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path: root/src/cmd/compile/internal/ssa/writebarrier.go
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-rw-r--r--src/cmd/compile/internal/ssa/writebarrier.go616
1 files changed, 616 insertions, 0 deletions
diff --git a/src/cmd/compile/internal/ssa/writebarrier.go b/src/cmd/compile/internal/ssa/writebarrier.go
new file mode 100644
index 0000000..849c9e8
--- /dev/null
+++ b/src/cmd/compile/internal/ssa/writebarrier.go
@@ -0,0 +1,616 @@
+// 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
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-09-27 04:21:44 +0000