go plive 源码
golang plive 代码
文件路径：/src/cmd/compile/internal/liveness/plive.go
// Copyright 2013 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.
// Garbage collector liveness bitmap generation.
// The command line flag -live causes this code to print debug information.
// The levels are:
//
// -live (aka -live=1): print liveness lists as code warnings at safe points
// -live=2: print an assembly listing with liveness annotations
//
// Each level includes the earlier output as well.
package liveness
import (
"fmt"
"os"
"sort"
"strings"
"cmd/compile/internal/abi"
"cmd/compile/internal/base"
"cmd/compile/internal/bitvec"
"cmd/compile/internal/ir"
"cmd/compile/internal/objw"
"cmd/compile/internal/reflectdata"
"cmd/compile/internal/ssa"
"cmd/compile/internal/typebits"
"cmd/compile/internal/types"
"cmd/internal/notsha256"
"cmd/internal/obj"
"cmd/internal/objabi"
"cmd/internal/src"
)
// OpVarDef is an annotation for the liveness analysis, marking a place
// where a complete initialization (definition) of a variable begins.
// Since the liveness analysis can see initialization of single-word
// variables quite easy, OpVarDef is only needed for multi-word
// variables satisfying isfat(n.Type). For simplicity though, buildssa
// emits OpVarDef regardless of variable width.
//
// An 'OpVarDef x' annotation in the instruction stream tells the liveness
// analysis to behave as though the variable x is being initialized at that
// point in the instruction stream. The OpVarDef must appear before the
// actual (multi-instruction) initialization, and it must also appear after
// any uses of the previous value, if any. For example, if compiling:
//
// x = x[1:]
//
// it is important to generate code like:
//
// base, len, cap = pieces of x[1:]
// OpVarDef x
// x = {base, len, cap}
//
// If instead the generated code looked like:
//
// OpVarDef x
// base, len, cap = pieces of x[1:]
// x = {base, len, cap}
//
// then the liveness analysis would decide the previous value of x was
// unnecessary even though it is about to be used by the x[1:] computation.
// Similarly, if the generated code looked like:
//
// base, len, cap = pieces of x[1:]
// x = {base, len, cap}
// OpVarDef x
//
// then the liveness analysis will not preserve the new value of x, because
// the OpVarDef appears to have "overwritten" it.
//
// OpVarDef is a bit of a kludge to work around the fact that the instruction
// stream is working on single-word values but the liveness analysis
// wants to work on individual variables, which might be multi-word
// aggregates. It might make sense at some point to look into letting
// the liveness analysis work on single-word values as well, although
// there are complications around interface values, slices, and strings,
// all of which cannot be treated as individual words.
//
// OpVarKill is the opposite of OpVarDef: it marks a value as no longer needed,
// even if its address has been taken. That is, an OpVarKill annotation asserts
// that its argument is certainly dead, for use when the liveness analysis
// would not otherwise be able to deduce that fact.
// TODO: get rid of OpVarKill here. It's useful for stack frame allocation
// so the compiler can allocate two temps to the same location. Here it's now
// useless, since the implementation of stack objects.
// blockEffects summarizes the liveness effects on an SSA block.
type blockEffects struct {
// Computed during Liveness.prologue using only the content of
// individual blocks:
//
// uevar: upward exposed variables (used before set in block)
// varkill: killed variables (set in block)
uevar bitvec.BitVec
varkill bitvec.BitVec
// Computed during Liveness.solve using control flow information:
//
// livein: variables live at block entry
// liveout: variables live at block exit
livein bitvec.BitVec
liveout bitvec.BitVec
}
// A collection of global state used by liveness analysis.
type liveness struct {
fn *ir.Func
f *ssa.Func
vars []*ir.Name
idx map[*ir.Name]int32
stkptrsize int64
be []blockEffects
// allUnsafe indicates that all points in this function are
// unsafe-points.
allUnsafe bool
// unsafePoints bit i is set if Value ID i is an unsafe-point
// (preemption is not allowed). Only valid if !allUnsafe.
unsafePoints bitvec.BitVec
// An array with a bit vector for each safe point in the
// current Block during liveness.epilogue. Indexed in Value
// order for that block. Additionally, for the entry block
// livevars[0] is the entry bitmap. liveness.compact moves
// these to stackMaps.
livevars []bitvec.BitVec
// livenessMap maps from safe points (i.e., CALLs) to their
// liveness map indexes.
livenessMap Map
stackMapSet bvecSet
stackMaps []bitvec.BitVec
cache progeffectscache
// partLiveArgs includes input arguments (PPARAM) that may
// be partially live. That is, it is considered live because
// a part of it is used, but we may not initialize all parts.
partLiveArgs map[*ir.Name]bool
doClobber bool // Whether to clobber dead stack slots in this function.
noClobberArgs bool // Do not clobber function arguments
}
// Map maps from *ssa.Value to LivenessIndex.
type Map struct {
Vals map[ssa.ID]objw.LivenessIndex
// The set of live, pointer-containing variables at the DeferReturn
// call (only set when open-coded defers are used).
DeferReturn objw.LivenessIndex
}
func (m *Map) reset() {
if m.Vals == nil {
m.Vals = make(map[ssa.ID]objw.LivenessIndex)
} else {
for k := range m.Vals {
delete(m.Vals, k)
}
}
m.DeferReturn = objw.LivenessDontCare
}
func (m *Map) set(v *ssa.Value, i objw.LivenessIndex) {
m.Vals[v.ID] = i
}
func (m Map) Get(v *ssa.Value) objw.LivenessIndex {
// If v isn't in the map, then it's a "don't care" and not an
// unsafe-point.
if idx, ok := m.Vals[v.ID]; ok {
return idx
}
return objw.LivenessIndex{StackMapIndex: objw.StackMapDontCare, IsUnsafePoint: false}
}
type progeffectscache struct {
retuevar []int32
tailuevar []int32
initialized bool
}
// shouldTrack reports whether the liveness analysis
// should track the variable n.
