
 		fmt.Printf("func %s\n", f.Name)
 		for mem, z := range zeroes {
 			fmt.Printf("  memory=v%d ptr=%v zeromask=%b\n", mem, z.base, z.mask)
 		}
 	}
 	return zeroes
 }

 // wbcall emits write barrier runtime call in b, returns memory.
 func wbcall(pos src.XPos, b *Block, fn, typ *obj.LSym, ptr, val, mem, sp, sb *Value) *Value {
 	config := b.Func.Config

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

 	var ACArgs []Param
 	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)
 		ACArgs = append(ACArgs, Param{Type: b.Func.Config.Types.Uintptr, Offset: int32(off)})
 		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)
 	ACArgs = append(ACArgs, Param{Type: ptr.Type, Offset: int32(off)})
 	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)
 		ACArgs = append(ACArgs, Param{Type: val.Type, Offset: int32(off)})
 		off += val.Type.Size()
 	}
 	off = round(off, config.PtrSize)

 	// issue call
 	mem = b.NewValue1A(pos, OpStaticCall, types.TypeMem, StaticAuxCall(fn, ACArgs, nil), mem)
 	mem.AuxInt = off - config.ctxt.FixedFrameSize()
 	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 reports 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, OpLocalAddr, OpSelectNAddr:
 		return true
 	}
 	return false
 }

 // IsGlobalAddr reports whether v is known to be an address of a global (or nil).
 func IsGlobalAddr(v *Value) bool {
 	if v.Op == OpAddr && v.Args[0].Op == OpSB {
 		return true // address of a global
 	}
 	if v.Op == OpConstNil {
 		return true
 	}
 	if v.Op == OpLoad && IsReadOnlyGlobalAddr(v.Args[0]) {
 		return true // loading from a read-only global - the resulting address can't be a heap address.
 	}
 	return false
 }

 // IsReadOnlyGlobalAddr reports whether v is known to be an address of a read-only global.
 func IsReadOnlyGlobalAddr(v *Value) bool {
 	if v.Op == OpConstNil {
 		// Nil pointers are read only. See issue 33438.
 		return true
 	}
 	if v.Op == OpAddr && v.Aux.(*obj.LSym).Type == objabi.SRODATA {
 		return true
 	}
 	return false
 }

 // IsNewObject reports whether v is a pointer to a freshly allocated & zeroed object at memory state mem.
 func IsNewObject(v *Value, mem *Value) bool {
 	if v.Op != OpLoad {
 		return false
 	}
 	if v.MemoryArg() != mem {
 		return false
 	}
 	if mem.Op != OpStaticCall {
 		return false
 	}
 	if !isSameCall(mem.Aux, "runtime.newobject") {
 		return false
 	}
 	if v.Args[0].Op != OpOffPtr {
 		return false
 	}
 	if v.Args[0].Args[0].Op != OpSP {
 		return false
 	}
 	c := v.Block.Func.Config
 	if v.Args[0].AuxInt != c.ctxt.FixedFrameSize()+c.RegSize { // offset of return value
 		return false
 	}
 	return true
 }

 // IsSanitizerSafeAddr reports whether v is known to be an address
 // that doesn't need instrumentation.
 func IsSanitizerSafeAddr(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, OpLocalAddr, OpSelectNAddr:
 		// Stack addresses are always safe.
 		return true
 	case OpITab, OpStringPtr, OpGetClosurePtr:
 		// Itabs, string data, and closure fields are
 		// read-only once initialized.
 		return true
 	case OpAddr:
 		return v.Aux.(*obj.LSym).Type == objabi.SRODATA
 	}
 	return false
 }

 // isVolatile reports 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.Op == OpSelectNAddr {
 		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/objabi"
	"cmd/internal/src"
	"fmt"
	)

	// A ZeroRegion records parts of an object which are known to be zero.
	// A ZeroRegion only applies to a single memory state.
	// Each bit in mask is set if the corresponding pointer-sized word of
	// the base object is known to be zero.
	// In other words, if mask & (1<<i) != 0, then [base+iptrSize, base+(i+1)ptrSize)
	// is known to be zero.
	type ZeroRegion struct {
	base *Value
	mask uint64
	}

