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Copy pathlift.go
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1261 lines (1138 loc) · 32.7 KB
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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 ir
// 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...
// https://doi.acm.org/10.1145/115372.115320
//
// Cooper, Harvey, Kennedy. 2001. A Simple, Fast Dominance Algorithm.
// Software Practice and Experience 2001, 4:1-10.
// https://www.hipersoft.rice.edu/grads/publications/dom14.pdf
//
// Daniel Berlin, llvmdev mailing list, 2012.
// https://lists.cs.uiuc.edu/pipermail/llvmdev/2012-January/046638.html
// (Be sure to expand the whole thread.)
//
// C. Scott Ananian. 1997. The static single information form.
//
// Jeremy Singer. 2006. Static program analysis based on virtual register renaming.
// 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"
"os"
"slices"
)
// 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 BlockMap[[]*BasicBlock]
func (df domFrontier) add(u, v *BasicBlock) {
df[u.Index] = append(df[u.Index], v)
}
// build builds the dominance frontier df for the dominator tree of
// fn, using the algorithm found in A Simple, Fast Dominance
// Algorithm, Figure 5.
//
// 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(fn *Function) {
for _, b := range fn.Blocks {
if len(b.Preds) >= 2 {
for _, p := range b.Preds {
runner := p
for runner != b.dom.idom {
df.add(runner, b)
runner = runner.dom.idom
}
}
}
}
}
func buildDomFrontier(fn *Function) domFrontier {
df := make(domFrontier, len(fn.Blocks))
df.build(fn)
return df
}
func removeInstr(refs []Instruction, instr Instruction) []Instruction {
s := refs
i := -1
for j := range s {
if instr == s[j] {
i = j
break
}
}
if i == -1 {
return s
}
// Don't start copying elements until we find one to delete.
for j := i + 1; j < len(s); j++ {
if v := s[j]; v != instr {
s[i] = v
i++
}
}
clear(s[i:]) // zero/nil out the obsolete elements, for GC
return s[:i]
}
func clearInstrs(instrs []Instruction) {
for i := range instrs {
instrs[i] = nil
}
}
func numberNodesPerBlock(f *Function) {
for _, b := range f.Blocks {
var base ID
for _, instr := range b.Instrs {
if instr == nil {
continue
}
instr.setID(base)
base++
}
}
}
// lift replaces local and new Allocs accessed only with
// load/store by IR registers, inserting φ-nodes where necessary.
// The result is a program in 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) bool {
// 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.
var df domFrontier
var newPhis BlockMap[[]newPhi]
// 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 and ssa:deferstack() in functions that contain no
// 'defer' instructions. Eliminate ssa:deferstack() if it does not
// escape.
usesDefer := false
deferstackAlloc, deferstackCall := deferstackPreamble(fn)
eliminateDeferStack := deferstackAlloc != nil && !deferstackAlloc.Heap
// Determine which allocs we can lift and number them densely.
// The renaming phase uses this numbering for compact maps.
numAllocs := 0
instructions := make(BlockMap[liftInstructions], len(fn.Blocks))
for i := range instructions {
instructions[i].insertInstructions = map[Instruction][]Instruction{}
}
// Number nodes, for liftable
numberNodesPerBlock(fn)
heads := make(BlockMap[int], len(fn.Blocks))
for i := range heads {
heads[i] = -1
}
for _, b := range fn.Blocks {
for i, instr := range b.Instrs {
if _, ok := instr.(*Phi); !ok {
heads[b.Index] = i
break
}
}
}
for _, b := range fn.Blocks {
b.gaps = 0
b.rundefers = 0
for _, instr := range b.Instrs {
switch instr := instr.(type) {
case *Alloc:
if !liftable(instr, instructions, heads) {
instr.index = -1
continue
}
if numAllocs == 0 {
df = buildDomFrontier(fn)
newPhis = make(BlockMap[[]newPhi], len(fn.Blocks))
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)
}
}
}
}
instr.index = numAllocs
numAllocs++
case *Defer:
usesDefer = true
if eliminateDeferStack {
// Clear _DeferStack and remove references to loads
if instr.DeferStack != nil {
if refs := instr.DeferStack.Referrers(); refs != nil {
*refs = removeInstr(*refs, instr)
}
instr.DeferStack = nil
}
}
case *RunDefers:
b.rundefers++
}
}
}
if numAllocs > 0 {
for _, b := range fn.Blocks {
work := instructions[b.Index]
for _, rename := range work.renameAllocs {
for _, instr_ := range b.Instrs[rename.startingAt:] {
replace(instr_, rename.from, rename.to)
}
}
}
for _, b := range fn.Blocks {
work := instructions[b.Index]
if len(work.insertInstructions) != 0 {
newInstrs := make([]Instruction, 0, len(fn.Blocks)+len(work.insertInstructions)*3)
for _, instr := range b.Instrs {
if add, ok := work.insertInstructions[instr]; ok {
newInstrs = append(newInstrs, add...)