// We don't care about variables that have no pointers,
// nor do we care about non-local variables,
// nor do we care about empty structs (handled by the pointer check),
// nor do we care about the fake PAUTOHEAP variables.
func shouldTrack(n *ir.Name) bool {
return (n.Class == ir.PAUTO && n.Esc() != ir.EscHeap || n.Class == ir.PPARAM || n.Class == ir.PPARAMOUT) && n.Type().HasPointers()
}
// getvariables returns the list of on-stack variables that we need to track
// and a map for looking up indices by *Node.
func getvariables(fn *ir.Func) ([]*ir.Name, map[*ir.Name]int32) {
var vars []*ir.Name
for _, n := range fn.Dcl {
if shouldTrack(n) {
vars = append(vars, n)
}
}
idx := make(map[*ir.Name]int32, len(vars))
for i, n := range vars {
idx[n] = int32(i)
}
return vars, idx
}
func (lv *liveness) initcache() {
if lv.cache.initialized {
base.Fatalf("liveness cache initialized twice")
return
}
lv.cache.initialized = true
for i, node := range lv.vars {
switch node.Class {
case ir.PPARAM:
// A return instruction with a p.to is a tail return, which brings
// the stack pointer back up (if it ever went down) and then jumps
// to a new function entirely. That form of instruction must read
// all the parameters for correctness, and similarly it must not
// read the out arguments - they won't be set until the new
// function runs.
lv.cache.tailuevar = append(lv.cache.tailuevar, int32(i))
case ir.PPARAMOUT:
// All results are live at every return point.
// Note that this point is after escaping return values
// are copied back to the stack using their PAUTOHEAP references.
lv.cache.retuevar = append(lv.cache.retuevar, int32(i))
}
}
}
// A liveEffect is a set of flags that describe an instruction's
// liveness effects on a variable.
//
// The possible flags are:
//
// uevar - used by the instruction
// varkill - killed by the instruction (set)
//
// A kill happens after the use (for an instruction that updates a value, for example).
type liveEffect int
const (
uevar liveEffect = 1 << iota
varkill
)
// valueEffects returns the index of a variable in lv.vars and the
// liveness effects v has on that variable.
// If v does not affect any tracked variables, it returns -1, 0.
func (lv *liveness) valueEffects(v *ssa.Value) (int32, liveEffect) {
n, e := affectedVar(v)
if e == 0 || n == nil { // cheapest checks first
return -1, 0
}
// AllocFrame has dropped unused variables from
// lv.fn.Func.Dcl, but they might still be referenced by
// OpVarFoo pseudo-ops. Ignore them to prevent "lost track of
// variable" ICEs (issue 19632).
switch v.Op {
case ssa.OpVarDef, ssa.OpVarKill, ssa.OpVarLive, ssa.OpKeepAlive:
if !n.Used() {
return -1, 0
}
}
if n.Class == ir.PPARAM && !n.Addrtaken() && n.Type().Size() > int64(types.PtrSize) {
// Only aggregate-typed arguments that are not address-taken can be
// partially live.
lv.partLiveArgs[n] = true
}
var effect liveEffect
// Read is a read, obviously.
//
// Addr is a read also, as any subsequent holder of the pointer must be able
// to see all the values (including initialization) written so far.
// This also prevents a variable from "coming back from the dead" and presenting
// stale pointers to the garbage collector. See issue 28445.
if e&(ssa.SymRead|ssa.SymAddr) != 0 {
effect |= uevar
}
if e&ssa.SymWrite != 0 && (!isfat(n.Type()) || v.Op == ssa.OpVarDef) {
effect |= varkill
}
if effect == 0 {
return -1, 0
}
if pos, ok := lv.idx[n]; ok {
return pos, effect
}
return -1, 0
}
// affectedVar returns the *ir.Name node affected by v
func affectedVar(v *ssa.Value) (*ir.Name, ssa.SymEffect) {
// Special cases.
switch v.Op {
case ssa.OpLoadReg:
n, _ := ssa.AutoVar(v.Args[0])
return n, ssa.SymRead
case ssa.OpStoreReg:
n, _ := ssa.AutoVar(v)
return n, ssa.SymWrite
case ssa.OpArgIntReg:
// This forces the spill slot for the register to be live at function entry.
// one of the following holds for a function F with pointer-valued register arg X:
// 0. No GC (so an uninitialized spill slot is okay)
// 1. GC at entry of F. GC is precise, but the spills around morestack initialize X's spill slot
// 2. Stack growth at entry of F. Same as GC.
// 3. GC occurs within F itself. This has to be from preemption, and thus GC is conservative.
// a. X is in a register -- then X is seen, and the spill slot is also scanned conservatively.
// b. X is spilled -- the spill slot is initialized, and scanned conservatively
// c. X is not live -- the spill slot is scanned conservatively, and it may contain X from an earlier spill.
// 4. GC within G, transitively called from F
// a. X is live at call site, therefore is spilled, to its spill slot (which is live because of subsequent LoadReg).
// b. X is not live at call site -- but neither is its spill slot.
n, _ := ssa.AutoVar(v)
return n, ssa.SymRead
case ssa.OpVarLive:
return v.Aux.(*ir.Name), ssa.SymRead
case ssa.OpVarDef, ssa.OpVarKill:
return v.Aux.(*ir.Name), ssa.SymWrite
case ssa.OpKeepAlive:
n, _ := ssa.AutoVar(v.Args[0])
return n, ssa.SymRead
}
e := v.Op.SymEffect()
if e == 0 {
return nil, 0
}
switch a := v.Aux.(type) {
case nil, *obj.LSym:
// ok, but no node
return nil, e
case *ir.Name:
return a, e
default:
base.Fatalf("weird aux: %s", v.LongString())
return nil, e
}
}
type livenessFuncCache struct {
be []blockEffects
livenessMap Map
}
// Constructs a new liveness structure used to hold the global state of the
// liveness computation. The cfg argument is a slice of *BasicBlocks and the
// vars argument is a slice of *Nodes.
func newliveness(fn *ir.Func, f *ssa.Func, vars []*ir.Name, idx map[*ir.Name]int32, stkptrsize int64) *liveness {
lv := &liveness{
fn: fn,
f: f,
vars: vars,
idx: idx,
stkptrsize: stkptrsize,
}
// Significant sources of allocation are kept in the ssa.Cache
// and reused. Surprisingly, the bit vectors themselves aren't
// a major source of allocation, but the liveness maps are.
