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// 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.
package ssa
// This file defines the lifting pass which tries to "lift" Alloc
// cells (new/local variables) into SSA registers, replacing loads
// with the dominating stored value, eliminating loads and stores, and
// inserting φ-nodes as needed.
// Cited papers and resources:
//
// Ron Cytron et al. 1991. Efficiently computing SSA form...
// http://doi.acm.org/10.1145/115372.115320
//
// Cooper, Harvey, Kennedy. 2001. A Simple, Fast Dominance Algorithm.
// Software Practice and Experience 2001, 4:1-10.
// http://www.hipersoft.rice.edu/grads/publications/dom14.pdf
//
// Daniel Berlin, llvmdev mailing list, 2012.
// http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/046638.html
// (Be sure to expand the whole thread.)
// TODO(adonovan): opt: there are many optimizations worth evaluating, and
// the conventional wisdom for SSA construction is that a simple
// algorithm well engineered often beats those of better asymptotic
// complexity on all but the most egregious inputs.
//
// Danny Berlin suggests that the Cooper et al. algorithm for
// computing the dominance frontier is superior to Cytron et al.
// Furthermore he recommends that rather than computing the DF for the
// whole function then renaming all alloc cells, it may be cheaper to
// compute the DF for each alloc cell separately and throw it away.
//
// Consider exploiting liveness information to avoid creating dead
// φ-nodes which we then immediately remove.
//
// Also see many other "TODO: opt" suggestions in the code.
import (
"fmt"
"go/token"
"go/types"
"math/big"
"os"
)
// If true, show diagnostic information at each step of lifting.
// Very verbose.
const debugLifting = false
// domFrontier maps each block to the set of blocks in its dominance
// frontier. The outer slice is conceptually a map keyed by
// Block.Index. The inner slice is conceptually a set, possibly
// containing duplicates.
//
// TODO(adonovan): opt: measure impact of dups; consider a packed bit
// representation, e.g. big.Int, and bitwise parallel operations for
// the union step in the Children loop.
//
// domFrontier's methods mutate the slice's elements but not its
// length, so their receivers needn't be pointers.
//
type domFrontier [][]*BasicBlock
func (df domFrontier) add(u, v *BasicBlock) {
p := &df[u.Index]
*p = append(*p, v)
}
// build builds the dominance frontier df for the dominator (sub)tree
// rooted at u, using the Cytron et al. algorithm.
//
// TODO(adonovan): opt: consider Berlin approach, computing pruned SSA
// by pruning the entire IDF computation, rather than merely pruning
// the DF -> IDF step.
func (df domFrontier) build(u *BasicBlock) {
// Encounter each node u in postorder of dom tree.
for _, child := range u.dom.children {
df.build(child)
}
for _, vb := range u.Succs {
if v := vb.dom; v.idom != u {
df.add(u, vb)
}
}
for _, w := range u.dom.children {
for _, vb := range df[w.Index] {
// TODO(adonovan): opt: use word-parallel bitwise union.
if v := vb.dom; v.idom != u {
df.add(u, vb)
}
}
}
}
func buildDomFrontier(fn *Function) domFrontier {
df := make(domFrontier, len(fn.Blocks))
df.build(fn.Blocks[0])
if fn.Recover != nil {
df.build(fn.Recover)
}
return df
}
func removeInstr(refs []Instruction, instr Instruction) []Instruction {
i := 0
for _, ref := range refs {
if ref == instr {
continue
}
refs[i] = ref
i++
}
for j := i; j != len(refs); j++ {
refs[j] = nil // aid GC
}
return refs[:i]
}
// lift replaces local and new Allocs accessed only with
// load/store by SSA registers, inserting φ-nodes where necessary.
// The result is a program in classical pruned SSA form.
//
// Preconditions:
// - fn has no dead blocks (blockopt has run).
// - Def/use info (Operands and Referrers) is up-to-date.
// - The dominator tree is up-to-date.
//
func lift(fn *Function) {
// TODO(adonovan): opt: lots of little optimizations may be
// worthwhile here, especially if they cause us to avoid
// buildDomFrontier. For example:
//
// - Alloc never loaded? Eliminate.
// - Alloc never stored? Replace all loads with a zero constant.
// - Alloc stored once? Replace loads with dominating store;
// don't forget that an Alloc is itself an effective store
// of zero.
// - Alloc used only within a single block?
// Use degenerate algorithm avoiding φ-nodes.
// - Consider synergy with scalar replacement of aggregates (SRA).
// e.g. *(&x.f) where x is an Alloc.
// Perhaps we'd get better results if we generated this as x.f
// i.e. Field(x, .f) instead of Load(FieldIndex(x, .f)).
// Unclear.
