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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
// This file contains code to compute the dominator tree
// of a control-flow graph.
// postorder computes a postorder traversal ordering for the
// basic blocks in f. Unreachable blocks will not appear.
func postorder(f *Func) []*Block {
return postorderWithNumbering(f, nil)
}
type blockAndIndex struct {
b *Block
index int // index is the number of successor edges of b that have already been explored.
}
// postorderWithNumbering provides a DFS postordering.
// This seems to make loop-finding more robust.
func postorderWithNumbering(f *Func, ponums []int32) []*Block {
seen := f.Cache.allocBoolSlice(f.NumBlocks())
defer f.Cache.freeBoolSlice(seen)
// result ordering
order := make([]*Block, 0, len(f.Blocks))
// stack of blocks and next child to visit
// A constant bound allows this to be stack-allocated. 32 is
// enough to cover almost every postorderWithNumbering call.
s := make([]blockAndIndex, 0, 32)
s = append(s, blockAndIndex{b: f.Entry})
seen[f.Entry.ID] = true
for len(s) > 0 {
tos := len(s) - 1
x := s[tos]
b := x.b
if i := x.index; i < len(b.Succs) {
s[tos].index++
bb := b.Succs[i].Block()
if !seen[bb.ID] {
seen[bb.ID] = true
s = append(s, blockAndIndex{b: bb})
}
continue
}
s = s[:tos]
if ponums != nil {
ponums[b.ID] = int32(len(order))
}
order = append(order, b)
}
return order
}
type linkedBlocks func(*Block) []Edge
func dominators(f *Func) []*Block {
preds := func(b *Block) []Edge { return b.Preds }
succs := func(b *Block) []Edge { return b.Succs }
//TODO: benchmark and try to find criteria for swapping between
// dominatorsSimple and dominatorsLT
return f.dominatorsLTOrig(f.Entry, preds, succs)
}
// dominatorsLTOrig runs Lengauer-Tarjan to compute a dominator tree starting at
// entry and using predFn/succFn to find predecessors/successors to allow
// computing both dominator and post-dominator trees.
func (f *Func) dominatorsLTOrig(entry *Block, predFn linkedBlocks, succFn linkedBlocks) []*Block {
// Adapted directly from the original TOPLAS article's "simple" algorithm
maxBlockID := entry.Func.NumBlocks()
scratch := f.Cache.allocIDSlice(7 * maxBlockID)
defer f.Cache.freeIDSlice(scratch)
semi := scratch[0*maxBlockID : 1*maxBlockID]
vertex := scratch[1*maxBlockID : 2*maxBlockID]
label := scratch[2*maxBlockID : 3*maxBlockID]
parent := scratch[3*maxBlockID : 4*maxBlockID]
ancestor := scratch[4*maxBlockID : 5*maxBlockID]
bucketHead := scratch[5*maxBlockID : 6*maxBlockID]
bucketLink := scratch[6*maxBlockID : 7*maxBlockID]
// This version uses integers for most of the computation,
// to make the work arrays smaller and pointer-free.
// fromID translates from ID to *Block where that is needed.
fromID := f.Cache.allocBlockSlice(maxBlockID)
defer f.Cache.freeBlockSlice(fromID)
for _, v := range f.Blocks {
fromID[v.ID] = v
}
idom := make([]*Block, maxBlockID)
// Step 1. Carry out a depth first search of the problem graph. Number
// the vertices from 1 to n as they are reached during the search.
n := f.dfsOrig(entry, succFn, semi, vertex, label, parent)
for i := n; i >= 2; i-- {
w := vertex[i]
// step2 in TOPLAS paper
for _, e := range predFn(fromID[w]) {
v := e.b
if semi[v.ID] == 0 {
// skip unreachable predecessor
// not in original, but we're using existing pred instead of building one.
continue
}
u := evalOrig(v.ID, ancestor, semi, label)
if semi[u] < semi[w] {
semi[w] = semi[u]
}
}
// add w to bucket[vertex[semi[w]]]
// implement bucket as a linked list implemented
// in a pair of arrays.