if lc, _ := f.Cache.Liveness.(*livenessFuncCache); lc == nil {
// Prep the cache so liveness can fill it later.
f.Cache.Liveness = new(livenessFuncCache)
} else {
if cap(lc.be) >= f.NumBlocks() {
lv.be = lc.be[:f.NumBlocks()]
}
lv.livenessMap = Map{Vals: lc.livenessMap.Vals, DeferReturn: objw.LivenessDontCare}
lc.livenessMap.Vals = nil
}
if lv.be == nil {
lv.be = make([]blockEffects, f.NumBlocks())
}
nblocks := int32(len(f.Blocks))
nvars := int32(len(vars))
bulk := bitvec.NewBulk(nvars, nblocks*7)
for _, b := range f.Blocks {
be := lv.blockEffects(b)
be.uevar = bulk.Next()
be.varkill = bulk.Next()
be.livein = bulk.Next()
be.liveout = bulk.Next()
}
lv.livenessMap.reset()
lv.markUnsafePoints()
lv.partLiveArgs = make(map[*ir.Name]bool)
lv.enableClobber()
return lv
}
func (lv *liveness) blockEffects(b *ssa.Block) *blockEffects {
return &lv.be[b.ID]
}
// Generates live pointer value maps for arguments and local variables. The
// this argument and the in arguments are always assumed live. The vars
// argument is a slice of *Nodes.
func (lv *liveness) pointerMap(liveout bitvec.BitVec, vars []*ir.Name, args, locals bitvec.BitVec) {
for i := int32(0); ; i++ {
i = liveout.Next(i)
if i < 0 {
break
}
node := vars[i]
switch node.Class {
case ir.PPARAM, ir.PPARAMOUT:
if !node.IsOutputParamInRegisters() {
if node.FrameOffset() < 0 {
lv.f.Fatalf("Node %v has frameoffset %d\n", node.Sym().Name, node.FrameOffset())
}
typebits.Set(node.Type(), node.FrameOffset(), args)
break
}
fallthrough // PPARAMOUT in registers acts memory-allocates like an AUTO
case ir.PAUTO:
typebits.Set(node.Type(), node.FrameOffset()+lv.stkptrsize, locals)
}
}
}
// IsUnsafe indicates that all points in this function are
// unsafe-points.
func IsUnsafe(f *ssa.Func) bool {
// The runtime assumes the only safe-points are function
// prologues (because that's how it used to be). We could and
// should improve that, but for now keep consider all points
// in the runtime unsafe. obj will add prologues and their
// safe-points.
//
// go:nosplit functions are similar. Since safe points used to
// be coupled with stack checks, go:nosplit often actually
// means "no safe points in this function".
return base.Flag.CompilingRuntime || f.NoSplit
}
// markUnsafePoints finds unsafe points and computes lv.unsafePoints.
func (lv *liveness) markUnsafePoints() {
if IsUnsafe(lv.f) {
// No complex analysis necessary.
lv.allUnsafe = true
return
}
lv.unsafePoints = bitvec.New(int32(lv.f.NumValues()))
// Mark architecture-specific unsafe points.
for _, b := range lv.f.Blocks {
for _, v := range b.Values {
if v.Op.UnsafePoint() {
lv.unsafePoints.Set(int32(v.ID))
}
}
}
// Mark write barrier unsafe points.
for _, wbBlock := range lv.f.WBLoads {
if wbBlock.Kind == ssa.BlockPlain && len(wbBlock.Values) == 0 {
// The write barrier block was optimized away
// but we haven't done dead block elimination.
// (This can happen in -N mode.)
continue
}
// Check that we have the expected diamond shape.
if len(wbBlock.Succs) != 2 {
lv.f.Fatalf("expected branch at write barrier block %v", wbBlock)
}
s0, s1 := wbBlock.Succs[0].Block(), wbBlock.Succs[1].Block()
if s0 == s1 {
// There's no difference between write barrier on and off.
// Thus there's no unsafe locations. See issue 26024.
continue
}
if s0.Kind != ssa.BlockPlain || s1.Kind != ssa.BlockPlain {
lv.f.Fatalf("expected successors of write barrier block %v to be plain", wbBlock)
}
if s0.Succs[0].Block() != s1.Succs[0].Block() {
lv.f.Fatalf("expected successors of write barrier block %v to converge", wbBlock)
}
// Flow backwards from the control value to find the
// flag load. We don't know what lowered ops we're
// looking for, but all current arches produce a
// single op that does the memory load from the flag
// address, so we look for that.
var load *ssa.Value
v := wbBlock.Controls[0]
for {
if sym, ok := v.Aux.(*obj.LSym); ok && sym == ir.Syms.WriteBarrier {
load = v
break
}
switch v.Op {
case ssa.Op386TESTL:
// 386 lowers Neq32 to (TESTL cond cond),
if v.Args[0] == v.Args[1] {
v = v.Args[0]
continue
}
case ssa.Op386MOVLload, ssa.OpARM64MOVWUload, ssa.OpPPC64MOVWZload, ssa.OpWasmI64Load32U:
// Args[0] is the address of the write
// barrier control. Ignore Args[1],
// which is the mem operand.
// TODO: Just ignore mem operands?
v = v.Args[0]
continue
}
// Common case: just flow backwards.
if len(v.Args) != 1 {
v.Fatalf("write barrier control value has more than one argument: %s", v.LongString())
}
v = v.Args[0]
}
// Mark everything after the load unsafe.
found := false
for _, v := range wbBlock.Values {
found = found || v == load
if found {
lv.unsafePoints.Set(int32(v.ID))
}
}
// Mark the two successor blocks unsafe. These come
// back together immediately after the direct write in
// one successor and the last write barrier call in
// the other, so there's no need to be more precise.
for _, succ := range wbBlock.Succs {
for _, v := range succ.Block().Values {
lv.unsafePoints.Set(int32(v.ID))
}
}
}
// Find uintptr -> unsafe.Pointer conversions and flood
// unsafeness back to a call (which is always a safe point).
//
// Looking for the uintptr -> unsafe.Pointer conversion has a
// few advantages over looking for unsafe.Pointer -> uintptr
// conversions:
//
// 1. We avoid needlessly blocking safe-points for
// unsafe.Pointer -> uintptr conversions that never go back to
// a Pointer.