}
newInstrs = append(newInstrs, instr)
}
b.Instrs = newInstrs
}
}
// TODO(dh): remove inserted allocs that end up unused after lifting.
for _, b := range fn.Blocks {
for _, instr := range b.Instrs {
if instr, ok := instr.(*Alloc); ok && instr.index >= 0 {
liftAlloc(df, instr, newPhis)
}
}
}
// 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)
simplifyPhis(newPhis)
// Eliminate dead φ-nodes.
markLiveNodes(fn.Blocks, newPhis)
// Eliminate ssa:deferstack() call.
if eliminateDeferStack {
b := deferstackCall.block
for i, instr := range b.Instrs {
if instr == deferstackCall {
b.Instrs[i] = nil
b.gaps++
break
}
}
}
}
// Prepend remaining live φ-nodes to each block and possibly kill rundefers.
for _, b := range fn.Blocks {
var head []Instruction
if numAllocs > 0 {
nps := newPhis[b.Index]
head = make([]Instruction, 0, len(nps))
for _, np := range nps {
if np.phi.live {
head = append(head, np.phi)
} else {
for _, edge := range np.phi.Edges {
if refs := edge.Referrers(); refs != nil {
*refs = removeInstr(*refs, np.phi)
}
}
np.phi.block = nil
}
}
}
rundefersToKill := b.rundefers
if usesDefer {
rundefersToKill = 0
}
j := len(head)
if j+b.gaps+rundefersToKill == 0 {
continue // fast path: no new phis or gaps
}
// We could do straight copies instead of element-wise copies
// when both b.gaps and rundefersToKill are zero. However,
// that seems to only be the case ~1% of the time, which
// doesn't seem worth the extra branch.
// Remove dead instructions, add phis
ns := len(b.Instrs) + j - b.gaps - rundefersToKill
if ns <= cap(b.Instrs) {
// b.Instrs has enough capacity to store all instructions
// OPT(dh): check cap vs the actually required space; if
// there is a big enough difference, it may be worth
// allocating a new slice, to avoid pinning memory.
dst := b.Instrs[:cap(b.Instrs)]
i := len(dst) - 1
for n := len(b.Instrs) - 1; n >= 0; n-- {
instr := dst[n]
if instr == nil {
continue
}
if !usesDefer {
if _, ok := instr.(*RunDefers); ok {
continue
}
}
dst[i] = instr
i--
}
off := i + 1 - len(head)
// aid GC
clearInstrs(dst[:off])
dst = dst[off:]
copy(dst, head)
b.Instrs = dst
} else {
// not enough space, so allocate a new slice and copy
// over.
dst := make([]Instruction, ns)
copy(dst, head)
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]
return numAllocs > 0
}
func hasDirectReferrer(instr Instruction) bool {
for _, instr := range *instr.Referrers() {
if _, ok := instr.(*Phi); !ok {
return true
}
}
return false
}
func markLiveNodes(blocks []*BasicBlock, newPhis BlockMap[[]newPhi]) {
// Phis may become dead due to optimization passes.
// Phi nodes are considered live if a non-phi
// node uses them. Once we find a node that is live, we mark all
// of its operands as used, too.
for _, npList := range newPhis {
for _, np := range npList {
phi := np.phi
if !phi.live && hasDirectReferrer(phi) {
markLivePhi(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(phi.(*Phi))
}
}
}
func markLivePhi(phi *Phi) {
phi.live = true
for _, rand := range phi.Edges {
if rand, ok := rand.(*Phi); ok {
if !rand.live {
markLivePhi(rand)
}
}
}
}
// simplifyPhis replaces trivial phis with non-phi alternatives. Phi
// nodes where all edges are identical, or consist of only the phi
// itself and one other value, may be replaced with the value.
func simplifyPhis(newPhis BlockMap[[]newPhi]) {
// find all phis that are trivial and can be replaced with a
// non-phi value. run until we reach a fixpoint, because replacing
// a phi may make other phis trivial.
