1 //===- ICF.cpp ------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // ICF is short for Identical Code Folding. That is a size optimization to
10 // identify and merge two or more read-only sections (typically functions)
11 // that happened to have the same contents. It usually reduces output size
12 // by a few percent.
13 //
14 // On Windows, ICF is enabled by default.
15 //
16 // See ELF/ICF.cpp for the details about the algorithm.
17 //
18 //===----------------------------------------------------------------------===//
19
20 #include "ICF.h"
21 #include "Chunks.h"
22 #include "Symbols.h"
23 #include "lld/Common/ErrorHandler.h"
24 #include "lld/Common/Threads.h"
25 #include "lld/Common/Timer.h"
26 #include "llvm/ADT/Hashing.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/Support/Parallel.h"
29 #include "llvm/Support/raw_ostream.h"
30 #include "llvm/Support/xxhash.h"
31 #include <algorithm>
32 #include <atomic>
33 #include <vector>
34
35 using namespace llvm;
36
37 namespace lld {
38 namespace coff {
39
40 static Timer icfTimer("ICF", Timer::root());
41
42 class ICF {
43 public:
44 void run(ArrayRef<Chunk *> v);
45
46 private:
47 void segregate(size_t begin, size_t end, bool constant);
48
49 bool assocEquals(const SectionChunk *a, const SectionChunk *b);
50
51 bool equalsConstant(const SectionChunk *a, const SectionChunk *b);
52 bool equalsVariable(const SectionChunk *a, const SectionChunk *b);
53
54 bool isEligible(SectionChunk *c);
55
56 size_t findBoundary(size_t begin, size_t end);
57
58 void forEachClassRange(size_t begin, size_t end,
59 std::function<void(size_t, size_t)> fn);
60
61 void forEachClass(std::function<void(size_t, size_t)> fn);
62
63 std::vector<SectionChunk *> chunks;
64 int cnt = 0;
65 std::atomic<bool> repeat = {false};
66 };
67
68 // Returns true if section S is subject of ICF.
69 //
70 // Microsoft's documentation
71 // (https://msdn.microsoft.com/en-us/library/bxwfs976.aspx; visited April
72 // 2017) says that /opt:icf folds both functions and read-only data.
73 // Despite that, the MSVC linker folds only functions. We found
74 // a few instances of programs that are not safe for data merging.
75 // Therefore, we merge only functions just like the MSVC tool. However, we also
76 // merge read-only sections in a couple of cases where the address of the
77 // section is insignificant to the user program and the behaviour matches that
78 // of the Visual C++ linker.
isEligible(SectionChunk * c)79 bool ICF::isEligible(SectionChunk *c) {
80 // Non-comdat chunks, dead chunks, and writable chunks are not eligible.
81 bool writable = c->getOutputCharacteristics() & llvm::COFF::IMAGE_SCN_MEM_WRITE;
82 if (!c->isCOMDAT() || !c->live || writable)
83 return false;
84
85 // Code sections are eligible.
86 if (c->getOutputCharacteristics() & llvm::COFF::IMAGE_SCN_MEM_EXECUTE)
87 return true;
88
89 // .pdata and .xdata unwind info sections are eligible.
90 StringRef outSecName = c->getSectionName().split('$').first;
91 if (outSecName == ".pdata" || outSecName == ".xdata")
92 return true;
93
94 // So are vtables.
95 if (c->sym && c->sym->getName().startswith("??_7"))
96 return true;
97
98 // Anything else not in an address-significance table is eligible.
99 return !c->keepUnique;
100 }
101
102 // Split an equivalence class into smaller classes.
segregate(size_t begin,size_t end,bool constant)103 void ICF::segregate(size_t begin, size_t end, bool constant) {
104 while (begin < end) {
105 // Divide [Begin, End) into two. Let Mid be the start index of the
106 // second group.
107 auto bound = std::stable_partition(
108 chunks.begin() + begin + 1, chunks.begin() + end, [&](SectionChunk *s) {
109 if (constant)
110 return equalsConstant(chunks[begin], s);
111 return equalsVariable(chunks[begin], s);
112 });
113 size_t mid = bound - chunks.begin();
114
115 // Split [Begin, End) into [Begin, Mid) and [Mid, End). We use Mid as an
116 // equivalence class ID because every group ends with a unique index.
