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
|
// 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
// tighten moves Values closer to the Blocks in which they are used.
// This can reduce the amount of register spilling required,
// if it doesn't also create more live values.
// A Value can be moved to any block that
// dominates all blocks in which it is used.
func tighten(f *Func) {
canMove := make([]bool, f.NumValues())
for _, b := range f.Blocks {
for _, v := range b.Values {
if v.Op.isLoweredGetClosurePtr() {
// Must stay in the entry block.
continue
}
switch v.Op {
case OpPhi, OpArg, OpArgIntReg, OpArgFloatReg, OpSelect0, OpSelect1, OpSelectN:
// Phis need to stay in their block.
// Arg must stay in the entry block.
// Tuple selectors must stay with the tuple generator.
// SelectN is typically, ultimately, a register.
continue
}
if v.MemoryArg() != nil {
// We can't move values which have a memory arg - it might
// make two memory values live across a block boundary.
continue
}
// Count arguments which will need a register.
narg := 0
for _, a := range v.Args {
if !a.rematerializeable() {
narg++
}
}
if narg >= 2 && !v.Type.IsFlags() {
// Don't move values with more than one input, as that may
// increase register pressure.
// We make an exception for flags, as we want flag generators
// moved next to uses (because we only have 1 flag register).
continue
}
canMove[v.ID] = true
}
}
// Build data structure for fast least-common-ancestor queries.
lca := makeLCArange(f)
// For each moveable value, record the block that dominates all uses found so far.
target := make([]*Block, f.NumValues())
// Grab loop information.
// We use this to make sure we don't tighten a value into a (deeper) loop.
idom := f.Idom()
loops := f.loopnest()
loops.calculateDepths()
changed := true
for changed {
changed = false
// Reset target
for i := range target {
target[i] = nil
}
// Compute target locations (for moveable values only).
// target location = the least common ancestor of all uses in the dominator tree.
for _, b := range f.Blocks {
for _, v := range b.Values {
for i, a := range v.Args {
if !canMove[a.ID] {
continue
}
use := b
if v.Op == OpPhi {
use = b.Preds[i].b
}
if target[a.ID] == nil {
target[a.ID] = use
} else {
target[a.ID] = lca.find(target[a.ID], use)
}
}
}
for _, c := range b.ControlValues() {
if !canMove[c.ID] {
continue
}
if target[c.ID] == nil {
target[c.ID] = b
} else {
target[c.ID] = lca.find(target[c.ID], b)
}
}
}
// If the target location is inside a loop,
// move the target location up to just before the loop head.
for _, b := range f.Blocks {
origloop := loops.b2l[b.ID]
for _, v := range b.Values {
t := target[v.ID]
if t == nil {
continue
}
targetloop := loops.b2l[t.ID]
for targetloop != nil && (origloop == nil || targetloop.depth > origloop.depth) {
t = idom[targetloop.header.ID]
target[v.ID] = t
targetloop = loops.b2l[t.ID]
}
}
}
// Move values to target locations.
for _, b := range f.Blocks {
for i := 0; i < len(b.Values); i++ {
v := b.Values[i]
t := target[v.ID]
if t == nil || t == b {
// v is not moveable, or is already in correct place.
continue
}
// Move v to the block which dominates its uses.
t.Values = append(t.Values, v)
v.Block = t
last := len(b.Values) - 1
b.Values[i] = b.Values[last]
b.Values[last] = nil
b.Values = b.Values[:last]
changed = true
i--
}
}
}
}
// phiTighten moves constants closer to phi users.
// This pass avoids having lots of constants live for lots of the program.
// See issue 16407.
func phiTighten(f *Func) {
for _, b := range f.Blocks {
for _, v := range b.Values {
if v.Op != OpPhi {
continue
}
for i, a := range v.Args {
if !a.rematerializeable() {
continue // not a constant we can move around
}
if a.Block == b.Preds[i].b {
continue // already in the right place
}
// Make a copy of a, put in predecessor block.
v.SetArg(i, a.copyInto(b.Preds[i].b))
}
}
}
}
|