for changed := true; changed; {
changed = false
for _, npList := range newPhis {
for _, np := range npList {
if np.phi.live {
// we're reusing 'live' to mean 'dead' in the context of simplifyPhis
continue
}
if r, ok := isUselessPhi(np.phi); ok {
// useless phi, replace its uses with the
// replacement value. the dead phi pass will clean
// up the phi afterwards.
replaceAll(np.phi, r)
np.phi.live = true
changed = true
}
}
}
}
}
type BlockSet struct {
idx int
values []bool
count int
}
func NewBlockSet(size int) *BlockSet {
return &BlockSet{values: make([]bool, size)}
}
func (s *BlockSet) Set(s2 *BlockSet) {
copy(s.values, s2.values)
s.count = 0
for _, v := range s.values {
if v {
s.count++
}
}
}
func (s *BlockSet) Num() int {
return s.count
}
func (s *BlockSet) Has(b *BasicBlock) bool {
if b.Index >= len(s.values) {
return false
}
return s.values[b.Index]
}
// add adds b to the set and returns true if the set changed.
func (s *BlockSet) Add(b *BasicBlock) bool {
if s.values[b.Index] {
return false
}
s.count++
s.values[b.Index] = true
s.idx = b.Index
return true
}
func (s *BlockSet) Clear() {
for j := range s.values {
s.values[j] = false
}
s.count = 0
}
// take removes an arbitrary element from a set s and
// returns its index, or returns -1 if empty.
func (s *BlockSet) Take() int {
// [i, end]
for i := s.idx; i < len(s.values); i++ {
if s.values[i] {
s.values[i] = false
s.idx = i
s.count--
return i
}
}
// [start, i)
for i := 0; i < s.idx; i++ {
if s.values[i] {
s.values[i] = false
s.idx = i
s.count--
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
}
type liftInstructions struct {
insertInstructions map[Instruction][]Instruction
renameAllocs []struct {
from *Alloc
to *Alloc
startingAt int
}
}
type liftableBlockDesc struct {
// is the block (partially) unliftable, because it contains unliftable
// instructions or is reachable by an unliftable block
isUnliftable bool
hasLiftableLoad bool
hasLiftableOther bool
// we need to emit stores in predecessors because the unliftable use is in
// a phi
storeInPreds bool
lastLiftable int
firstUnliftable int
}
// liftable determines if alloc can be lifted, and records instructions to split partially liftable allocs.
//
// In the trivial case, all uses of the alloc can be lifted. This is the case when it is only used for storing into and
// loading from. In that case, no instructions are recorded.
//
// In the more complex case, the alloc is used for storing into and loading from, but it is also used as a value, for
// example because it gets passed to a function, e.g. fn(&x). In this case, uses of the alloc fall into one of two
// categories: those that can be lifted and those that can't. A boundary forms between these two categories in the
// function's control flow: Once an unliftable use is encountered, the alloc is no longer liftable for the remainder of
// the basic block the use is in, nor in any blocks reachable from it.
//
// We record instructions that split the alloc into two allocs: one that is used in liftable uses, and one that is used
// in unliftable uses. Whenever we encounter a boundary between liftable and unliftable uses or blocks, we emit a pair
// of Load and Store that copy the value from the liftable alloc into the unliftable alloc. Taking these instructions
// into account, the normal lifting machinery will completely lift the liftable alloc, store the correct lifted values
// into the unliftable alloc, and will not at all lift the unliftable alloc.
//
// In Go syntax, the transformation looks somewhat like this:
//
// func foo() {
// x := 32
// if cond {
// println(x)
// escape(&x)
// println(x)
// } else {
// println(x)
// }
// println(x)
// }
//
// transforms into
//
// func fooSplitAlloc() {
// x := 32
// var x_ int
// if cond {
// println(x)
// x_ = x
// escape(&x_)
// println(x_)
// } else {
// println(x)
// x_ = x
// }
// println(x_)
// }
func liftable(alloc *Alloc, instructions BlockMap[liftInstructions], heads BlockMap[int]) bool {
fn := alloc.block.parent
// Don't lift result values in functions that defer
// calls that may recover from panic.