	// needwb reports whether we need write barrier for store op v.
	// v must be Store/Move/Zero.
	// zeroes provides known zero information (keyed by ID of memory-type values).
	func needwb(v *Value, zeroes map[ID]ZeroRegion) bool {
	t, ok := v.Aux.(*types.Type)
	if !ok {
	v.Fatalf("store aux is not a type: %s", v.LongString())
	}
	if !t.HasPointers() {
	return false
	}
	if IsStackAddr(v.Args[0]) {
	return false // write on stack doesn't need write barrier
	}
	if v.Op == OpMove && IsReadOnlyGlobalAddr(v.Args[1]) && IsNewObject(v.Args[0], v.MemoryArg()) {
	// Copying data from readonly memory into a fresh object doesn't need a write barrier.
	return false
	}
	if v.Op == OpStore && IsGlobalAddr(v.Args[1]) {
	// Storing pointers to non-heap locations into zeroed memory doesn't need a write barrier.
	ptr := v.Args[0]
	var off int64
	size := v.Aux.(*types.Type).Size()
	for ptr.Op == OpOffPtr {
	off += ptr.AuxInt
	ptr = ptr.Args[0]
	}
	ptrSize := v.Block.Func.Config.PtrSize
	if off%ptrSize != 0 \|\| size%ptrSize != 0 {
	v.Fatalf("unaligned pointer write")
	}
	if off < 0 \|\| off+size > 64*ptrSize {
	// write goes off end of tracked offsets
	return true
	}
	z := zeroes[v.MemoryArg().ID]
	if ptr != z.base {
	return true
	}
	for i := off; i < off+size; i += ptrSize {
	if z.mask>>uint(i/ptrSize)&1 == 0 {
	return true // not known to be zero
	}
	}
	// All written locations are known to be zero - write barrier not needed.
	return false
	}
	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 {
	// gcWriteBarrier(ptr, val) // Not a regular Go call
	// } 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 typedmemmove, typedmemclr, gcWriteBarrier *obj.LSym
	var stores, after []*Value
	var sset *sparseSet
	var storeNumber []int32

	zeroes := f.computeZeroMap()
	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.
	nWBops := 0 // count of temporarily created WB ops remaining to be rewritten in the current block
	for _, v := range b.Values {
	switch v.Op {
	case OpStore, OpMove, OpZero:
	if needwb(v, zeroes) {
	switch v.Op {
	case OpStore:
	v.Op = OpStoreWB
	case OpMove:
	v.Op = OpMoveWB
	case OpZero:
	v.Op = OpZeroWB
	}
	nWBops++
	}
	}
	}
	if nWBops == 0 {
	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
	sp, sb = f.spSb()
	wbsym := f.fe.Syslook("writeBarrier")
	wbaddr = f.Entry.NewValue1A(initpos, OpAddr, f.Config.Types.UInt32Ptr, wbsym, sb)
	gcWriteBarrier = f.fe.Syslook("gcWriteBarrier")
	typedmemmove = f.fe.Syslook("typedmemmove")
	typedmemclr = f.fe.Syslook("typedmemclr")
	const0 = f.ConstInt32(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)

	firstSplit := true
	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.CopyControls(b)
	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)
	// TODO: For OpStoreWB and the buffered write barrier,
	// we could move the write out of the write barrier,
	// which would lead to fewer branches. We could do
	// something similar to OpZeroWB, since the runtime
	// could provide just the barrier half and then we
	// could unconditionally do an OpZero (which could
	// also generate better zeroing code). OpMoveWB is
	// trickier and would require changing how
	// cgoCheckMemmove works.
	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

	// If the source of a MoveWB is volatile (will be clobbered by a
	// function call), we need to copy it to a temporary location, as
	// marshaling the args of typedmemmove might clobber the value we're
	// trying to move.
	// Look for volatile source, copy it to temporary before we emit any
	// call.
	// It is unlikely to have more than one of them. Just do a linear
	// search instead of using a map.
	type volatileCopy struct {
	src *Value // address of original volatile value
	tmp *Value // address of temporary we've copied the volatile value into
	}
	var volatiles []volatileCopy
	copyLoop:
	for _, w := range stores {
	if w.Op == OpMoveWB {
	val := w.Args[1]
	if isVolatile(val) {
	for _, c := range volatiles {
	if val == c.src {
	continue copyLoop // already copied
	}
	}