//
// But we will start with the simplest correct code.
df := buildDomFrontier(fn)
if debugLifting {
title := false
for i, blocks := range df {
if blocks != nil {
if !title {
fmt.Fprintf(os.Stderr, "Dominance frontier of %s:\n", fn)
title = true
}
fmt.Fprintf(os.Stderr, "\t%s: %s\n", fn.Blocks[i], blocks)
}
}
}
newPhis := make(newPhiMap)
// During this pass we will replace some BasicBlock.Instrs
// (allocs, loads and stores) with nil, keeping a count in
// BasicBlock.gaps. At the end we will reset Instrs to the
// concatenation of all non-dead newPhis and non-nil Instrs
// for the block, reusing the original array if space permits.
// While we're here, we also eliminate 'rundefers'
// instructions in functions that contain no 'defer'
// instructions.
usesDefer := false
// A counter used to generate ~unique ids for Phi nodes, as an
// aid to debugging. We use large numbers to make them highly
// visible. All nodes are renumbered later.
fresh := 1000
// Determine which allocs we can lift and number them densely.
// The renaming phase uses this numbering for compact maps.
numAllocs := 0
for _, b := range fn.Blocks {
b.gaps = 0
b.rundefers = 0
for _, instr := range b.Instrs {
switch instr := instr.(type) {
case *Alloc:
index := -1
if liftAlloc(df, instr, newPhis, &fresh) {
index = numAllocs
numAllocs++
}
instr.index = index
case *Defer:
usesDefer = true
case *RunDefers:
b.rundefers++
}
}
}
// renaming maps an alloc (keyed by index) to its replacement
// value. Initially the renaming contains nil, signifying the
// zero constant of the appropriate type; we construct the
// Const lazily at most once on each path through the domtree.
// TODO(adonovan): opt: cache per-function not per subtree.
renaming := make([]Value, numAllocs)
// Renaming.
rename(fn.Blocks[0], renaming, newPhis)
// Eliminate dead φ-nodes.
removeDeadPhis(fn.Blocks, newPhis)
// Prepend remaining live φ-nodes to each block.
for _, b := range fn.Blocks {
nps := newPhis[b]
j := len(nps)
rundefersToKill := b.rundefers
if usesDefer {
rundefersToKill = 0
}
if j+b.gaps+rundefersToKill == 0 {
continue // fast path: no new phis or gaps
}
// Compact nps + non-nil Instrs into a new slice.
// TODO(adonovan): opt: compact in situ (rightwards)
// if Instrs has sufficient space or slack.
dst := make([]Instruction, len(b.Instrs)+j-b.gaps-rundefersToKill)
for i, np := range nps {
dst[i] = np.phi
}
for _, instr := range b.Instrs {
if instr == nil {
continue
}
if !usesDefer {
if _, ok := instr.(*RunDefers); ok {
continue
}
}
dst[j] = instr
j++
}
b.Instrs = dst
}
// Remove any fn.Locals that were lifted.
j := 0
for _, l := range fn.Locals {
if l.index < 0 {
fn.Locals[j] = l
j++
}
}
// Nil out fn.Locals[j:] to aid GC.
for i := j; i < len(fn.Locals); i++ {
fn.Locals[i] = nil
}
fn.Locals = fn.Locals[:j]
}
// removeDeadPhis removes φ-nodes not transitively needed by a
// non-Phi, non-DebugRef instruction.
func removeDeadPhis(blocks []*BasicBlock, newPhis newPhiMap) {
// First pass: find the set of "live" φ-nodes: those reachable
// from some non-Phi instruction.
//
// We compute reachability in reverse, starting from each φ,
// rather than forwards, starting from each live non-Phi
// instruction, because this way visits much less of the
// Value graph.
livePhis := make(map[*Phi]bool)
for _, npList := range newPhis {
for _, np := range npList {
phi := np.phi
if !livePhis[phi] && phiHasDirectReferrer(phi) {
markLivePhi(livePhis, phi)
}
}
}
// Existing φ-nodes due to && and || operators
// are all considered live (see Go issue 19622).
for _, b := range blocks {
for _, phi := range b.phis() {
markLivePhi(livePhis, phi.(*Phi))
}
}
// Second pass: eliminate unused phis from newPhis.
for block, npList := range newPhis {
j := 0
for _, np := range npList {
if livePhis[np.phi] {
npList[j] = np
j++
} else {
// discard it, first removing it from referrers
for _, val := range np.phi.Edges {
if refs := val.Referrers(); refs != nil {
*refs = removeInstr(*refs, np.phi)
}
}
np.phi.block = nil
}
}
newPhis[block] = npList[:j]
}
}
// markLivePhi marks phi, and all φ-nodes transitively reachable via
// its Operands, live.