117 for (size_t i = begin; i < mid; ++i)
118 chunks[i]->eqClass[(cnt + 1) % 2] = mid;
119
120 // If we created a group, we need to iterate the main loop again.
121 if (mid != end)
122 repeat = true;
123
124 begin = mid;
125 }
126 }
127
128 // Returns true if two sections' associative children are equal.
assocEquals(const SectionChunk * a,const SectionChunk * b)129 bool ICF::assocEquals(const SectionChunk *a, const SectionChunk *b) {
130 auto childClasses = [&](const SectionChunk *sc) {
131 std::vector<uint32_t> classes;
132 for (const SectionChunk &c : sc->children())
133 if (!c.getSectionName().startswith(".debug") &&
134 c.getSectionName() != ".gfids$y" && c.getSectionName() != ".gljmp$y")
135 classes.push_back(c.eqClass[cnt % 2]);
136 return classes;
137 };
138 return childClasses(a) == childClasses(b);
139 }
140
141 // Compare "non-moving" part of two sections, namely everything
142 // except relocation targets.
equalsConstant(const SectionChunk * a,const SectionChunk * b)143 bool ICF::equalsConstant(const SectionChunk *a, const SectionChunk *b) {
144 if (a->relocsSize != b->relocsSize)
145 return false;
146
147 // Compare relocations.
148 auto eq = [&](const coff_relocation &r1, const coff_relocation &r2) {
149 if (r1.Type != r2.Type ||
150 r1.VirtualAddress != r2.VirtualAddress) {
151 return false;
152 }
153 Symbol *b1 = a->file->getSymbol(r1.SymbolTableIndex);
154 Symbol *b2 = b->file->getSymbol(r2.SymbolTableIndex);
155 if (b1 == b2)
156 return true;
157 if (auto *d1 = dyn_cast<DefinedRegular>(b1))
158 if (auto *d2 = dyn_cast<DefinedRegular>(b2))
159 return d1->getValue() == d2->getValue() &&
160 d1->getChunk()->eqClass[cnt % 2] == d2->getChunk()->eqClass[cnt % 2];
161 return false;
162 };
163 if (!std::equal(a->getRelocs().begin(), a->getRelocs().end(),
164 b->getRelocs().begin(), eq))
165 return false;
166
167 // Compare section attributes and contents.
168 return a->getOutputCharacteristics() == b->getOutputCharacteristics() &&
169 a->getSectionName() == b->getSectionName() &&
170 a->header->SizeOfRawData == b->header->SizeOfRawData &&
171 a->checksum == b->checksum && a->getContents() == b->getContents() &&
172 assocEquals(a, b);
173 }
174
175 // Compare "moving" part of two sections, namely relocation targets.
equalsVariable(const SectionChunk * a,const SectionChunk * b)176 bool ICF::equalsVariable(const SectionChunk *a, const SectionChunk *b) {
177 // Compare relocations.
178 auto eq = [&](const coff_relocation &r1, const coff_relocation &r2) {
179 Symbol *b1 = a->file->getSymbol(r1.SymbolTableIndex);
180 Symbol *b2 = b->file->getSymbol(r2.SymbolTableIndex);
181 if (b1 == b2)
182 return true;
183 if (auto *d1 = dyn_cast<DefinedRegular>(b1))
184 if (auto *d2 = dyn_cast<DefinedRegular>(b2))
185 return d1->getChunk()->eqClass[cnt % 2] == d2->getChunk()->eqClass[cnt % 2];
186 return false;
187 };
188 return std::equal(a->getRelocs().begin(), a->getRelocs().end(),
189 b->getRelocs().begin(), eq) &&
190 assocEquals(a, b);
191 }
192
193 // Find the first Chunk after Begin that has a different class from Begin.
findBoundary(size_t begin,size_t end)194 size_t ICF::findBoundary(size_t begin, size_t end) {
195 for (size_t i = begin + 1; i < end; ++i)
196 if (chunks[begin]->eqClass[cnt % 2] != chunks[i]->eqClass[cnt % 2])
197 return i;
198 return end;
199 }
200
forEachClassRange(size_t begin,size_t end,std::function<void (size_t,size_t)> fn)201 void ICF::forEachClassRange(size_t begin, size_t end,
202 std::function<void(size_t, size_t)> fn) {
203 while (begin < end) {
204 size_t mid = findBoundary(begin, end);
205 fn(begin, mid);
206 begin = mid;
207 }
208 }
209
210 // Call Fn on each class group.
forEachClass(std::function<void (size_t,size_t)> fn)211 void ICF::forEachClass(std::function<void(size_t, size_t)> fn) {
212 // If the number of sections are too small to use threading,
213 // call Fn sequentially.