	t := val.Type.Elem()
	tmp := f.fe.Auto(w.Pos, t)
	memThen = bThen.NewValue1A(w.Pos, OpVarDef, types.TypeMem, tmp, memThen)
	tmpaddr := bThen.NewValue2A(w.Pos, OpLocalAddr, t.PtrTo(), tmp, sp, memThen)
	siz := t.Size()
	memThen = bThen.NewValue3I(w.Pos, OpMove, types.TypeMem, siz, tmpaddr, val, memThen)
	memThen.Aux = t
	volatiles = append(volatiles, volatileCopy{val, tmpaddr})
	}
	}
	}

	for _, w := range stores {
	ptr := w.Args[0]
	pos := w.Pos

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

	// then block: emit write barrier call
	switch w.Op {
	case OpStoreWB, OpMoveWB, OpZeroWB:
	if w.Op == OpStoreWB {
	memThen = bThen.NewValue3A(pos, OpWB, types.TypeMem, gcWriteBarrier, ptr, val, memThen)
	} else {
	srcval := val
	if w.Op == OpMoveWB && isVolatile(srcval) {
	for _, c := range volatiles {
	if srcval == c.src {
	srcval = c.tmp
	break
	}
	}
	}
	memThen = wbcall(pos, bThen, fn, typ, ptr, srcval, memThen, sp, sb)
	}
	// Note that we set up a writebarrier function call.
	f.fe.SetWBPos(pos)
	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)
	}
	}

	// mark volatile temps dead
	for _, c := range volatiles {
	tmpNode := c.tmp.Aux
	memThen = bThen.NewValue1A(memThen.Pos, OpVarKill, types.TypeMem, tmpNode, memThen)
	}

	// 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.Pos = last.Pos.WithNotStmt()
	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
	}

	// Preemption is unsafe between loading the write
	// barrier-enabled flag and performing the write
	// because that would allow a GC phase transition,
	// which would invalidate the flag. Remember the
	// conditional block so liveness analysis can disable
	// safe-points. This is somewhat subtle because we're
	// splitting b bottom-up.
	if firstSplit {
	// Add b itself.
	b.Func.WBLoads = append(b.Func.WBLoads, b)
	firstSplit = false
	} else {
	// We've already split b, so we just pushed a
	// write barrier test into bEnd.
	b.Func.WBLoads = append(b.Func.WBLoads, bEnd)
	}

	// if we have more stores in this block, do this block again
	if nWBops > 0 {
	goto again
	}
	}
	}

	// computeZeroMap returns a map from an ID of a memory value to
	// a set of locations that are known to be zeroed at that memory value.
	func (f *Func) computeZeroMap() map[ID]ZeroRegion {
	ptrSize := f.Config.PtrSize
	// Keep track of which parts of memory are known to be zero.
	// This helps with removing write barriers for various initialization patterns.
	// This analysis is conservative. We only keep track, for each memory state, of
	// which of the first 64 words of a single object are known to be zero.
	zeroes := map[ID]ZeroRegion{}
	// Find new objects.
	for _, b := range f.Blocks {
	for _, v := range b.Values {
	if v.Op != OpLoad {
	continue
	}
	mem := v.MemoryArg()
	if IsNewObject(v, mem) {
	nptr := v.Type.Elem().Size() / ptrSize
	if nptr > 64 {
	nptr = 64
	}
	zeroes[mem.ID] = ZeroRegion{base: v, mask: 1<<uint(nptr) - 1}
	}
	}
	}
	// Find stores to those new objects.
	for {
	changed := false
	for _, b := range f.Blocks {
	// Note: iterating forwards helps convergence, as values are
	// typically (but not always!) in store order.
	for _, v := range b.Values {
	if v.Op != OpStore {
	continue
	}
	z, ok := zeroes[v.MemoryArg().ID]
	if !ok {
	continue
	}
	ptr := v.Args[0]
	var off int64
	size := v.Aux.(*types.Type).Size()
	for ptr.Op == OpOffPtr {
	off += ptr.AuxInt
	ptr = ptr.Args[0]
	}
	if ptr != z.base {
	// Different base object - we don't know anything.
	// We could even be writing to the base object we know
	// about, but through an aliased but offset pointer.
	// So we have to throw all the zero information we have away.
	continue
	}
	// Round to cover any partially written pointer slots.
	// Pointer writes should never be unaligned like this, but non-pointer
	// writes to pointer-containing types will do this.
	if d := off % ptrSize; d != 0 {
	off -= d
	size += d
	}
	if d := size % ptrSize; d != 0 {
	size += ptrSize - d
	}
	// Clip to the 64 words that we track.
	min := off
	max := off + size
	if min < 0 {
	min = 0
	}
	if max > 64*ptrSize {
	max = 64 * ptrSize
	}
	// Clear bits for parts that we are writing (and hence
	// will no longer necessarily be zero).
	for i := min; i < max; i += ptrSize {
	bit := i / ptrSize
	z.mask &^= 1 << uint(bit)
	}
	if z.mask == 0 {
	// No more known zeros - don't bother keeping.
	continue
	}
	// Save updated known zero contents for new store.
	if zeroes[v.ID] != z {
	zeroes[v.ID] = z
	changed = true
	}
	}
	}
	if !changed {
	break
	}
	}
	if f.pass.debug > 0 {
	fmt.Printf("func %s\n", f.Name)
	for mem, z := range zeroes {
	fmt.Printf(" memory=v%d ptr=%v zeromask=%b\n", mem, z.base, z.mask)
	}
	}
	return zeroes
	}