func markLivePhi(livePhis map[*Phi]bool, phi *Phi) {
livePhis[phi] = true
for _, rand := range phi.Operands(nil) {
if q, ok := (*rand).(*Phi); ok {
if !livePhis[q] {
markLivePhi(livePhis, q)
}
}
}
}
// phiHasDirectReferrer reports whether phi is directly referred to by
// a non-Phi instruction. Such instructions are the
// roots of the liveness traversal.
func phiHasDirectReferrer(phi *Phi) bool {
for _, instr := range *phi.Referrers() {
if _, ok := instr.(*Phi); !ok {
return true
}
}
return false
}
type blockSet struct{ big.Int } // (inherit methods from Int)
// add adds b to the set and returns true if the set changed.
func (s *blockSet) add(b *BasicBlock) bool {
i := b.Index
if s.Bit(i) != 0 {
return false
}
s.SetBit(&s.Int, i, 1)
return true
}
// take removes an arbitrary element from a set s and
// returns its index, or returns -1 if empty.
func (s *blockSet) take() int {
l := s.BitLen()
for i := 0; i < l; i++ {
if s.Bit(i) == 1 {
s.SetBit(&s.Int, i, 0)
return i
}
}
return -1
}
// newPhi is a pair of a newly introduced φ-node and the lifted Alloc
// it replaces.
type newPhi struct {
phi *Phi
alloc *Alloc
}
// newPhiMap records for each basic block, the set of newPhis that
// must be prepended to the block.
type newPhiMap map[*BasicBlock][]newPhi
// liftAlloc determines whether alloc can be lifted into registers,
// and if so, it populates newPhis with all the φ-nodes it may require
// and returns true.
//
// fresh is a source of fresh ids for phi nodes.
//
func liftAlloc(df domFrontier, alloc *Alloc, newPhis newPhiMap, fresh *int) bool {
// Don't lift aggregates into registers, because we don't have
// a way to express their zero-constants.
switch deref(alloc.Type()).Underlying().(type) {
case *types.Array, *types.Struct:
return false
}
// Don't lift named return values in functions that defer
// calls that may recover from panic.
if fn := alloc.Parent(); fn.Recover != nil {
for _, nr := range fn.namedResults {
if nr == alloc {
return false
}
}
}
// Compute defblocks, the set of blocks containing a
// definition of the alloc cell.
var defblocks blockSet
for _, instr := range *alloc.Referrers() {
// Bail out if we discover the alloc is not liftable;
// the only operations permitted to use the alloc are
// loads/stores into the cell, and DebugRef.
switch instr := instr.(type) {
case *Store:
if instr.Val == alloc {
return false // address used as value
}
if instr.Addr != alloc {
panic("Alloc.Referrers is inconsistent")
}
defblocks.add(instr.Block())
case *UnOp:
if instr.Op != token.MUL {
return false // not a load
}
if instr.X != alloc {
panic("Alloc.Referrers is inconsistent")
}
case *DebugRef:
// ok
default:
return false // some other instruction
}
}
// The Alloc itself counts as a (zero) definition of the cell.
defblocks.add(alloc.Block())
if debugLifting {
fmt.Fprintln(os.Stderr, "\tlifting ", alloc, alloc.Name())
}
fn := alloc.Parent()
// Φ-insertion.
//
// What follows is the body of the main loop of the insert-φ
// function described by Cytron et al, but instead of using
// counter tricks, we just reset the 'hasAlready' and 'work'
// sets each iteration. These are bitmaps so it's pretty cheap.
//
// TODO(adonovan): opt: recycle slice storage for W,
// hasAlready, defBlocks across liftAlloc calls.
var hasAlready blockSet
// Initialize W and work to defblocks.
var work blockSet = defblocks // blocks seen
var W blockSet // blocks to do
W.Set(&defblocks.Int)
// Traverse iterated dominance frontier, inserting φ-nodes.
for i := W.take(); i != -1; i = W.take() {
u := fn.Blocks[i]
for _, v := range df[u.Index] {
if hasAlready.add(v) {
// Create φ-node.
// It will be prepended to v.Instrs later, if needed.
phi := &Phi{
Edges: make([]Value, len(v.Preds)),
Comment: alloc.Comment,
}
// This is merely a debugging aid:
phi.setNum(*fresh)
*fresh++
phi.pos = alloc.Pos()
phi.setType(deref(alloc.Type()))
phi.block = v
if debugLifting {
fmt.Fprintf(os.Stderr, "\tplace %s = %s at block %s\n", phi.Name(), phi, v)
}
newPhis[v] = append(newPhis[v], newPhi{phi, alloc})
if work.add(v) {
W.add(v)
}
}
}
}
return true
}
// replaceAll replaces all intraprocedural uses of x with y,
// updating x.Referrers and y.Referrers.