vsw := vertex[semi[w]]
bucketLink[w] = bucketHead[vsw]
bucketHead[vsw] = w
linkOrig(parent[w], w, ancestor)
// step3 in TOPLAS paper
for v := bucketHead[parent[w]]; v != 0; v = bucketLink[v] {
u := evalOrig(v, ancestor, semi, label)
if semi[u] < semi[v] {
idom[v] = fromID[u]
} else {
idom[v] = fromID[parent[w]]
}
}
}
// step 4 in toplas paper
for i := ID(2); i <= n; i++ {
w := vertex[i]
if idom[w].ID != vertex[semi[w]] {
idom[w] = idom[idom[w].ID]
}
}
return idom
}
// dfsOrig performs a depth first search over the blocks starting at entry block
// (in arbitrary order). This is a de-recursed version of dfs from the
// original Tarjan-Lengauer TOPLAS article. It's important to return the
// same values for parent as the original algorithm.
func (f *Func) dfsOrig(entry *Block, succFn linkedBlocks, semi, vertex, label, parent []ID) ID {
n := ID(0)
s := make([]*Block, 0, 256)
s = append(s, entry)
for len(s) > 0 {
v := s[len(s)-1]
s = s[:len(s)-1]
// recursing on v
if semi[v.ID] != 0 {
continue // already visited
}
n++
semi[v.ID] = n
vertex[n] = v.ID
label[v.ID] = v.ID
// ancestor[v] already zero
for _, e := range succFn(v) {
w := e.b
// if it has a dfnum, we've already visited it
if semi[w.ID] == 0 {
// yes, w can be pushed multiple times.
s = append(s, w)
parent[w.ID] = v.ID // keep overwriting this till it is visited.
}
}
}
return n
}
// compressOrig is the "simple" compress function from LT paper.
func compressOrig(v ID, ancestor, semi, label []ID) {
if ancestor[ancestor[v]] != 0 {
compressOrig(ancestor[v], ancestor, semi, label)
if semi[label[ancestor[v]]] < semi[label[v]] {
label[v] = label[ancestor[v]]
}
ancestor[v] = ancestor[ancestor[v]]
}
}
// evalOrig is the "simple" eval function from LT paper.
func evalOrig(v ID, ancestor, semi, label []ID) ID {
if ancestor[v] == 0 {
return v
}
compressOrig(v, ancestor, semi, label)
return label[v]
}
func linkOrig(v, w ID, ancestor []ID) {
ancestor[w] = v
}
// dominatorsSimple computes the dominator tree for f. It returns a slice
// which maps block ID to the immediate dominator of that block.
// Unreachable blocks map to nil. The entry block maps to nil.
func dominatorsSimple(f *Func) []*Block {
// A simple algorithm for now
// Cooper, Harvey, Kennedy
idom := make([]*Block, f.NumBlocks())
// Compute postorder walk
post := f.postorder()
// Make map from block id to order index (for intersect call)
postnum := f.Cache.allocIntSlice(f.NumBlocks())
defer f.Cache.freeIntSlice(postnum)
for i, b := range post {
postnum[b.ID] = i
}
// Make the entry block a self-loop
idom[f.Entry.ID] = f.Entry
if postnum[f.Entry.ID] != len(post)-1 {
f.Fatalf("entry block %v not last in postorder", f.Entry)
}
// Compute relaxation of idom entries
for {
changed := false
for i := len(post) - 2; i >= 0; i-- {
b := post[i]
var d *Block
for _, e := range b.Preds {
p := e.b
if idom[p.ID] == nil {
continue
}
if d == nil {
d = p
continue
}
d = intersect(d, p, postnum, idom)
}
if d != idom[b.ID] {
idom[b.ID] = d
changed = true
}
}
if !changed {
break
}
}
// Set idom of entry block to nil instead of itself.
idom[f.Entry.ID] = nil
return idom
}
// intersect finds the closest dominator of both b and c.
// It requires a postorder numbering of all the blocks.
func intersect(b, c *Block, postnum []int, idom []*Block) *Block {
// TODO: This loop is O(n^2). It used to be used in nilcheck,
// see BenchmarkNilCheckDeep*.
for b != c {
if postnum[b.ID] < postnum[c.ID] {
b = idom[b.ID]
} else {
c = idom[c.ID]
}
}
return b
}