//
// 2. We don't have to detect calls to reflect.Value.Pointer,
// reflect.Value.UnsafeAddr, and reflect.Value.InterfaceData,
// which are implicit unsafe.Pointer -> uintptr conversions.
// We can't even reliably detect this if there's an indirect
// call to one of these methods.
//
// TODO: For trivial unsafe.Pointer arithmetic, it would be
// nice to only flood as far as the unsafe.Pointer -> uintptr
// conversion, but it's hard to know which argument of an Add
// or Sub to follow.
var flooded bitvec.BitVec
var flood func(b *ssa.Block, vi int)
flood = func(b *ssa.Block, vi int) {
if flooded.N == 0 {
flooded = bitvec.New(int32(lv.f.NumBlocks()))
}
if flooded.Get(int32(b.ID)) {
return
}
for i := vi - 1; i >= 0; i-- {
v := b.Values[i]
if v.Op.IsCall() {
// Uintptrs must not contain live
// pointers across calls, so stop
// flooding.
return
}
lv.unsafePoints.Set(int32(v.ID))
}
if vi == len(b.Values) {
// We marked all values in this block, so no
// need to flood this block again.
flooded.Set(int32(b.ID))
}
for _, pred := range b.Preds {
flood(pred.Block(), len(pred.Block().Values))
}
}
for _, b := range lv.f.Blocks {
for i, v := range b.Values {
if !(v.Op == ssa.OpConvert && v.Type.IsPtrShaped()) {
continue
}
// Flood the unsafe-ness of this backwards
// until we hit a call.
flood(b, i+1)
}
}
}
// Returns true for instructions that must have a stack map.
//
// This does not necessarily mean the instruction is a safe-point. In
// particular, call Values can have a stack map in case the callee
// grows the stack, but not themselves be a safe-point.
func (lv *liveness) hasStackMap(v *ssa.Value) bool {
if !v.Op.IsCall() {
return false
}
// typedmemclr and typedmemmove are write barriers and
// deeply non-preemptible. They are unsafe points and
// hence should not have liveness maps.
if sym, ok := v.Aux.(*ssa.AuxCall); ok && (sym.Fn == ir.Syms.Typedmemclr || sym.Fn == ir.Syms.Typedmemmove) {
return false
}
return true
}
// Initializes the sets for solving the live variables. Visits all the
// instructions in each basic block to summarizes the information at each basic
// block
func (lv *liveness) prologue() {
lv.initcache()
for _, b := range lv.f.Blocks {
be := lv.blockEffects(b)
// Walk the block instructions backward and update the block
// effects with the each prog effects.
for j := len(b.Values) - 1; j >= 0; j-- {
pos, e := lv.valueEffects(b.Values[j])
if e&varkill != 0 {
be.varkill.Set(pos)
be.uevar.Unset(pos)
}
if e&uevar != 0 {
be.uevar.Set(pos)
}
}
}
}
// Solve the liveness dataflow equations.
func (lv *liveness) solve() {
// These temporary bitvectors exist to avoid successive allocations and
// frees within the loop.
nvars := int32(len(lv.vars))
newlivein := bitvec.New(nvars)
newliveout := bitvec.New(nvars)
// Walk blocks in postorder ordering. This improves convergence.
po := lv.f.Postorder()
// Iterate through the blocks in reverse round-robin fashion. A work
// queue might be slightly faster. As is, the number of iterations is
// so low that it hardly seems to be worth the complexity.
for change := true; change; {
change = false
for _, b := range po {
be := lv.blockEffects(b)
newliveout.Clear()
switch b.Kind {
case ssa.BlockRet:
for _, pos := range lv.cache.retuevar {
newliveout.Set(pos)
}
case ssa.BlockRetJmp:
for _, pos := range lv.cache.tailuevar {
newliveout.Set(pos)
}
case ssa.BlockExit:
// panic exit - nothing to do
default:
// A variable is live on output from this block
// if it is live on input to some successor.
//
// out[b] = \bigcup_{s \in succ[b]} in[s]
newliveout.Copy(lv.blockEffects(b.Succs[0].Block()).livein)
for _, succ := range b.Succs[1:] {
newliveout.Or(newliveout, lv.blockEffects(succ.Block()).livein)
}
}
if !be.liveout.Eq(newliveout) {
change = true
be.liveout.Copy(newliveout)
}
// A variable is live on input to this block
// if it is used by this block, or live on output from this block and
// not set by the code in this block.
//
// in[b] = uevar[b] \cup (out[b] \setminus varkill[b])
newlivein.AndNot(be.liveout, be.varkill)
be.livein.Or(newlivein, be.uevar)
}
}
}
// Visits all instructions in a basic block and computes a bit vector of live
// variables at each safe point locations.
func (lv *liveness) epilogue() {
nvars := int32(len(lv.vars))
liveout := bitvec.New(nvars)
livedefer := bitvec.New(nvars) // always-live variables
// If there is a defer (that could recover), then all output
// parameters are live all the time. In addition, any locals
// that are pointers to heap-allocated output parameters are
// also always live (post-deferreturn code needs these
// pointers to copy values back to the stack).
// TODO: if the output parameter is heap-allocated, then we
// don't need to keep the stack copy live?
if lv.fn.HasDefer() {
for i, n := range lv.vars {
if n.Class == ir.PPARAMOUT {
if n.IsOutputParamHeapAddr() {
// Just to be paranoid. Heap addresses are PAUTOs.
base.Fatalf("variable %v both output param and heap output param", n)
}
if n.Heapaddr != nil {
// If this variable moved to the heap, then
// its stack copy is not live.
continue
}
// Note: zeroing is handled by zeroResults in walk.go.
livedefer.Set(int32(i))
}
if n.IsOutputParamHeapAddr() {
// This variable will be overwritten early in the function
// prologue (from the result of a mallocgc) but we need to
// zero it in case that malloc causes a stack scan.
n.SetNeedzero(true)
livedefer.Set(int32(i))
}
if n.OpenDeferSlot() {
// Open-coded defer args slots must be live
// everywhere in a function, since a panic can
// occur (almost) anywhere. Because it is live
// everywhere, it must be zeroed on entry.
