Repository navigation
Expand file tree
/
Copy pathblockopt.go
More file actions
205 lines (181 loc) · 5.02 KB
/
Copy pathblockopt.go
File metadata and controls
205 lines (181 loc) · 5.02 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
// 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
// Simple block optimizations to simplify the control flow graph.
// TODO(adonovan): opt: instead of creating several "unreachable" blocks
// per function in the Builder, reuse a single one (e.g. at Blocks[1])
// to reduce garbage.
import (
"fmt"
"os"
)
// If true, perform sanity checking and show progress at each
// successive iteration of optimizeBlocks. Very verbose.
const debugBlockOpt = false
// markReachable sets Index=-1 for all blocks reachable from b.
func markReachable(b *BasicBlock) {
b.gaps = -1
for _, succ := range b.Succs {
if succ.gaps == 0 {
markReachable(succ)
}
}
}
// deleteUnreachableBlocks marks all reachable blocks of f and
// eliminates (nils) all others, including possibly cyclic subgraphs.
func deleteUnreachableBlocks(f *Function) {
const white, black = 0, -1
// We borrow b.gaps temporarily as the mark bit.
for _, b := range f.Blocks {
b.gaps = white
}
markReachable(f.Blocks[0])
// In SSI form, we need the exit to be reachable for correct
// post-dominance information. In original form, however, we
// cannot unconditionally mark it reachable because we won't
// be adding fake edges, and this breaks the calculation of
// dominance information.
markReachable(f.Exit)
for i, b := range f.Blocks {
if b.gaps == white {
for _, c := range b.Succs {
if c.gaps == black {
c.removePred(b) // delete white->black edge
}
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "unreachable", b)
}
f.Blocks[i] = nil // delete b
}
}
f.removeNilBlocks()
}
// jumpThreading attempts to apply simple jump-threading to block b,
// in which a->b->c become a->c if b is just a Jump.
// The result is true if the optimization was applied.
func jumpThreading(f *Function, b *BasicBlock) bool {
if b.Index == 0 {
return false // don't apply to entry block
}
if b.Instrs == nil {
return false
}
for _, pred := range b.Preds {
switch pred.Control().(type) {
case *ConstantSwitch:
// don't optimize away the head blocks of switch statements
return false
}
}
if _, ok := b.Instrs[0].(*Jump); !ok {
return false // not just a jump
}
c := b.Succs[0]
if c == b {
return false // don't apply to degenerate jump-to-self.
}
if c.hasPhi() {
return false // not sound without more effort
}
for j, a := range b.Preds {
a.replaceSucc(b, c)
// If a now has two edges to c, replace its degenerate If by Jump.
if len(a.Succs) == 2 && a.Succs[0] == c && a.Succs[1] == c {
jump := new(Jump)
jump.setBlock(a)
a.Instrs[len(a.Instrs)-1] = jump
a.Succs = a.Succs[:1]
c.removePred(b)
} else {
if j == 0 {
c.replacePred(b, a)
} else {
c.Preds = append(c.Preds, a)
}
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "jumpThreading", a, b, c)
}
}
f.Blocks[b.Index] = nil // delete b
return true
}
// fuseBlocks attempts to apply the block fusion optimization to block
// a, in which a->b becomes ab if len(a.Succs)==len(b.Preds)==1.
// The result is true if the optimization was applied.
func fuseBlocks(f *Function, a *BasicBlock) bool {
if len(a.Succs) != 1 {
return false
}
if a.Succs[0] == f.Exit {
return false
}
b := a.Succs[0]
if len(b.Preds) != 1 {
return false
}
if _, ok := a.Instrs[len(a.Instrs)-1].(*Panic); ok {
// panics aren't simple jumps, they have side effects.
return false
}
// Degenerate &&/|| ops may result in a straight-line CFG
// containing φ-nodes. (Ideally we'd replace such them with
// their sole operand but that requires Referrers, built later.)
if b.hasPhi() {
return false // not sound without further effort
}
// Eliminate jump at end of A, then copy all of B across.
a.Instrs = append(a.Instrs[:len(a.Instrs)-1], b.Instrs...)
for _, instr := range b.Instrs {
instr.setBlock(a)
}
// A inherits B's successors
a.Succs = append(a.succs2[:0], b.Succs...)
// Fix up Preds links of all successors of B.
for _, c := range b.Succs {
c.replacePred(b, a)
}
if debugBlockOpt {
fmt.Fprintln(os.Stderr, "fuseBlocks", a, b)
}
f.Blocks[b.Index] = nil // delete b
return true
}
// optimizeBlocks() performs some simple block optimizations on a
// completed function: dead block elimination, block fusion, jump
// threading.
func optimizeBlocks(f *Function) {
if debugBlockOpt {
f.WriteTo(os.Stderr)
mustSanityCheck(f, nil)
}
deleteUnreachableBlocks(f)
// Loop until no further progress.
changed := true
for changed {
changed = false
if debugBlockOpt {
f.WriteTo(os.Stderr)
mustSanityCheck(f, nil)
}
for _, b := range f.Blocks {
// f.Blocks will temporarily contain nils to indicate
// deleted blocks; we remove them at the end.
if b == nil {
continue
}
// Fuse blocks. b->c becomes bc.
if fuseBlocks(f, b) {
changed = true
}
// a->b->c becomes a->c if b contains only a Jump.
if jumpThreading(f, b) {
changed = true
continue // (b was disconnected)
}
}
}
f.removeNilBlocks()
}