// Precondition: x.Referrers() != nil, i.e. x must be local to some function.
//
func replaceAll(x, y Value) {
var rands []*Value
pxrefs := x.Referrers()
pyrefs := y.Referrers()
for _, instr := range *pxrefs {
rands = instr.Operands(rands[:0]) // recycle storage
for _, rand := range rands {
if *rand != nil {
if *rand == x {
*rand = y
}
}
}
if pyrefs != nil {
*pyrefs = append(*pyrefs, instr) // dups ok
}
}
*pxrefs = nil // x is now unreferenced
}
// renamed returns the value to which alloc is being renamed,
// constructing it lazily if it's the implicit zero initialization.
//
func renamed(renaming []Value, alloc *Alloc) Value {
v := renaming[alloc.index]
if v == nil {
v = zeroConst(deref(alloc.Type()))
renaming[alloc.index] = v
}
return v
}
// rename implements the (Cytron et al) SSA renaming algorithm, a
// preorder traversal of the dominator tree replacing all loads of
// Alloc cells with the value stored to that cell by the dominating
// store instruction. For lifting, we need only consider loads,
// stores and φ-nodes.
//
// renaming is a map from *Alloc (keyed by index number) to its
// dominating stored value; newPhis[x] is the set of new φ-nodes to be
// prepended to block x.
//
func rename(u *BasicBlock, renaming []Value, newPhis newPhiMap) {
// Each φ-node becomes the new name for its associated Alloc.
for _, np := range newPhis[u] {
phi := np.phi
alloc := np.alloc
renaming[alloc.index] = phi
}
// Rename loads and stores of allocs.
for i, instr := range u.Instrs {
switch instr := instr.(type) {
case *Alloc:
if instr.index >= 0 { // store of zero to Alloc cell
// Replace dominated loads by the zero value.
renaming[instr.index] = nil
if debugLifting {
fmt.Fprintf(os.Stderr, "\tkill alloc %s\n", instr)
}
// Delete the Alloc.
u.Instrs[i] = nil
u.gaps++
}
case *Store:
if alloc, ok := instr.Addr.(*Alloc); ok && alloc.index >= 0 { // store to Alloc cell
// Replace dominated loads by the stored value.
renaming[alloc.index] = instr.Val
if debugLifting {
fmt.Fprintf(os.Stderr, "\tkill store %s; new value: %s\n",
instr, instr.Val.Name())
}
// Remove the store from the referrer list of the stored value.
if refs := instr.Val.Referrers(); refs != nil {
*refs = removeInstr(*refs, instr)
}
// Delete the Store.
u.Instrs[i] = nil
u.gaps++
}
case *UnOp:
if instr.Op == token.MUL {
if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // load of Alloc cell
newval := renamed(renaming, alloc)
if debugLifting {
fmt.Fprintf(os.Stderr, "\tupdate load %s = %s with %s\n",
instr.Name(), instr, newval.Name())
}
// Replace all references to
// the loaded value by the
// dominating stored value.
replaceAll(instr, newval)
// Delete the Load.
u.Instrs[i] = nil
u.gaps++
}
}
case *DebugRef:
if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // ref of Alloc cell
if instr.IsAddr {
instr.X = renamed(renaming, alloc)
instr.IsAddr = false
// Add DebugRef to instr.X's referrers.
if refs := instr.X.Referrers(); refs != nil {
*refs = append(*refs, instr)
}
} else {
// A source expression denotes the address
// of an Alloc that was optimized away.
instr.X = nil
// Delete the DebugRef.
u.Instrs[i] = nil
u.gaps++
}
}
}
}
// For each φ-node in a CFG successor, rename the edge.
for _, v := range u.Succs {
phis := newPhis[v]
if len(phis) == 0 {
continue
}
i := v.predIndex(u)
for _, np := range phis {
phi := np.phi
alloc := np.alloc
newval := renamed(renaming, alloc)
if debugLifting {
fmt.Fprintf(os.Stderr, "\tsetphi %s edge %s -> %s (#%d) (alloc=%s) := %s\n",
phi.Name(), u, v, i, alloc.Name(), newval.Name())
}
phi.Edges[i] = newval
if prefs := newval.Referrers(); prefs != nil {
*prefs = append(*prefs, phi)
}
}
}
// Continue depth-first recursion over domtree, pushing a
// fresh copy of the renaming map for each subtree.
for i, v := range u.dom.children {
r := renaming
if i < len(u.dom.children)-1 {
// On all but the final iteration, we must make
// a copy to avoid destructive update.
r = make([]Value, len(renaming))
copy(r, renaming)
}
rename(v, r, newPhis)
}
}