if fn.Recover != nil {
if slices.Contains(fn.results, alloc) {
return false
}
}
blocks := fn.liftableBlockMap
if len(blocks) != len(fn.Blocks) {
blocks = make(BlockMap[liftableBlockDesc], len(fn.Blocks))
fn.liftableBlockMap = blocks
} else {
clear(blocks)
}
for _, b := range fn.Blocks {
blocks[b.Index].lastLiftable = -1
blocks[b.Index].firstUnliftable = len(b.Instrs) + 1
}
// Look at all uses of the alloc and deduce which blocks have liftable or unliftable instructions.
for _, instr := range alloc.referrers {
// Find the first unliftable use
desc := &blocks[instr.Block().Index]
hasUnliftable := false
inHead := false
switch instr := instr.(type) {
case *Store:
if instr.Val == alloc {
hasUnliftable = true
}
case *Load:
case *debugRef:
case *Phi:
inHead = true
hasUnliftable = true
default:
hasUnliftable = true
}
if hasUnliftable {
desc.isUnliftable = true
if int(instr.ID()) < desc.firstUnliftable {
desc.firstUnliftable = int(instr.ID())
}
if inHead {
desc.storeInPreds = true
desc.firstUnliftable = 0
}
}
}
for _, instr := range alloc.referrers {
// Find the last liftable use, taking the previously calculated firstUnliftable into consideration
desc := &blocks[instr.Block().Index]
if int(instr.ID()) >= desc.firstUnliftable {
continue
}
hasLiftable := false
switch instr := instr.(type) {
case *Store:
if instr.Val != alloc {
desc.hasLiftableOther = true
hasLiftable = true
}
case *Load:
desc.hasLiftableLoad = true
hasLiftable = true
case *debugRef:
desc.hasLiftableOther = true
}
if hasLiftable {
if int(instr.ID()) > desc.lastLiftable {
desc.lastLiftable = int(instr.ID())
}
}
}
for i := range blocks {
// Update firstUnliftable to be one after lastLiftable. We do this to include the unliftable's preceding
// DebugRefs in the renaming.
if blocks[i].lastLiftable == -1 && !blocks[i].storeInPreds {
// There are no liftable instructions (for this alloc) in this block. Set firstUnliftable to the
// first non-head instruction to avoid inserting the store before phi instructions, which would
// fail validation.
first := heads[i]
blocks[i].firstUnliftable = first
} else {
blocks[i].firstUnliftable = blocks[i].lastLiftable + 1
}
}
// If a block is reachable by a (partially) unliftable block, then the entirety of the block is unliftable. In that
// case, stores have to be inserted in the predecessors.
//
// TODO(dh): this isn't always necessary. If the block is reachable by itself, i.e. part of a loop, then if the
// Alloc instruction is itself part of that loop, then there is a subset of instructions in the loop that can be
// lifted. For example:
//
// for {
// x := 42
// println(x)
// escape(&x)
// }
//
// The x that escapes in one iteration of the loop isn't the same x that we read from on the next iteration.
seen := make(BlockMap[bool], len(fn.Blocks))
var dfs func(b *BasicBlock)
dfs = func(b *BasicBlock) {
if seen[b.Index] {
return
}
seen[b.Index] = true
desc := &blocks[b.Index]
desc.hasLiftableLoad = false
desc.hasLiftableOther = false
desc.isUnliftable = true
desc.firstUnliftable = 0
desc.storeInPreds = true
for _, succ := range b.Succs {
dfs(succ)
}
}
for _, b := range fn.Blocks {
if blocks[b.Index].isUnliftable {
for _, succ := range b.Succs {
dfs(succ)
}
}
}
hasLiftableLoad := false
hasLiftableOther := false
hasUnliftable := false
for _, b := range fn.Blocks {
desc := &blocks[b.Index]
hasLiftableLoad = hasLiftableLoad || desc.hasLiftableLoad
hasLiftableOther = hasLiftableOther || desc.hasLiftableOther
if desc.isUnliftable {
hasUnliftable = true
}
}
if !hasLiftableLoad && !hasLiftableOther {
// There are no liftable uses
return false
} else if !hasUnliftable {
// The alloc is entirely liftable without splitting
return true
} else if !hasLiftableLoad {
// The alloc is not entirely liftable, and the only liftable uses are stores. While some of those stores could
// get lifted away, it would also lead to an infinite loop when lifting to a fixpoint, because the newly created
// allocs also get stored into repeatable and that's their only liftable uses.
return false
}
// We need to insert stores for the new alloc. If a (partially) unliftable block has no unliftable
// predecessors and the use isn't in a phi node, then the store can be inserted right before the unliftable use.