214 if (chunks.size() < 1024) {
215 forEachClassRange(0, chunks.size(), fn);
216 ++cnt;
217 return;
218 }
219
220 // Shard into non-overlapping intervals, and call Fn in parallel.
221 // The sharding must be completed before any calls to Fn are made
222 // so that Fn can modify the Chunks in its shard without causing data
223 // races.
224 const size_t numShards = 256;
225 size_t step = chunks.size() / numShards;
226 size_t boundaries[numShards + 1];
227 boundaries[0] = 0;
228 boundaries[numShards] = chunks.size();
229 parallelForEachN(1, numShards, [&](size_t i) {
230 boundaries[i] = findBoundary((i - 1) * step, chunks.size());
231 });
232 parallelForEachN(1, numShards + 1, [&](size_t i) {
233 if (boundaries[i - 1] < boundaries[i]) {
234 forEachClassRange(boundaries[i - 1], boundaries[i], fn);
235 }
236 });
237 ++cnt;
238 }
239
240 // Merge identical COMDAT sections.
241 // Two sections are considered the same if their section headers,
242 // contents and relocations are all the same.
run(ArrayRef<Chunk * > vec)243 void ICF::run(ArrayRef<Chunk *> vec) {
244 ScopedTimer t(icfTimer);
245
246 // Collect only mergeable sections and group by hash value.
247 uint32_t nextId = 1;
248 for (Chunk *c : vec) {
249 if (auto *sc = dyn_cast<SectionChunk>(c)) {
250 if (isEligible(sc))
251 chunks.push_back(sc);
252 else
253 sc->eqClass[0] = nextId++;
254 }
255 }
256
257 // Make sure that ICF doesn't merge sections that are being handled by string
258 // tail merging.
259 for (MergeChunk *mc : MergeChunk::instances)
260 if (mc)
261 for (SectionChunk *sc : mc->sections)
262 sc->eqClass[0] = nextId++;
263
264 // Initially, we use hash values to partition sections.
265 parallelForEach(chunks, [&](SectionChunk *sc) {
266 sc->eqClass[0] = xxHash64(sc->getContents());
267 });
268
269 // Combine the hashes of the sections referenced by each section into its
270 // hash.
271 for (unsigned cnt = 0; cnt != 2; ++cnt) {
272 parallelForEach(chunks, [&](SectionChunk *sc) {
273 uint32_t hash = sc->eqClass[cnt % 2];
274 for (Symbol *b : sc->symbols())
275 if (auto *sym = dyn_cast_or_null<DefinedRegular>(b))
276 hash += sym->getChunk()->eqClass[cnt % 2];
277 // Set MSB to 1 to avoid collisions with non-hash classes.
278 sc->eqClass[(cnt + 1) % 2] = hash | (1U << 31);
279 });
280 }
281
282 // From now on, sections in Chunks are ordered so that sections in
283 // the same group are consecutive in the vector.
284 llvm::stable_sort(chunks, [](const SectionChunk *a, const SectionChunk *b) {
285 return a->eqClass[0] < b->eqClass[0];
286 });
287
288 // Compare static contents and assign unique IDs for each static content.
289 forEachClass([&](size_t begin, size_t end) { segregate(begin, end, true); });
290
291 // Split groups by comparing relocations until convergence is obtained.
292 do {
293 repeat = false;
294 forEachClass(
295 [&](size_t begin, size_t end) { segregate(begin, end, false); });
296 } while (repeat);
297
298 log("ICF needed " + Twine(cnt) + " iterations");
299
300 // Merge sections in the same classes.
301 forEachClass([&](size_t begin, size_t end) {
302 if (end - begin == 1)
303 return;
304
305 log("Selected " + chunks[begin]->getDebugName());
306 for (size_t i = begin + 1; i < end; ++i) {
307 log(" Removed " + chunks[i]->getDebugName());
308 chunks[begin]->replace(chunks[i]);
309 }
310 });
311 }
312
313 // Entry point to ICF.
doICF(ArrayRef<Chunk * > chunks)314 void doICF(ArrayRef<Chunk *> chunks) { ICF().run(chunks); }
315
316 } // namespace coff
317 } // namespace lld
318