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
206
207
208
209
210
211
212
213
|
// Copyright 2014 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "src/compiler/backend/jump-threading.h"
#include "src/compiler/backend/code-generator-impl.h"
namespace v8 {
namespace internal {
namespace compiler {
#define TRACE(...) \
do { \
if (FLAG_trace_turbo_jt) PrintF(__VA_ARGS__); \
} while (false)
namespace {
struct JumpThreadingState {
bool forwarded;
ZoneVector<RpoNumber>& result;
ZoneStack<RpoNumber>& stack;
void Clear(size_t count) { result.assign(count, unvisited()); }
void PushIfUnvisited(RpoNumber num) {
if (result[num.ToInt()] == unvisited()) {
stack.push(num);
result[num.ToInt()] = onstack();
}
}
void Forward(RpoNumber to) {
RpoNumber from = stack.top();
RpoNumber to_to = result[to.ToInt()];
bool pop = true;
if (to == from) {
TRACE(" xx %d\n", from.ToInt());
result[from.ToInt()] = from;
} else if (to_to == unvisited()) {
TRACE(" fw %d -> %d (recurse)\n", from.ToInt(), to.ToInt());
stack.push(to);
result[to.ToInt()] = onstack();
pop = false; // recurse.
} else if (to_to == onstack()) {
TRACE(" fw %d -> %d (cycle)\n", from.ToInt(), to.ToInt());
result[from.ToInt()] = to; // break the cycle.
forwarded = true;
} else {
TRACE(" fw %d -> %d (forward)\n", from.ToInt(), to.ToInt());
result[from.ToInt()] = to_to; // forward the block.
forwarded = true;
}
if (pop) stack.pop();
}
RpoNumber unvisited() { return RpoNumber::FromInt(-1); }
RpoNumber onstack() { return RpoNumber::FromInt(-2); }
};
bool IsBlockWithBranchPoisoning(InstructionSequence* code,
InstructionBlock* block) {
if (block->PredecessorCount() != 1) return false;
RpoNumber pred_rpo = (block->predecessors())[0];
const InstructionBlock* pred = code->InstructionBlockAt(pred_rpo);
if (pred->code_start() == pred->code_end()) return false;
Instruction* instr = code->InstructionAt(pred->code_end() - 1);
FlagsMode mode = FlagsModeField::decode(instr->opcode());
return mode == kFlags_branch_and_poison;
}
} // namespace
bool JumpThreading::ComputeForwarding(Zone* local_zone,
ZoneVector<RpoNumber>* result,
InstructionSequence* code,
bool frame_at_start) {
ZoneStack<RpoNumber> stack(local_zone);
JumpThreadingState state = {false, *result, stack};
state.Clear(code->InstructionBlockCount());
// Iterate over the blocks forward, pushing the blocks onto the stack.
for (auto const block : code->instruction_blocks()) {
RpoNumber current = block->rpo_number();
state.PushIfUnvisited(current);
// Process the stack, which implements DFS through empty blocks.
while (!state.stack.empty()) {
InstructionBlock* block = code->InstructionBlockAt(state.stack.top());
// Process the instructions in a block up to a non-empty instruction.
TRACE("jt [%d] B%d\n", static_cast<int>(stack.size()),
block->rpo_number().ToInt());
RpoNumber fw = block->rpo_number();
if (!IsBlockWithBranchPoisoning(code, block)) {
bool fallthru = true;
for (int i = block->code_start(); i < block->code_end(); ++i) {
Instruction* instr = code->InstructionAt(i);
if (!instr->AreMovesRedundant()) {
// can't skip instructions with non redundant moves.
TRACE(" parallel move\n");
fallthru = false;
} else if (FlagsModeField::decode(instr->opcode()) != kFlags_none) {
// can't skip instructions with flags continuations.
TRACE(" flags\n");
fallthru = false;
} else if (instr->IsNop()) {
// skip nops.
TRACE(" nop\n");
continue;
} else if (instr->arch_opcode() == kArchJmp) {
// try to forward the jump instruction.
TRACE(" jmp\n");
// if this block deconstructs the frame, we can't forward it.
// TODO(mtrofin): we can still forward if we end up building
// the frame at start. So we should move the decision of whether
// to build a frame or not in the register allocator, and trickle it
// here and to the code generator.
if (frame_at_start || !(block->must_deconstruct_frame() ||
block->must_construct_frame())) {
fw = code->InputRpo(instr, 0);
}
fallthru = false;
} else {
// can't skip other instructions.
TRACE(" other\n");
fallthru = false;
}
break;
}
if (fallthru) {
int next = 1 + block->rpo_number().ToInt();
if (next < code->InstructionBlockCount())
fw = RpoNumber::FromInt(next);
}
}
state.Forward(fw);
}
}
#ifdef DEBUG
for (RpoNumber num : *result) {
DCHECK(num.IsValid());
}
#endif
if (FLAG_trace_turbo_jt) {
for (int i = 0; i < static_cast<int>(result->size()); i++) {
TRACE("B%d ", i);
int to = (*result)[i].ToInt();
if (i != to) {
TRACE("-> B%d\n", to);
} else {
TRACE("\n");
}
}
}
return state.forwarded;
}
void JumpThreading::ApplyForwarding(Zone* local_zone,
ZoneVector<RpoNumber> const& result,
InstructionSequence* code) {
if (!FLAG_turbo_jt) return;
ZoneVector<bool> skip(static_cast<int>(result.size()), false, local_zone);
// Skip empty blocks when the previous block doesn't fall through.
bool prev_fallthru = true;
for (auto const block : code->instruction_blocks()) {
int block_num = block->rpo_number().ToInt();
skip[block_num] = !prev_fallthru && result[block_num].ToInt() != block_num;
bool fallthru = true;
for (int i = block->code_start(); i < block->code_end(); ++i) {
Instruction* instr = code->InstructionAt(i);
FlagsMode mode = FlagsModeField::decode(instr->opcode());
if (mode == kFlags_branch || mode == kFlags_branch_and_poison) {
fallthru = false; // branches don't fall through to the next block.
} else if (instr->arch_opcode() == kArchJmp) {
if (skip[block_num]) {
// Overwrite a redundant jump with a nop.
TRACE("jt-fw nop @%d\n", i);
instr->OverwriteWithNop();
}
fallthru = false; // jumps don't fall through to the next block.
}
}
prev_fallthru = fallthru;
}
// Patch RPO immediates.
InstructionSequence::Immediates& immediates = code->immediates();
for (size_t i = 0; i < immediates.size(); i++) {
Constant constant = immediates[i];
if (constant.type() == Constant::kRpoNumber) {
RpoNumber rpo = constant.ToRpoNumber();
RpoNumber fw = result[rpo.ToInt()];
if (!(fw == rpo)) immediates[i] = Constant(fw);
}
}
// Renumber the blocks so that IsNextInAssemblyOrder() will return true,
// even if there are skipped blocks in-between.
int ao = 0;
for (auto const block : code->ao_blocks()) {
block->set_ao_number(RpoNumber::FromInt(ao));
if (!skip[block->rpo_number().ToInt()]) ao++;
}
}
#undef TRACE
} // namespace compiler
} // namespace internal
} // namespace v8
|
