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/
cse.go
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/
cse.go
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// Copyright 2015 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
import (
"fmt"
"sort"
)
const (
cmpDepth = 4
)
// cse does common-subexpression elimination on the Function.
// Values are just relinked, nothing is deleted. A subsequent deadcode
// pass is required to actually remove duplicate expressions.
func cse(f *Func) {
// Two values are equivalent if they satisfy the following definition:
// equivalent(v, w):
// v.op == w.op
// v.type == w.type
// v.aux == w.aux
// v.auxint == w.auxint
// len(v.args) == len(w.args)
// v.block == w.block if v.op == OpPhi
// equivalent(v.args[i], w.args[i]) for i in 0..len(v.args)-1
// The algorithm searches for a partition of f's values into
// equivalence classes using the above definition.
// It starts with a coarse partition and iteratively refines it
// until it reaches a fixed point.
// Make initial coarse partitions by using a subset of the conditions above.
a := make([]*Value, 0, f.NumValues())
auxIDs := auxmap{}
for _, b := range f.Blocks {
for _, v := range b.Values {
if auxIDs[v.Aux] == 0 {
auxIDs[v.Aux] = int32(len(auxIDs)) + 1
}
if v.Type.IsMemory() {
continue // memory values can never cse
}
if opcodeTable[v.Op].commutative && len(v.Args) == 2 && v.Args[1].ID < v.Args[0].ID {
// Order the arguments of binary commutative operations.
v.Args[0], v.Args[1] = v.Args[1], v.Args[0]
}
a = append(a, v)
}
}
partition := partitionValues(a, auxIDs)
// map from value id back to eqclass id
valueEqClass := make([]ID, f.NumValues())
for _, b := range f.Blocks {
for _, v := range b.Values {
// Use negative equivalence class #s for unique values.
valueEqClass[v.ID] = -v.ID
}
}
for i, e := range partition {
if f.pass.debug > 1 && len(e) > 500 {
fmt.Printf("CSE.large partition (%d): ", len(e))
for j := 0; j < 3; j++ {
fmt.Printf("%s ", e[j].LongString())
}
fmt.Println()
}
for _, v := range e {
valueEqClass[v.ID] = ID(i)
}
if f.pass.debug > 2 && len(e) > 1 {
fmt.Printf("CSE.partition #%d:", i)
for _, v := range e {
fmt.Printf(" %s", v.String())
}
fmt.Printf("\n")
}
}
// Find an equivalence class where some members of the class have
// non-equivalent arguments. Split the equivalence class appropriately.
// Repeat until we can't find any more splits.
for {
changed := false
// partition can grow in the loop. By not using a range loop here,
// we process new additions as they arrive, avoiding O(n^2) behavior.
for i := 0; i < len(partition); i++ {
e := partition[i]
v := e[0]
// all values in this equiv class that are not equivalent to v get moved
// into another equiv class.
// To avoid allocating while building that equivalence class,
// move the values equivalent to v to the beginning of e
// and other values to the end of e.
allvals := e
eqloop:
for j := 1; j < len(e); {
w := e[j]
equivalent := true
for i := 0; i < len(v.Args); i++ {
if valueEqClass[v.Args[i].ID] != valueEqClass[w.Args[i].ID] {
equivalent = false
break
}
}
if !equivalent || v.Type.Compare(w.Type) != CMPeq {
// w is not equivalent to v.
// move it to the end and shrink e.
e[j], e[len(e)-1] = e[len(e)-1], e[j]
e = e[:len(e)-1]
valueEqClass[w.ID] = ID(len(partition))
changed = true
continue eqloop
}
// v and w are equivalent. Keep w in e.
j++
}
partition[i] = e
if len(e) < len(allvals) {
partition = append(partition, allvals[len(e):])
}
}
if !changed {
break
}
}
// Dominator tree (f.sdom) is computed by the generic domtree pass.
