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
|
//===- MarkLive.cpp -------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements --gc-sections, which is a feature to remove unused
// chunks from the output. Unused chunks are those that are not reachable from
// known root symbols or chunks. This feature is implemented as a mark-sweep
// garbage collector.
//
// Here's how it works. Each InputChunk has a "Live" bit. The bit is off by
// default. Starting with the GC-roots, visit all reachable chunks and set their
// Live bits. The Writer will then ignore chunks whose Live bits are off, so
// that such chunk are not appear in the output.
//
//===----------------------------------------------------------------------===//
#include "MarkLive.h"
#include "Config.h"
#include "InputChunks.h"
#include "InputElement.h"
#include "SymbolTable.h"
#include "Symbols.h"
#define DEBUG_TYPE "lld"
using namespace llvm;
using namespace llvm::wasm;
namespace lld::wasm {
namespace {
class MarkLive {
public:
void run();
private:
void enqueue(Symbol *sym);
void enqueueInitFunctions(const ObjFile *sym);
void mark();
bool isCallCtorsLive();
// A list of chunks to visit.
SmallVector<InputChunk *, 256> queue;
};
} // namespace
void MarkLive::enqueue(Symbol *sym) {
if (!sym || sym->isLive())
return;
LLVM_DEBUG(dbgs() << "markLive: " << sym->getName() << "\n");
InputFile *file = sym->getFile();
bool needInitFunctions = file && !file->isLive() && sym->isDefined();
sym->markLive();
// Mark ctor functions in the object that defines this symbol live.
// The ctor functions are all referenced by the synthetic callCtors
// function. However, this function does not contain relocations so we
// have to manually mark the ctors as live.
if (needInitFunctions)
enqueueInitFunctions(cast<ObjFile>(file));
if (InputChunk *chunk = sym->getChunk())
queue.push_back(chunk);
}
// The ctor functions are all referenced by the synthetic callCtors
// function. However, this function does not contain relocations so we
// have to manually mark the ctors as live.
void MarkLive::enqueueInitFunctions(const ObjFile *obj) {
const WasmLinkingData &l = obj->getWasmObj()->linkingData();
for (const WasmInitFunc &f : l.InitFunctions) {
auto *initSym = obj->getFunctionSymbol(f.Symbol);
if (!initSym->isDiscarded())
enqueue(initSym);
}
}
void MarkLive::run() {
// Add GC root symbols.
if (!config->entry.empty())
enqueue(symtab->find(config->entry));
// We need to preserve any no-strip or exported symbol
for (Symbol *sym : symtab->symbols())
if (sym->isNoStrip() || sym->isExported())
enqueue(sym);
if (WasmSym::callDtors)
enqueue(WasmSym::callDtors);
// Enqueue constructors in objects explicitly live from the command-line.
for (const ObjFile *obj : symtab->objectFiles)
if (obj->isLive())
enqueueInitFunctions(obj);
mark();
// If we have any non-discarded init functions, mark `__wasm_call_ctors` as
// live so that we assign it an index and call it.
if (isCallCtorsLive())
WasmSym::callCtors->markLive();
}
void MarkLive::mark() {
// Follow relocations to mark all reachable chunks.
while (!queue.empty()) {
InputChunk *c = queue.pop_back_val();
for (const WasmRelocation reloc : c->getRelocations()) {
if (reloc.Type == R_WASM_TYPE_INDEX_LEB)
continue;
Symbol *sym = c->file->getSymbol(reloc.Index);
// If the function has been assigned the special index zero in the table,
// the relocation doesn't pull in the function body, since the function
// won't actually go in the table (the runtime will trap attempts to call
// that index, since we don't use it). A function with a table index of
// zero is only reachable via "call", not via "call_indirect". The stub
// functions used for weak-undefined symbols have this behaviour (compare
// equal to null pointer, only reachable via direct call).
if (reloc.Type == R_WASM_TABLE_INDEX_SLEB ||
reloc.Type == R_WASM_TABLE_INDEX_SLEB64 ||
reloc.Type == R_WASM_TABLE_INDEX_I32 ||
reloc.Type == R_WASM_TABLE_INDEX_I64) {
auto *funcSym = cast<FunctionSymbol>(sym);
if (funcSym->isStub)
continue;
}
enqueue(sym);
}
}
}
void markLive() {
if (!config->gcSections)
return;
LLVM_DEBUG(dbgs() << "markLive\n");
MarkLive marker;
marker.run();
// Report garbage-collected sections.
if (config->printGcSections) {
for (const ObjFile *obj : symtab->objectFiles) {
for (InputChunk *c : obj->functions)
if (!c->live)
message("removing unused section " + toString(c));
for (InputChunk *c : obj->segments)
if (!c->live)
message("removing unused section " + toString(c));
for (InputGlobal *g : obj->globals)
if (!g->live)
message("removing unused section " + toString(g));
for (InputTag *t : obj->tags)
if (!t->live)
message("removing unused section " + toString(t));
for (InputTable *t : obj->tables)
if (!t->live)
message("removing unused section " + toString(t));
}
for (InputChunk *c : symtab->syntheticFunctions)
if (!c->live)
message("removing unused section " + toString(c));
for (InputGlobal *g : symtab->syntheticGlobals)
if (!g->live)
message("removing unused section " + toString(g));
for (InputTable *t : symtab->syntheticTables)
if (!t->live)
message("removing unused section " + toString(t));
}
}
bool MarkLive::isCallCtorsLive() {
// In a reloctable link, we don't call `__wasm_call_ctors`.
if (config->relocatable)
return false;
// In Emscripten-style PIC, we call `__wasm_call_ctors` which calls
// `__wasm_apply_data_relocs`.
if (config->isPic)
return true;
// If there are any init functions, mark `__wasm_call_ctors` live so that
// it can call them.
for (const ObjFile *file : symtab->objectFiles) {
const WasmLinkingData &l = file->getWasmObj()->linkingData();
for (const WasmInitFunc &f : l.InitFunctions) {
auto *sym = file->getFunctionSymbol(f.Symbol);
if (!sym->isDiscarded() && sym->isLive())
return true;
}
}
return false;
}
} // namespace lld::wasm
|