// Otherwise, stores have to be inserted at the end of all liftable predecessors.
newAlloc := &Alloc{Heap: true}
newAlloc.setBlock(alloc.block)
newAlloc.setType(alloc.typ)
newAlloc.setSource(alloc.source)
newAlloc.index = -1
newAlloc.comment = "split alloc"
{
work := instructions[alloc.block.Index]
work.insertInstructions[alloc] = append(work.insertInstructions[alloc], newAlloc)
}
predHasStore := make(BlockMap[bool], len(fn.Blocks))
for _, b := range fn.Blocks {
desc := &blocks[b.Index]
bWork := &instructions[b.Index]
if desc.isUnliftable {
bWork.renameAllocs = append(bWork.renameAllocs, struct {
from *Alloc
to *Alloc
startingAt int
}{
alloc, newAlloc, int(desc.firstUnliftable),
})
}
if !desc.isUnliftable {
continue
}
propagate := func(in *BasicBlock, before Instruction) {
load := &Load{
X: alloc,
}
store := &Store{
Addr: newAlloc,
Val: load,
}
load.setType(deref(alloc.typ))
load.setBlock(in)
load.comment = "split alloc"
store.setBlock(in)
updateOperandReferrers(load)
updateOperandReferrers(store)
store.comment = "split alloc"
entry := &instructions[in.Index]
entry.insertInstructions[before] = append(entry.insertInstructions[before], load, store)
}
if desc.storeInPreds {
// emit stores at the end of liftable preds
for _, pred := range b.Preds {
if blocks[pred.Index].isUnliftable {
continue
}
if !alloc.block.Dominates(pred) {
// Consider this cfg:
//
// 1
// /|
// / |
// ↙ ↓
// 2--→3
//
// with an Alloc in block 2. It doesn't make sense to insert a store in block 1 for the jump to
// block 3, because 1 can never see the Alloc in the first place.
//
// Ignoring phi nodes, an Alloc always dominates all of its uses, and phi nodes don't matter here,
// because for the incoming edges that do matter, we do emit the stores.
continue
}
if predHasStore[pred.Index] {
// Don't generate redundant propagations. Not only is it unnecessary, it can lead to infinite loops
// when trying to lift to a fix point, because redundant stores are liftable.
continue
}
predHasStore[pred.Index] = true
before := pred.Instrs[len(pred.Instrs)-1]
propagate(pred, before)
}
} else {
// emit store before the first unliftable use
before := b.Instrs[desc.firstUnliftable]
propagate(b, before)
}
}
return true
}
// liftAlloc lifts alloc into registers and populates newPhis with all the φ-nodes it may require.
func liftAlloc(df domFrontier, alloc *Alloc, newPhis BlockMap[[]newPhi]) {
fn := alloc.Parent()
defblocks := fn.blockset(0)
Aphi := fn.blockset(2)
W := fn.blockset(3)
// Compute defblocks, the set of blocks containing a
// definition of the alloc cell.
for _, instr := range *alloc.Referrers() {
switch instr := instr.(type) {
case *Store:
defblocks.Add(instr.Block())
}
}
// 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())
}
// 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.
// Initialize W and work to defblocks.
for change := true; change; {
change = false
{
// Traverse iterated dominance frontier, inserting φ-nodes.
W.Set(defblocks)
for i := W.Take(); i != -1; i = W.Take() {
n := fn.Blocks[i]
for _, y := range df[n.Index] {
if Aphi.Add(y) {
if len(*alloc.Referrers()) == 0 {
continue
}
live := false
for _, ref := range *alloc.Referrers() {
if _, ok := ref.(*Load); ok {
if y.Reaches(ref.Block()) {
live = true
break
}
}
}
if !live {
continue
}
// Create φ-node.
// It will be prepended to v.Instrs later, if needed.
phi := &Phi{
Edges: make([]Value, len(y.Preds)),
}
phi.comment = alloc.comment
phi.source = alloc.source
phi.setType(deref(alloc.Type()))
phi.block = y
if debugLifting {
fmt.Fprintf(os.Stderr, "\tplace %s = %s at block %s\n", phi.Name(), phi, y)
}
newPhis[y.Index] = append(newPhis[y.Index], newPhi{phi, alloc})
change = true
if defblocks.Add(y) {
W.Add(y)
}
}
}
}
}
}
}
// 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 {
switch instr := instr.(type) {
case *CompositeValue: