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Copy pathlift.go
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1075 lines (977 loc) · 26.7 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- and σ-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.)
//
// 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"
"go/types"
"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) {
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 {
preds := b.Preds[0:len(b.Preds):len(b.Preds)]
if b == fn.Exit {
for i, v := range fn.fakeExits.values {
if v {
preds = append(preds, fn.Blocks[i])
}
}
}
if len(preds) >= 2 {
for _, p := range 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
}
type postDomFrontier [][]*BasicBlock
func (rdf postDomFrontier) add(u, v *BasicBlock) {
rdf[u.Index] = append(rdf[u.Index], v)
}
func (rdf postDomFrontier) build(fn *Function) {
for _, b := range fn.Blocks {
succs := b.Succs[0:len(b.Succs):len(b.Succs)]
if fn.fakeExits.Has(b) {
succs = append(succs, fn.Exit)
}
if len(succs) >= 2 {
for _, s := range succs {
runner := s
for runner != b.pdom.idom {
rdf.add(runner, b)
runner = runner.pdom.idom
}
}
}
}
}
func buildPostDomFrontier(fn *Function) postDomFrontier {
rdf := make(postDomFrontier, len(fn.Blocks))
rdf.build(fn)
return rdf
}
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]
}
func clearInstrs(instrs []Instruction) {
for i := range instrs {
instrs[i] = nil
}
}
// lift replaces local and new Allocs accessed only with
// load/store by IR registers, inserting φ- and σ-nodes where necessary.
// The result is a program in pruned SSI 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.
var df domFrontier
var rdf postDomFrontier
var closure *closure
var newPhis newPhiMap
var newSigmas newSigmaMap
// 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
// 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:
if !liftable(instr) {
instr.index = -1
continue
}
index := -1
if numAllocs == 0 {
df = buildDomFrontier(fn)
rdf = buildPostDomFrontier(fn)
if len(fn.Blocks) > 2 {
closure = transitiveClosure(fn)
}
newPhis = make(newPhiMap, len(fn.Blocks))
newSigmas = make(newSigmaMap, 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)
}
}
}
}
liftAlloc(closure, df, rdf, instr, newPhis, newSigmas)
index = numAllocs
numAllocs++
instr.index = index
case *Defer:
usesDefer = true
case *RunDefers:
b.rundefers++
}
}
}
if numAllocs > 0 {
// 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, newSigmas)
simplifyPhis(newPhis)
// Eliminate dead φ- and σ-nodes.
markLiveNodes(fn.Blocks, newPhis, newSigmas)
}
// 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 _, pred := range b.Preds {
nss := newSigmas[pred.Index]
idx := pred.succIndex(b)
for _, newSigma := range nss {
if sigma := newSigma.sigmas[idx]; sigma != nil && sigma.live {
head = append(head, sigma)
// we didn't populate referrers before, as most
// sigma nodes will be killed
if refs := sigma.X.Referrers(); refs != nil {
*refs = append(*refs, sigma)
}
} else if sigma != nil {
sigma.block = nil
}
}
}
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 and sigmas
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]
}
func hasDirectReferrer(instr Instruction) bool {
for _, instr := range *instr.Referrers() {
switch instr.(type) {
case *Phi, *Sigma:
// ignore
default:
return true
}
}
return false
}
func markLiveNodes(blocks []*BasicBlock, newPhis newPhiMap, newSigmas newSigmaMap) {
// Phi and sigma nodes are considered live if a non-phi, non-sigma
// 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)
}
}
}
for _, npList := range newSigmas {
for _, np := range npList {
for _, sigma := range np.sigmas {
if sigma != nil && !sigma.live && hasDirectReferrer(sigma) {
markLiveSigma(sigma)
}
}
}
}
// 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 {
switch rand := rand.(type) {
case *Phi:
if !rand.live {
markLivePhi(rand)
}
case *Sigma:
if !rand.live {
markLiveSigma(rand)
}
}
}
}
func markLiveSigma(sigma *Sigma) {
sigma.live = true
switch rand := sigma.X.(type) {
case *Phi:
if !rand.live {
markLivePhi(rand)
}
case *Sigma:
if !rand.live {
markLiveSigma(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 newPhiMap) {
// 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
}
}
}
}
for _, npList := range newPhis {
for _, np := range npList {
np.phi.live = false
}
}
}
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
}
type closure struct {
span []uint32
reachables []interval
}
type interval uint32
const (
flagMask = 1 << 31
numBits = 20
lengthBits = 32 - numBits - 1
lengthMask = (1<<lengthBits - 1) << numBits
numMask = 1<<numBits - 1
)
func (c closure) has(s, v *BasicBlock) bool {
idx := uint32(v.Index)
if idx == 1 || s.Dominates(v) {
return true
}
r := c.reachable(s.Index)
for i := 0; i < len(r); i++ {
inv := r[i]
var start, end uint32
if inv&flagMask == 0 {
// small interval
start = uint32(inv & numMask)
end = start + uint32(inv&lengthMask)>>numBits
} else {
// large interval
i++
start = uint32(inv & numMask)
end = uint32(r[i])
}
if idx >= start && idx <= end {
return true
}
}
return false
}
func (c closure) reachable(id int) []interval {
return c.reachables[c.span[id]:c.span[id+1]]
}
func (c closure) walk(current *BasicBlock, b *BasicBlock, visited []bool) {
visited[b.Index] = true
for _, succ := range b.Succs {
if visited[succ.Index] {
continue
}
visited[succ.Index] = true
c.walk(current, succ, visited)
}
}
func transitiveClosure(fn *Function) *closure {
reachable := make([]bool, len(fn.Blocks))
c := &closure{}
c.span = make([]uint32, len(fn.Blocks)+1)
addInterval := func(start, end uint32) {
if l := end - start; l <= 1<<lengthBits-1 {
n := interval(l<<numBits | start)
c.reachables = append(c.reachables, n)
} else {
n1 := interval(1<<31 | start)
n2 := interval(end)
c.reachables = append(c.reachables, n1, n2)
}
}
for i, b := range fn.Blocks[1:] {
for i := range reachable {
reachable[i] = false
}
c.walk(b, b, reachable)
start := ^uint32(0)
for id, isReachable := range reachable {
if !isReachable {
if start != ^uint32(0) {
end := uint32(id) - 1
addInterval(start, end)
start = ^uint32(0)
}
continue
} else if start == ^uint32(0) {
start = uint32(id)
}
}
if start != ^uint32(0) {
addInterval(start, uint32(len(reachable))-1)
}
c.span[i+2] = uint32(len(c.reachables))
}
return c
}
// newPhi is a pair of a newly introduced φ-node and the lifted Alloc
// it replaces.
type newPhi struct {
phi *Phi
alloc *Alloc
}
type newSigma struct {
alloc *Alloc
sigmas []*Sigma
}
// newPhiMap records for each basic block, the set of newPhis that
// must be prepended to the block.
type newPhiMap [][]newPhi
type newSigmaMap [][]newSigma
func liftable(alloc *Alloc) 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
}
fn := alloc.Parent()
// Don't lift named return values in functions that defer
// calls that may recover from panic.
if fn.hasDefer {
for _, nr := range fn.namedResults {
if nr == alloc {
return false
}
}
}
for _, instr := range *alloc.Referrers() {
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")
}
case *Load:
if instr.X != alloc {
panic("Alloc.Referrers is inconsistent")
}
case *DebugRef:
// ok
default:
return false
}
}
return true
}
// 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.
func liftAlloc(closure *closure, df domFrontier, rdf postDomFrontier, alloc *Alloc, newPhis newPhiMap, newSigmas newSigmaMap) {
fn := alloc.Parent()
defblocks := fn.blockset(0)
useblocks := fn.blockset(1)
Aphi := fn.blockset(2)
Asigma := fn.blockset(3)
W := fn.blockset(4)
// Compute defblocks, the set of blocks containing a
// definition of the alloc cell.
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:
defblocks.Add(instr.Block())
case *Load:
useblocks.Add(instr.Block())
for _, ref := range *instr.Referrers() {
useblocks.Add(ref.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
if closure == nil {
live = true
} else {
for _, ref := range *alloc.Referrers() {
if _, ok := ref.(*Load); ok {
if closure.has(y, 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.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})
for _, p := range y.Preds {
useblocks.Add(p)
}
change = true
if defblocks.Add(y) {
W.Add(y)
}
}
}
}
}
{
W.Set(useblocks)
for i := W.Take(); i != -1; i = W.Take() {
n := fn.Blocks[i]
for _, y := range rdf[n.Index] {
if Asigma.Add(y) {
sigmas := make([]*Sigma, 0, len(y.Succs))
anyLive := false
for _, succ := range y.Succs {
live := false
for _, ref := range *alloc.Referrers() {
if closure == nil || closure.has(succ, ref.Block()) {
live = true
anyLive = true
break
}
}
if live {
sigma := &Sigma{
From: y,
X: alloc,
}
sigma.source = alloc.source
sigma.setType(deref(alloc.Type()))
sigma.block = succ
sigmas = append(sigmas, sigma)
} else {
sigmas = append(sigmas, nil)
}
}
if anyLive {
newSigmas[y.Index] = append(newSigmas[y.Index], newSigma{alloc, sigmas})
for _, s := range y.Succs {
defblocks.Add(s)
}
change = true
if useblocks.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 {
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(fn *Function, renaming []Value, alloc *Alloc) Value {
v := renaming[alloc.index]
if v == nil {
v = emitConst(fn, zeroConst(deref(alloc.Type())))
renaming[alloc.index] = v
}
return v
}
// rename implements the Cytron et al-based SSI 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.
//
// 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, newSigmas newSigmaMap) {
// Each φ-node becomes the new name for its associated Alloc.
for _, np := range newPhis[u.Index] {
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())
}
if refs := instr.Addr.Referrers(); refs != nil {
*refs = removeInstr(*refs, instr)
}
if refs := instr.Val.Referrers(); refs != nil {
*refs = removeInstr(*refs, instr)
}
// Delete the Store.
u.Instrs[i] = nil
u.gaps++
}
case *Load:
if alloc, ok := instr.X.(*Alloc); ok && alloc.index >= 0 { // load of Alloc cell
// In theory, we wouldn't be able to replace loads
// directly, because a loaded value could be used in
// different branches, in which case it should be
// replaced with different sigma nodes. But we can't
// simply defer replacement, either, because then
// later stores might incorrectly affect this load.
//
// To avoid doing renaming on _all_ values (instead of
// just loads and stores like we're doing), we make
// sure during code generation that each load is only
// used in one block. For example, in constant switch
// statements, where the tag is only evaluated once,
// we store it in a temporary and load it for each
// comparison, so that we have individual loads to
// replace.
newval := renamed(u.Parent(), renaming, alloc)
if debugLifting {
fmt.Fprintf(os.Stderr, "\tupdate load %s = %s with %s\n",
instr.Name(), instr, newval)
}
replaceAll(instr, newval)
u.Instrs[i] = nil
u.gaps++
}
case *DebugRef:
if x, ok := instr.X.(*Alloc); ok && x.index >= 0 {
if instr.IsAddr {
instr.X = renamed(u.Parent(), renaming, x)
instr.IsAddr = false
// Add DebugRef to instr.X's referrers.
if refs := instr.X.Referrers(); refs != nil {
*refs = append(*refs, instr)