livedefer.Set(int32(i))
// It was already marked as Needzero when created.
if !n.Needzero() {
base.Fatalf("all pointer-containing defer arg slots should have Needzero set")
}
}
}
}
// We must analyze the entry block first. The runtime assumes
// the function entry map is index 0. Conveniently, layout
// already ensured that the entry block is first.
if lv.f.Entry != lv.f.Blocks[0] {
lv.f.Fatalf("entry block must be first")
}
{
// Reserve an entry for function entry.
live := bitvec.New(nvars)
lv.livevars = append(lv.livevars, live)
}
for _, b := range lv.f.Blocks {
be := lv.blockEffects(b)
// Walk forward through the basic block instructions and
// allocate liveness maps for those instructions that need them.
for _, v := range b.Values {
if !lv.hasStackMap(v) {
continue
}
live := bitvec.New(nvars)
lv.livevars = append(lv.livevars, live)
}
// walk backward, construct maps at each safe point
index := int32(len(lv.livevars) - 1)
liveout.Copy(be.liveout)
for i := len(b.Values) - 1; i >= 0; i-- {
v := b.Values[i]
if lv.hasStackMap(v) {
// Found an interesting instruction, record the
// corresponding liveness information.
live := &lv.livevars[index]
live.Or(*live, liveout)
live.Or(*live, livedefer) // only for non-entry safe points
index--
}
// Update liveness information.
pos, e := lv.valueEffects(v)
if e&varkill != 0 {
liveout.Unset(pos)
}
if e&uevar != 0 {
liveout.Set(pos)
}
}
if b == lv.f.Entry {
if index != 0 {
base.Fatalf("bad index for entry point: %v", index)
}
// Check to make sure only input variables are live.
for i, n := range lv.vars {
if !liveout.Get(int32(i)) {
continue
}
if n.Class == ir.PPARAM {
continue // ok
}
base.FatalfAt(n.Pos(), "bad live variable at entry of %v: %L", lv.fn.Nname, n)
}
// Record live variables.
live := &lv.livevars[index]
live.Or(*live, liveout)
}
if lv.doClobber {
lv.clobber(b)
}
// The liveness maps for this block are now complete. Compact them.
lv.compact(b)
}
// If we have an open-coded deferreturn call, make a liveness map for it.
if lv.fn.OpenCodedDeferDisallowed() {
lv.livenessMap.DeferReturn = objw.LivenessDontCare
} else {
idx, _ := lv.stackMapSet.add(livedefer)
lv.livenessMap.DeferReturn = objw.LivenessIndex{
StackMapIndex: idx,
IsUnsafePoint: false,
}
}
// Done compacting. Throw out the stack map set.
lv.stackMaps = lv.stackMapSet.extractUnique()
lv.stackMapSet = bvecSet{}
// Useful sanity check: on entry to the function,
// the only things that can possibly be live are the
// input parameters.
for j, n := range lv.vars {
if n.Class != ir.PPARAM && lv.stackMaps[0].Get(int32(j)) {
lv.f.Fatalf("%v %L recorded as live on entry", lv.fn.Nname, n)
}
}
}
// Compact coalesces identical bitmaps from lv.livevars into the sets
// lv.stackMapSet.
//
// Compact clears lv.livevars.
//
// There are actually two lists of bitmaps, one list for the local variables and one
// list for the function arguments. Both lists are indexed by the same PCDATA
// index, so the corresponding pairs must be considered together when
// merging duplicates. The argument bitmaps change much less often during
// function execution than the local variable bitmaps, so it is possible that
// we could introduce a separate PCDATA index for arguments vs locals and
// then compact the set of argument bitmaps separately from the set of
// local variable bitmaps. As of 2014-04-02, doing this to the godoc binary
// is actually a net loss: we save about 50k of argument bitmaps but the new
// PCDATA tables cost about 100k. So for now we keep using a single index for
// both bitmap lists.
func (lv *liveness) compact(b *ssa.Block) {
pos := 0
if b == lv.f.Entry {
// Handle entry stack map.
lv.stackMapSet.add(lv.livevars[0])
pos++
}
for _, v := range b.Values {
hasStackMap := lv.hasStackMap(v)
isUnsafePoint := lv.allUnsafe || v.Op != ssa.OpClobber && lv.unsafePoints.Get(int32(v.ID))
idx := objw.LivenessIndex{StackMapIndex: objw.StackMapDontCare, IsUnsafePoint: isUnsafePoint}
if hasStackMap {
idx.StackMapIndex, _ = lv.stackMapSet.add(lv.livevars[pos])
pos++
}
if hasStackMap || isUnsafePoint {
lv.livenessMap.set(v, idx)
}
}
// Reset livevars.
lv.livevars = lv.livevars[:0]
}
func (lv *liveness) enableClobber() {
// The clobberdead experiment inserts code to clobber pointer slots in all
// the dead variables (locals and args) at every synchronous safepoint.
if !base.Flag.ClobberDead {
return
}
if lv.fn.Pragma&ir.CgoUnsafeArgs != 0 {
// C or assembly code uses the exact frame layout. Don't clobber.
return
}
if len(lv.vars) > 10000 || len(lv.f.Blocks) > 10000 {
// Be careful to avoid doing too much work.
// Bail if >10000 variables or >10000 blocks.
// Otherwise, giant functions make this experiment generate too much code.
return
}
if lv.f.Name == "forkAndExecInChild" {
// forkAndExecInChild calls vfork on some platforms.
// The code we add here clobbers parts of the stack in the child.
// When the parent resumes, it is using the same stack frame. But the
// child has clobbered stack variables that the parent needs. Boom!
// In particular, the sys argument gets clobbered.
return
}
if lv.f.Name == "wbBufFlush" ||
((lv.f.Name == "callReflect" || lv.f.Name == "callMethod") && lv.fn.ABIWrapper()) {
// runtime.wbBufFlush must not modify its arguments. See the comments
// in runtime/mwbbuf.go:wbBufFlush.
//
// reflect.callReflect and reflect.callMethod are called from special
// functions makeFuncStub and methodValueCall. The runtime expects
// that it can find the first argument (ctxt) at 0(SP) in makeFuncStub
// and methodValueCall's frame (see runtime/traceback.go:getArgInfo).
