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// Copyright 2021 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 liveness
import (
"fmt"
"internal/abi"
"cmd/compile/internal/base"
"cmd/compile/internal/bitvec"
"cmd/compile/internal/ir"
"cmd/compile/internal/objw"
"cmd/compile/internal/ssa"
"cmd/internal/obj"
)
// Argument liveness tracking.
//
// For arguments passed in registers, this file tracks if their spill slots
// are live for runtime traceback. An argument spill slot is live at a PC
// if we know that an actual value has stored into it at or before this point.
//
// Stack args are always live and not tracked in this code. Stack args are
// laid out before register spill slots, so we emit the smallest offset that
// needs tracking. Slots before that offset are always live. That offset is
// usually the offset of the first spill slot. But if the first spill slot is
// always live (e.g. if it is address-taken), it will be the offset of a later
// one.
//
// The liveness information is emitted as a FUNCDATA and a PCDATA.
//
// FUNCDATA format:
// - start (smallest) offset that needs tracking (1 byte)
// - a list of bitmaps.
// In a bitmap bit i is set if the i-th spill slot is live.
//
// At a PC where the liveness info changes, a PCDATA indicates the
// byte offset of the liveness map in the FUNCDATA. PCDATA -1 is a
// special case indicating all slots are live (for binary size
// saving).
const allLiveIdx = -1
// name and offset
type nameOff struct {
n *ir.Name
off int64
}
func (a nameOff) FrameOffset() int64 { return a.n.FrameOffset() + a.off }
func (a nameOff) String() string { return fmt.Sprintf("%v+%d", a.n, a.off) }
type blockArgEffects struct {
livein bitvec.BitVec // variables live at block entry
liveout bitvec.BitVec // variables live at block exit
}
type argLiveness struct {
fn *ir.Func
f *ssa.Func
args []nameOff // name and offset of spill slots
idx map[nameOff]int32 // index in args
be []blockArgEffects // indexed by block ID
bvset bvecSet // Set of liveness bitmaps, used for uniquifying.
// Liveness map indices at each Value (where it changes) and Block entry.
// During the computation the indices are temporarily index to bvset.
// At the end they will be index (offset) to the output funcdata (changed
// in (*argLiveness).emit).
blockIdx map[ssa.ID]int
valueIdx map[ssa.ID]int
}
// ArgLiveness computes the liveness information of register argument spill slots.
// An argument's spill slot is "live" if we know it contains a meaningful value,
// that is, we have stored the register value to it.
// Returns the liveness map indices at each Block entry and at each Value (where
// it changes).
func ArgLiveness(fn *ir.Func, f *ssa.Func, pp *objw.Progs) (blockIdx, valueIdx map[ssa.ID]int) {
if f.OwnAux.ABIInfo().InRegistersUsed() == 0 || base.Flag.N != 0 {
// No register args. Nothing to emit.
// Or if -N is used we spill everything upfront so it is always live.
return nil, nil
}
lv := &argLiveness{
fn: fn,
f: f,
idx: make(map[nameOff]int32),
be: make([]blockArgEffects, f.NumBlocks()),
blockIdx: make(map[ssa.ID]int),
valueIdx: make(map[ssa.ID]int),
}
// Gather all register arg spill slots.
for _, a := range f.OwnAux.ABIInfo().InParams() {
n := a.Name
if n == nil || len(a.Registers) == 0 {
continue
}
_, offs := a.RegisterTypesAndOffsets()
for _, off := range offs {
if n.FrameOffset()+off > 0xff {
// We only print a limited number of args, with stack
// offsets no larger than 255.
continue
}
lv.args = append(lv.args, nameOff{n, off})
}
}
if len(lv.args) > 10 {
lv.args = lv.args[:10] // We print no more than 10 args.
}
// We spill address-taken or non-SSA-able value upfront, so they are always live.
alwaysLive := func(n *ir.Name) bool { return n.Addrtaken() || !ssa.CanSSA(n.Type()) }
// We'll emit the smallest offset for the slots that need liveness info.
// No need to include a slot with a lower offset if it is always live.
for len(lv.args) > 0 && alwaysLive(lv.args[0].n) {
lv.args = lv.args[1:]
}
if len(lv.args) == 0 {
return // everything is always live
}
for i, a := range lv.args {
lv.idx[a] = int32(i)
}
nargs := int32(len(lv.args))
bulk := bitvec.NewBulk(nargs, int32(len(f.Blocks)*2), fn.Pos())
for _, b := range f.Blocks {
be := &lv.be[b.ID]
be.livein = bulk.Next()
be.liveout = bulk.Next()
// initialize to all 1s, so we can AND them
be.livein.Not()
be.liveout.Not()
}
entrybe := &lv.be[f.Entry.ID]
entrybe.livein.Clear()
for i, a := range lv.args {
if alwaysLive(a.n) {
entrybe.livein.Set(int32(i))
}
}
// Visit blocks in reverse-postorder, compute block effects.
po := f.Postorder()
for i := len(po) - 1; i >= 0; i-- {
b := po[i]
be := &lv.be[b.ID]
// A slot is live at block entry if it is live in all predecessors.
for _, pred := range b.Preds {
pb := pred.Block()
be.livein.And(be.livein, lv.be[pb.ID].liveout)
}
be.liveout.Copy(be.livein)
for _, v := range b.Values {
lv.valueEffect(v, be.liveout)
}
}
// Coalesce identical live vectors. Compute liveness indices at each PC
// where it changes.
live := bitvec.New(nargs)
addToSet := func(bv bitvec.BitVec) (int, bool) {
if bv.Count() == int(nargs) { // special case for all live
return allLiveIdx, false
}
return lv.bvset.add(bv)
}
for _, b := range lv.f.Blocks {
be := &lv.be[b.ID]
lv.blockIdx[b.ID], _ = addToSet(be.livein)
live.Copy(be.livein)
var lastv *ssa.Value
for i, v := range b.Values {
if lv.valueEffect(v, live) {
// Record that liveness changes but not emit a map now.