	// wbcall emits write barrier runtime call in b, returns memory.
	func wbcall(pos src.XPos, b Block, fn, typ obj.LSym, ptr, val, mem, sp, sb Value) Value {
	config := b.Func.Config

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

	var ACArgs []Param
	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)
	ACArgs = append(ACArgs, Param{Type: b.Func.Config.Types.Uintptr, Offset: int32(off)})
	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)
	ACArgs = append(ACArgs, Param{Type: ptr.Type, Offset: int32(off)})
	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)
	ACArgs = append(ACArgs, Param{Type: val.Type, Offset: int32(off)})
	off += val.Type.Size()
	}
	off = round(off, config.PtrSize)

	// issue call
	mem = b.NewValue1A(pos, OpStaticCall, types.TypeMem, StaticAuxCall(fn, ACArgs, nil), mem)
	mem.AuxInt = off - config.ctxt.FixedFrameSize()
	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 reports 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, OpLocalAddr, OpSelectNAddr:
	return true
	}
	return false
	}

	// IsGlobalAddr reports whether v is known to be an address of a global (or nil).
	func IsGlobalAddr(v *Value) bool {
	if v.Op == OpAddr && v.Args[0].Op == OpSB {
	return true // address of a global
	}
	if v.Op == OpConstNil {
	return true
	}
	if v.Op == OpLoad && IsReadOnlyGlobalAddr(v.Args[0]) {
	return true // loading from a read-only global - the resulting address can't be a heap address.
	}
	return false
	}

	// IsReadOnlyGlobalAddr reports whether v is known to be an address of a read-only global.
	func IsReadOnlyGlobalAddr(v *Value) bool {
	if v.Op == OpConstNil {
	// Nil pointers are read only. See issue 33438.
	return true
	}
	if v.Op == OpAddr && v.Aux.(*obj.LSym).Type == objabi.SRODATA {
	return true
	}
	return false
	}

	// IsNewObject reports whether v is a pointer to a freshly allocated & zeroed object at memory state mem.
	func IsNewObject(v Value, mem Value) bool {
	if v.Op != OpLoad {
	return false
	}
	if v.MemoryArg() != mem {
	return false
	}
	if mem.Op != OpStaticCall {
	return false
	}
	if !isSameCall(mem.Aux, "runtime.newobject") {
	return false
	}
	if v.Args[0].Op != OpOffPtr {
	return false
	}
	if v.Args[0].Args[0].Op != OpSP {
	return false
	}
	c := v.Block.Func.Config
	if v.Args[0].AuxInt != c.ctxt.FixedFrameSize()+c.RegSize { // offset of return value
	return false
	}
	return true
	}

	// IsSanitizerSafeAddr reports whether v is known to be an address
	// that doesn't need instrumentation.
	func IsSanitizerSafeAddr(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, OpLocalAddr, OpSelectNAddr:
	// Stack addresses are always safe.
	return true
	case OpITab, OpStringPtr, OpGetClosurePtr:
	// Itabs, string data, and closure fields are
	// read-only once initialized.
	return true
	case OpAddr:
	return v.Aux.(*obj.LSym).Type == objabi.SRODATA
	}
	return false
	}

	// isVolatile reports 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.Op == OpSelectNAddr {
	v = v.Args[0]
	}
	return v.Op == OpSP
	}