// Normally callReflect and callMethod already do not modify the
// argument, and keep it alive. But the compiler-generated ABI wrappers
// don't do that. Special case the wrappers to not clobber its arguments.
lv.noClobberArgs = true
}
if h := os.Getenv("GOCLOBBERDEADHASH"); h != "" {
// Clobber only functions where the hash of the function name matches a pattern.
// Useful for binary searching for a miscompiled function.
hstr := ""
for _, b := range notsha256.Sum256([]byte(lv.f.Name)) {
hstr += fmt.Sprintf("%08b", b)
}
if !strings.HasSuffix(hstr, h) {
return
}
fmt.Printf("\t\t\tCLOBBERDEAD %s\n", lv.f.Name)
}
lv.doClobber = true
}
// Inserts code to clobber pointer slots in all the dead variables (locals and args)
// at every synchronous safepoint in b.
func (lv *liveness) clobber(b *ssa.Block) {
// Copy block's values to a temporary.
oldSched := append([]*ssa.Value{}, b.Values...)
b.Values = b.Values[:0]
idx := 0
// Clobber pointer slots in all dead variables at entry.
if b == lv.f.Entry {
for len(oldSched) > 0 && len(oldSched[0].Args) == 0 {
// Skip argless ops. We need to skip at least
// the lowered ClosurePtr op, because it
// really wants to be first. This will also
// skip ops like InitMem and SP, which are ok.
b.Values = append(b.Values, oldSched[0])
oldSched = oldSched[1:]
}
clobber(lv, b, lv.livevars[0])
idx++
}
// Copy values into schedule, adding clobbering around safepoints.
for _, v := range oldSched {
if !lv.hasStackMap(v) {
b.Values = append(b.Values, v)
continue
}
clobber(lv, b, lv.livevars[idx])
b.Values = append(b.Values, v)
idx++
}
}
// clobber generates code to clobber pointer slots in all dead variables
// (those not marked in live). Clobbering instructions are added to the end
// of b.Values.
func clobber(lv *liveness, b *ssa.Block, live bitvec.BitVec) {
for i, n := range lv.vars {
if !live.Get(int32(i)) && !n.Addrtaken() && !n.OpenDeferSlot() && !n.IsOutputParamHeapAddr() {
// Don't clobber stack objects (address-taken). They are
// tracked dynamically.
// Also don't clobber slots that are live for defers (see
// the code setting livedefer in epilogue).
if lv.noClobberArgs && n.Class == ir.PPARAM {
continue
}
clobberVar(b, n)
}
}
}
// clobberVar generates code to trash the pointers in v.
// Clobbering instructions are added to the end of b.Values.
func clobberVar(b *ssa.Block, v *ir.Name) {
clobberWalk(b, v, 0, v.Type())
}
// b = block to which we append instructions
// v = variable
// offset = offset of (sub-portion of) variable to clobber (in bytes)
// t = type of sub-portion of v.
func clobberWalk(b *ssa.Block, v *ir.Name, offset int64, t *types.Type) {
if !t.HasPointers() {
return
}
switch t.Kind() {
case types.TPTR,
types.TUNSAFEPTR,
types.TFUNC,
types.TCHAN,
types.TMAP:
clobberPtr(b, v, offset)
case types.TSTRING:
// struct { byte *str; int len; }
clobberPtr(b, v, offset)
case types.TINTER:
// struct { Itab *tab; void *data; }
// or, when isnilinter(t)==true:
// struct { Type *type; void *data; }
clobberPtr(b, v, offset)
clobberPtr(b, v, offset+int64(types.PtrSize))
case types.TSLICE:
// struct { byte *array; int len; int cap; }
clobberPtr(b, v, offset)
case types.TARRAY:
for i := int64(0); i < t.NumElem(); i++ {
clobberWalk(b, v, offset+i*t.Elem().Size(), t.Elem())
}
case types.TSTRUCT:
for _, t1 := range t.Fields().Slice() {
clobberWalk(b, v, offset+t1.Offset, t1.Type)
}
default:
base.Fatalf("clobberWalk: unexpected type, %v", t)
}
}
// clobberPtr generates a clobber of the pointer at offset offset in v.
// The clobber instruction is added at the end of b.
func clobberPtr(b *ssa.Block, v *ir.Name, offset int64) {
b.NewValue0IA(src.NoXPos, ssa.OpClobber, types.TypeVoid, offset, v)
}
func (lv *liveness) showlive(v *ssa.Value, live bitvec.BitVec) {
if base.Flag.Live == 0 || ir.FuncName(lv.fn) == "init" || strings.HasPrefix(ir.FuncName(lv.fn), ".") {
return
}
if lv.fn.Wrapper() || lv.fn.Dupok() {
// Skip reporting liveness information for compiler-generated wrappers.
return
}
if !(v == nil || v.Op.IsCall()) {
// Historically we only printed this information at
// calls. Keep doing so.
return
}
if live.IsEmpty() {
return
}
pos := lv.fn.Nname.Pos()
if v != nil {
pos = v.Pos
}
s := "live at "
if v == nil {
s += fmt.Sprintf("entry to %s:", ir.FuncName(lv.fn))
} else if sym, ok := v.Aux.(*ssa.AuxCall); ok && sym.Fn != nil {
fn := sym.Fn.Name
if pos := strings.Index(fn, "."); pos >= 0 {
fn = fn[pos+1:]
}
s += fmt.Sprintf("call to %s:", fn)
} else {
s += "indirect call:"
}
for j, n := range lv.vars {
if live.Get(int32(j)) {
s += fmt.Sprintf(" %v", n)
}
}
base.WarnfAt(pos, s)
}
func (lv *liveness) printbvec(printed bool, name string, live bitvec.BitVec) bool {
if live.IsEmpty() {
return printed
}
if !printed {
fmt.Printf("\t")
} else {
fmt.Printf(" ")
}
fmt.Printf("%s=", name)
comma := ""
for i, n := range lv.vars {
if !live.Get(int32(i)) {
continue
}
fmt.Printf("%s%s", comma, n.Sym().Name)
comma = ","
}
return true
}
// printeffect is like printbvec, but for valueEffects.
func (lv *liveness) printeffect(printed bool, name string, pos int32, x bool) bool {
if !x {
return printed
}
if !printed {
fmt.Printf("\t")
} else {
fmt.Printf(" ")
}
fmt.Printf("%s=", name)
if x {
fmt.Printf("%s", lv.vars[pos].Sym().Name)
}
return true
}
// Prints the computed liveness information and inputs, for debugging.
// This format synthesizes the information used during the multiple passes
// into a single presentation.
func (lv *liveness) printDebug() {
fmt.Printf("liveness: %s\n", ir.FuncName(lv.fn))
for i, b := range lv.f.Blocks {
if i > 0 {
fmt.Printf("\n")
}
// bb#0 pred=1,2 succ=3,4
fmt.Printf("bb#%d pred=", b.ID)
for j, pred := range b.Preds {
if j > 0 {
fmt.Printf(",")
}
fmt.Printf("%d", pred.Block().ID)
}
fmt.Printf(" succ=")
for j, succ := range b.Succs {
if j > 0 {
fmt.Printf(",")
}
fmt.Printf("%d", succ.Block().ID)
}
fmt.Printf("\n")
be := lv.blockEffects(b)
// initial settings
printed := false
printed = lv.printbvec(printed, "uevar", be.uevar)
printed = lv.printbvec(printed, "livein", be.livein)
if printed {
fmt.Printf("\n")
}
// program listing, with individual effects listed
if b == lv.f.Entry {
live := lv.stackMaps[0]
fmt.Printf("(%s) function entry\n", base.FmtPos(lv.fn.Nname.Pos()))
fmt.Printf("\tlive=")
printed = false
for j, n := range lv.vars {
if !live.Get(int32(j)) {
continue
}
if printed {
fmt.Printf(",")
}
fmt.Printf("%v", n)
printed = true
}
fmt.Printf("\n")
}
for _, v := range b.Values {
fmt.Printf("(%s) %v\n", base.FmtPos(v.Pos), v.LongString())
pcdata := lv.livenessMap.Get(v)
pos, effect := lv.valueEffects(v)
printed = false
printed = lv.printeffect(printed, "uevar", pos, effect&uevar != 0)
printed = lv.printeffect(printed, "varkill", pos, effect&varkill != 0)
if printed {
fmt.Printf("\n")
}
if pcdata.StackMapValid() {
fmt.Printf("\tlive=")
printed = false
if pcdata.StackMapValid() {
live := lv.stackMaps[pcdata.StackMapIndex]
for j, n := range lv.vars {
if !live.Get(int32(j)) {
continue
}
if printed {
fmt.Printf(",")
}
fmt.Printf("%v", n)
printed = true
}
}
fmt.Printf("\n")
}
if pcdata.IsUnsafePoint {
fmt.Printf("\tunsafe-point\n")
}
}
// bb bitsets
fmt.Printf("end\n")
printed = false
printed = lv.printbvec(printed, "varkill", be.varkill)
printed = lv.printbvec(printed, "liveout", be.liveout)
if printed {
fmt.Printf("\n")
}
}
fmt.Printf("\n")
}
// Dumps a slice of bitmaps to a symbol as a sequence of uint32 values. The
// first word dumped is the total number of bitmaps. The second word is the
// length of the bitmaps. All bitmaps are assumed to be of equal length. The
// remaining bytes are the raw bitmaps.
func (lv *liveness) emit() (argsSym, liveSym *obj.LSym) {
// Size args bitmaps to be just large enough to hold the largest pointer.
// First, find the largest Xoffset node we care about.
// (Nodes without pointers aren't in lv.vars; see ShouldTrack.)
var maxArgNode *ir.Name
for _, n := range lv.vars {
switch n.Class {
case ir.PPARAM, ir.PPARAMOUT:
if !n.IsOutputParamInRegisters() {
if maxArgNode == nil || n.FrameOffset() > maxArgNode.FrameOffset() {
maxArgNode = n
}
}
}
}
// Next, find the offset of the largest pointer in the largest node.
var maxArgs int64
if maxArgNode != nil {
maxArgs = maxArgNode.FrameOffset() + types.PtrDataSize(maxArgNode.Type())
}
// Size locals bitmaps to be stkptrsize sized.
// We cannot shrink them to only hold the largest pointer,
// because their size is used to calculate the beginning
// of the local variables frame.
// Further discussion in https://golang.org/cl/104175.
// TODO: consider trimming leading zeros.
// This would require shifting all bitmaps.
maxLocals := lv.stkptrsize
// Temporary symbols for encoding bitmaps.
var argsSymTmp, liveSymTmp obj.LSym
args := bitvec.New(int32(maxArgs / int64(types.PtrSize)))
aoff := objw.Uint32(&argsSymTmp, 0, uint32(len(lv.stackMaps))) // number of bitmaps
aoff = objw.Uint32(&argsSymTmp, aoff, uint32(args.N)) // number of bits in each bitmap
locals := bitvec.New(int32(maxLocals / int64(types.PtrSize)))
loff := objw.Uint32(&liveSymTmp, 0, uint32(len(lv.stackMaps))) // number of bitmaps
loff = objw.Uint32(&liveSymTmp, loff, uint32(locals.N)) // number of bits in each bitmap
for _, live := range lv.stackMaps {
args.Clear()
locals.Clear()
lv.pointerMap(live, lv.vars, args, locals)
aoff = objw.BitVec(&argsSymTmp, aoff, args)
loff = objw.BitVec(&liveSymTmp, loff, locals)
}
// These symbols will be added to Ctxt.Data by addGCLocals
// after parallel compilation is done.
return base.Ctxt.GCLocalsSym(argsSymTmp.P), base.Ctxt.GCLocalsSym(liveSymTmp.P)
}
// Entry pointer for Compute analysis. Solves for the Compute of
// pointer variables in the function and emits a runtime data
// structure read by the garbage collector.