// For a sequence of StoreRegs we only need to emit one
// at last.
lastv = v
}
if lastv != nil && (mayFault(v) || i == len(b.Values)-1) {
// Emit the liveness map if it may fault or at the end of
// the block. We may need a traceback if the instruction
// may cause a panic.
var added bool
lv.valueIdx[lastv.ID], added = addToSet(live)
if added {
// live is added to bvset and we cannot modify it now.
// Make a copy.
t := live
live = bitvec.New(nargs)
live.Copy(t)
}
lastv = nil
}
}
// Sanity check.
if !live.Eq(be.liveout) {
panic("wrong arg liveness map at block end")
}
}
// Emit funcdata symbol, update indices to offsets in the symbol data.
lsym := lv.emit()
fn.LSym.Func().ArgLiveInfo = lsym
//lv.print()
p := pp.Prog(obj.AFUNCDATA)
p.From.SetConst(abi.FUNCDATA_ArgLiveInfo)
p.To.Type = obj.TYPE_MEM
p.To.Name = obj.NAME_EXTERN
p.To.Sym = lsym
return lv.blockIdx, lv.valueIdx
}
// valueEffect applies the effect of v to live, return whether it is changed.
func (lv *argLiveness) valueEffect(v *ssa.Value, live bitvec.BitVec) bool {
if v.Op != ssa.OpStoreReg { // TODO: include other store instructions?
return false
}
n, off := ssa.AutoVar(v)
if n.Class != ir.PPARAM {
return false
}
i, ok := lv.idx[nameOff{n, off}]
if !ok || live.Get(i) {
return false
}
live.Set(i)
return true
}
func mayFault(v *ssa.Value) bool {
switch v.Op {
case ssa.OpLoadReg, ssa.OpStoreReg, ssa.OpCopy, ssa.OpPhi,
ssa.OpVarDef, ssa.OpVarLive, ssa.OpKeepAlive,
ssa.OpSelect0, ssa.OpSelect1, ssa.OpSelectN, ssa.OpMakeResult,
ssa.OpConvert, ssa.OpInlMark, ssa.OpGetG:
return false
}
if len(v.Args) == 0 {
return false // assume constant op cannot fault
}
return true // conservatively assume all other ops could fault
}
func (lv *argLiveness) print() {
fmt.Println("argument liveness:", lv.f.Name)
live := bitvec.New(int32(len(lv.args)))
for _, b := range lv.f.Blocks {
be := &lv.be[b.ID]
fmt.Printf("%v: live in: ", b)
lv.printLivenessVec(be.livein)
if idx, ok := lv.blockIdx[b.ID]; ok {
fmt.Printf(" #%d", idx)
}
fmt.Println()
for _, v := range b.Values {
if lv.valueEffect(v, live) {
fmt.Printf(" %v: ", v)
lv.printLivenessVec(live)
if idx, ok := lv.valueIdx[v.ID]; ok {
fmt.Printf(" #%d", idx)
}
fmt.Println()
}
}
fmt.Printf("%v: live out: ", b)
lv.printLivenessVec(be.liveout)
fmt.Println()
}
fmt.Println("liveness maps data:", lv.fn.LSym.Func().ArgLiveInfo.P)
}
func (lv *argLiveness) printLivenessVec(bv bitvec.BitVec) {
for i, a := range lv.args {
if bv.Get(int32(i)) {
fmt.Printf("%v ", a)
}
}
}
func (lv *argLiveness) emit() *obj.LSym {
livenessMaps := lv.bvset.extractUnique()
// stack offsets of register arg spill slots
argOffsets := make([]uint8, len(lv.args))
for i, a := range lv.args {
off := a.FrameOffset()
if off > 0xff {
panic("offset too large")
}
argOffsets[i] = uint8(off)
}
idx2off := make([]int, len(livenessMaps))
lsym := base.Ctxt.Lookup(lv.fn.LSym.Name + ".argliveinfo")
lsym.Set(obj.AttrContentAddressable, true)
off := objw.Uint8(lsym, 0, argOffsets[0]) // smallest offset that needs liveness info.
for idx, live := range livenessMaps {
idx2off[idx] = off
off = objw.BitVec(lsym, off, live)
}
// Update liveness indices to offsets.
for i, x := range lv.blockIdx {
if x != allLiveIdx {
lv.blockIdx[i] = idx2off[x]
}
}
for i, x := range lv.valueIdx {
if x != allLiveIdx {
lv.valueIdx[i] = idx2off[x]
}
}
return lsym
}