// Compute substitutions we would like to do. We substitute v for w
// if v and w are in the same equivalence class and v dominates w.
rewrite := make([]*Value, f.NumValues())
for _, e := range partition {
sort.Sort(partitionByDom{e, f.sdom})
for i := 0; i < len(e)-1; i++ {
// e is sorted by domorder, so a maximal dominant element is first in the slice
v := e[i]
if v == nil {
continue
}
e[i] = nil
// Replace all elements of e which v dominates
for j := i + 1; j < len(e); j++ {
w := e[j]
if w == nil {
continue
}
if f.sdom.isAncestorEq(v.Block, w.Block) {
rewrite[w.ID] = v
e[j] = nil
} else {
// e is sorted by domorder, so v.Block doesn't dominate any subsequent blocks in e
break
}
}
}
}
rewrites := int64(0)
// Apply substitutions
for _, b := range f.Blocks {
for _, v := range b.Values {
for i, w := range v.Args {
if x := rewrite[w.ID]; x != nil {
v.SetArg(i, x)
rewrites++
}
}
}
if v := b.Control; v != nil {
if x := rewrite[v.ID]; x != nil {
if v.Op == OpNilCheck {
// nilcheck pass will remove the nil checks and log
// them appropriately, so don't mess with them here.
continue
}
b.SetControl(x)
}
}
}
if f.pass.stats > 0 {
f.LogStat("CSE REWRITES", rewrites)
}
}
// An eqclass approximates an equivalence class. During the
// algorithm it may represent the union of several of the
// final equivalence classes.
type eqclass []*Value
// partitionValues partitions the values into equivalence classes
// based on having all the following features match:
// - opcode
// - type
// - auxint
// - aux
// - nargs
// - block # if a phi op
// - first two arg's opcodes and auxint
// - NOT first two arg's aux; that can break CSE.
// partitionValues returns a list of equivalence classes, each
// being a sorted by ID list of *Values. The eqclass slices are
// backed by the same storage as the input slice.
// Equivalence classes of size 1 are ignored.
func partitionValues(a []*Value, auxIDs auxmap) []eqclass {
sort.Sort(sortvalues{a, auxIDs})
var partition []eqclass
for len(a) > 0 {
v := a[0]
j := 1
for ; j < len(a); j++ {
w := a[j]
if cmpVal(v, w, auxIDs, cmpDepth) != CMPeq {
break
}
}
if j > 1 {
partition = append(partition, a[:j])
}
a = a[j:]
}
return partition
}
func lt2Cmp(isLt bool) Cmp {
if isLt {
return CMPlt
}
return CMPgt
}
type auxmap map[interface{}]int32
func cmpVal(v, w *Value, auxIDs auxmap, depth int) Cmp {
// Try to order these comparison by cost (cheaper first)
if v.Op != w.Op {
return lt2Cmp(v.Op < w.Op)
}
if v.AuxInt != w.AuxInt {
return lt2Cmp(v.AuxInt < w.AuxInt)
}
if len(v.Args) != len(w.Args) {
return lt2Cmp(len(v.Args) < len(w.Args))
}
if v.Op == OpPhi && v.Block != w.Block {
return lt2Cmp(v.Block.ID < w.Block.ID)
}
if v.Type.IsMemory() {
// We will never be able to CSE two values
// that generate memory.
return lt2Cmp(v.ID < w.ID)
}
if tc := v.Type.Compare(w.Type); tc != CMPeq {
return tc
}
if v.Aux != w.Aux {
if v.Aux == nil {
return CMPlt
}
if w.Aux == nil {
return CMPgt
}
return lt2Cmp(auxIDs[v.Aux] < auxIDs[w.Aux])
}
if depth > 0 {
for i := range v.Args {
if v.Args[i] == w.Args[i] {
// skip comparing equal args
continue
}
if ac := cmpVal(v.Args[i], w.Args[i], auxIDs, depth-1); ac != CMPeq {
return ac
}
}
}
return CMPeq
}
// Sort values to make the initial partition.
type sortvalues struct {
a []*Value // array of values
auxIDs auxmap // aux -> aux ID map
}
func (sv sortvalues) Len() int { return len(sv.a) }
func (sv sortvalues) Swap(i, j int) { sv.a[i], sv.a[j] = sv.a[j], sv.a[i] }
func (sv sortvalues) Less(i, j int) bool {
v := sv.a[i]
w := sv.a[j]
if cmp := cmpVal(v, w, sv.auxIDs, cmpDepth); cmp != CMPeq {
return cmp == CMPlt
}
// Sort by value ID last to keep the sort result deterministic.
return v.ID < w.ID
}
type partitionByDom struct {
a []*Value // array of values
sdom SparseTree
}
func (sv partitionByDom) Len() int { return len(sv.a) }
func (sv partitionByDom) Swap(i, j int) { sv.a[i], sv.a[j] = sv.a[j], sv.a[i] }
func (sv partitionByDom) Less(i, j int) bool {
v := sv.a[i]
w := sv.a[j]
return sv.sdom.domorder(v.Block) < sv.sdom.domorder(w.Block)
}