// Returns a map from GC safe points to their corresponding stack map index,
// and a map that contains all input parameters that may be partially live.
func Compute(curfn *ir.Func, f *ssa.Func, stkptrsize int64, pp *objw.Progs) (Map, map[*ir.Name]bool) {
// Construct the global liveness state.
vars, idx := getvariables(curfn)
lv := newliveness(curfn, f, vars, idx, stkptrsize)
// Run the dataflow framework.
lv.prologue()
lv.solve()
lv.epilogue()
if base.Flag.Live > 0 {
lv.showlive(nil, lv.stackMaps[0])
for _, b := range f.Blocks {
for _, val := range b.Values {
if idx := lv.livenessMap.Get(val); idx.StackMapValid() {
lv.showlive(val, lv.stackMaps[idx.StackMapIndex])
}
}
}
}
if base.Flag.Live >= 2 {
lv.printDebug()
}
// Update the function cache.
{
cache := f.Cache.Liveness.(*livenessFuncCache)
if cap(lv.be) < 2000 { // Threshold from ssa.Cache slices.
for i := range lv.be {
lv.be[i] = blockEffects{}
}
cache.be = lv.be
}
if len(lv.livenessMap.Vals) < 2000 {
cache.livenessMap = lv.livenessMap
}
}
// Emit the live pointer map data structures
ls := curfn.LSym
fninfo := ls.Func()
fninfo.GCArgs, fninfo.GCLocals = lv.emit()
p := pp.Prog(obj.AFUNCDATA)
p.From.SetConst(objabi.FUNCDATA_ArgsPointerMaps)
p.To.Type = obj.TYPE_MEM
p.To.Name = obj.NAME_EXTERN
p.To.Sym = fninfo.GCArgs
p = pp.Prog(obj.AFUNCDATA)
p.From.SetConst(objabi.FUNCDATA_LocalsPointerMaps)
p.To.Type = obj.TYPE_MEM
p.To.Name = obj.NAME_EXTERN
p.To.Sym = fninfo.GCLocals
if x := lv.emitStackObjects(); x != nil {
p := pp.Prog(obj.AFUNCDATA)
p.From.SetConst(objabi.FUNCDATA_StackObjects)
p.To.Type = obj.TYPE_MEM
p.To.Name = obj.NAME_EXTERN
p.To.Sym = x
}
return lv.livenessMap, lv.partLiveArgs
}
func (lv *liveness) emitStackObjects() *obj.LSym {
var vars []*ir.Name
for _, n := range lv.fn.Dcl {
if shouldTrack(n) && n.Addrtaken() && n.Esc() != ir.EscHeap {
vars = append(vars, n)
}
}
if len(vars) == 0 {
return nil
}
// Sort variables from lowest to highest address.
sort.Slice(vars, func(i, j int) bool { return vars[i].FrameOffset() < vars[j].FrameOffset() })
// Populate the stack object data.
// Format must match runtime/stack.go:stackObjectRecord.
x := base.Ctxt.Lookup(lv.fn.LSym.Name + ".stkobj")
x.Set(obj.AttrContentAddressable, true)
lv.fn.LSym.Func().StackObjects = x
off := 0
off = objw.Uintptr(x, off, uint64(len(vars)))
for _, v := range vars {
// Note: arguments and return values have non-negative Xoffset,
// in which case the offset is relative to argp.
// Locals have a negative Xoffset, in which case the offset is relative to varp.
// We already limit the frame size, so the offset and the object size
// should not be too big.
frameOffset := v.FrameOffset()
if frameOffset != int64(int32(frameOffset)) {
base.Fatalf("frame offset too big: %v %d", v, frameOffset)
}
off = objw.Uint32(x, off, uint32(frameOffset))
t := v.Type()
sz := t.Size()
if sz != int64(int32(sz)) {
base.Fatalf("stack object too big: %v of type %v, size %d", v, t, sz)
}
lsym, useGCProg, ptrdata := reflectdata.GCSym(t)
if useGCProg {
ptrdata = -ptrdata
}
off = objw.Uint32(x, off, uint32(sz))
off = objw.Uint32(x, off, uint32(ptrdata))
off = objw.SymPtrOff(x, off, lsym)
}
if base.Flag.Live != 0 {
for _, v := range vars {
base.WarnfAt(v.Pos(), "stack object %v %v", v, v.Type())
}
}
return x
}
// isfat reports whether a variable of type t needs multiple assignments to initialize.
// For example:
//
// type T struct { x, y int }
// x := T{x: 0, y: 1}
//
// Then we need:
//
// var t T
// t.x = 0
// t.y = 1
//
// to fully initialize t.
func isfat(t *types.Type) bool {
if t != nil {
switch t.Kind() {
case types.TSLICE, types.TSTRING,
types.TINTER: // maybe remove later
return true
case types.TARRAY:
// Array of 1 element, check if element is fat
if t.NumElem() == 1 {
return isfat(t.Elem())
}
return true
case types.TSTRUCT:
// Struct with 1 field, check if field is fat
if t.NumFields() == 1 {
return isfat(t.Field(0).Type)
}
return true
}
}
return false
}
// WriteFuncMap writes the pointer bitmaps for bodyless function fn's
// inputs and outputs as the value of symbol <fn>.args_stackmap.
// If fn has outputs, two bitmaps are written, otherwise just one.
func WriteFuncMap(fn *ir.Func, abiInfo *abi.ABIParamResultInfo) {
if ir.FuncName(fn) == "_" || fn.Sym().Linkname != "" {
return
}
nptr := int(abiInfo.ArgWidth() / int64(types.PtrSize))
bv := bitvec.New(int32(nptr) * 2)
for _, p := range abiInfo.InParams() {
typebits.Set(p.Type, p.FrameOffset(abiInfo), bv)
}
nbitmap := 1
if fn.Type().NumResults() > 0 {
nbitmap = 2
}
lsym := base.Ctxt.Lookup(fn.LSym.Name + ".args_stackmap")
off := objw.Uint32(lsym, 0, uint32(nbitmap))
off = objw.Uint32(lsym, off, uint32(bv.N))
off = objw.BitVec(lsym, off, bv)
if fn.Type().NumResults() > 0 {
for _, p := range abiInfo.OutParams() {
if len(p.Registers) == 0 {
typebits.Set(p.Type, p.FrameOffset(abiInfo), bv)
}
}
off = objw.BitVec(lsym, off, bv)
}
objw.Global(lsym, int32(off), obj.RODATA|obj.LOCAL)
}
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