1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
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 // This file defines the interface to a pass that merges duplicate global
10 // constants together into a single constant that is shared.  This is useful
11 // because some passes (ie TraceValues) insert a lot of string constants into
12 // the program, regardless of whether or not an existing string is available.
13 //
14 // Algorithm: ConstantMerge is designed to build up a map of available constants
15 // and eliminate duplicates when it is initialized.
16 //
17 //===----------------------------------------------------------------------===//
18 
19 #include "llvm/Transforms/IPO/ConstantMerge.h"
20 #include "llvm/ADT/DenseMap.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DataLayout.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/GlobalValue.h"
28 #include "llvm/IR/GlobalVariable.h"
29 #include "llvm/IR/LLVMContext.h"
30 #include "llvm/IR/Module.h"
31 #include "llvm/InitializePasses.h"
32 #include "llvm/Pass.h"
33 #include "llvm/Support/Casting.h"
34 #include "llvm/Transforms/IPO.h"
35 #include <algorithm>
36 #include <cassert>
37 #include <utility>
38 
39 using namespace llvm;
40 
41 #define DEBUG_TYPE "constmerge"
42 
43 STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
44 
45 /// Find values that are marked as llvm.used.
46 static void FindUsedValues(GlobalVariable *LLVMUsed,
47                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
48   if (!LLVMUsed) return;
49   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
50 
51   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
52     Value *Operand = Inits->getOperand(i)->stripPointerCasts();
53     GlobalValue *GV = cast<GlobalValue>(Operand);
54     UsedValues.insert(GV);
55   }
56 }
57 
58 // True if A is better than B.
59 static bool IsBetterCanonical(const GlobalVariable &A,
60                               const GlobalVariable &B) {
61   if (!A.hasLocalLinkage() && B.hasLocalLinkage())
62     return true;
63 
64   if (A.hasLocalLinkage() && !B.hasLocalLinkage())
65     return false;
66 
67   return A.hasGlobalUnnamedAddr();
68 }
69 
70 static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
71   SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
72   GV->getAllMetadata(MDs);
73   for (const auto &V : MDs)
74     if (V.first != LLVMContext::MD_dbg)
75       return true;
76   return false;
77 }
78 
79 static void copyDebugLocMetadata(const GlobalVariable *From,
80                                  GlobalVariable *To) {
81   SmallVector<DIGlobalVariableExpression *, 1> MDs;
82   From->getDebugInfo(MDs);
83   for (auto MD : MDs)
84     To->addDebugInfo(MD);
85 }
86 
87 static unsigned getAlignment(GlobalVariable *GV) {
88   unsigned Align = GV->getAlignment();
89   if (Align)
90     return Align;
91   return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
92 }
93 
94 static bool
95 isUnmergeableGlobal(GlobalVariable *GV,
96                     const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
97   // Only process constants with initializers in the default address space.
98   return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
99          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
100          // Don't touch values marked with attribute(used).
101          UsedGlobals.count(GV);
102 }
103 
104 enum class CanMerge { No, Yes };
105 static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
106   if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
107     return CanMerge::No;
108   if (hasMetadataOtherThanDebugLoc(Old))
109     return CanMerge::No;
110   assert(!hasMetadataOtherThanDebugLoc(New));
111   if (!Old->hasGlobalUnnamedAddr())
112     New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
113   return CanMerge::Yes;
114 }
115 
116 static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
117   Constant *NewConstant = New;
118 
119   LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
120                     << New->getName() << "\n");
121 
122   // Bump the alignment if necessary.
123   if (Old->getAlignment() || New->getAlignment())
124     New->setAlignment(Align(std::max(getAlignment(Old), getAlignment(New))));
125 
126   copyDebugLocMetadata(Old, New);
127   Old->replaceAllUsesWith(NewConstant);
128 
129   // Delete the global value from the module.
130   assert(Old->hasLocalLinkage() &&
131          "Refusing to delete an externally visible global variable.");
132   Old->eraseFromParent();
133 }
134 
135 static bool mergeConstants(Module &M) {
136   // Find all the globals that are marked "used".  These cannot be merged.
137   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
138   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
139   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
140 
141   // Map unique constants to globals.
142   DenseMap<Constant *, GlobalVariable *> CMap;
143 
144   SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
145       SameContentReplacements;
146 
147   size_t ChangesMade = 0;
148   size_t OldChangesMade = 0;
149 
150   // Iterate constant merging while we are still making progress.  Merging two
151   // constants together may allow us to merge other constants together if the
152   // second level constants have initializers which point to the globals that
153   // were just merged.
154   while (true) {
155     // Find the canonical constants others will be merged with.
156     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
157          GVI != E; ) {
158       GlobalVariable *GV = &*GVI++;
159 
160       // If this GV is dead, remove it.
161       GV->removeDeadConstantUsers();
162       if (GV->use_empty() && GV->hasLocalLinkage()) {
163         GV->eraseFromParent();
164         ++ChangesMade;
165         continue;
166       }
167 
168       if (isUnmergeableGlobal(GV, UsedGlobals))
169         continue;
170 
171       // This transformation is legal for weak ODR globals in the sense it
172       // doesn't change semantics, but we really don't want to perform it
173       // anyway; it's likely to pessimize code generation, and some tools
174       // (like the Darwin linker in cases involving CFString) don't expect it.
175       if (GV->isWeakForLinker())
176         continue;
177 
178       // Don't touch globals with metadata other then !dbg.
179       if (hasMetadataOtherThanDebugLoc(GV))
180         continue;
181 
182       Constant *Init = GV->getInitializer();
183 
184       // Check to see if the initializer is already known.
185       GlobalVariable *&Slot = CMap[Init];
186 
187       // If this is the first constant we find or if the old one is local,
188       // replace with the current one. If the current is externally visible
189       // it cannot be replace, but can be the canonical constant we merge with.
190       bool FirstConstantFound = !Slot;
191       if (FirstConstantFound || IsBetterCanonical(*GV, *Slot)) {
192         Slot = GV;
193         LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV->getName()
194                           << (FirstConstantFound ? "\n" : " (updated)\n"));
195       }
196     }
197 
198     // Identify all globals that can be merged together, filling in the
199     // SameContentReplacements vector. We cannot do the replacement in this pass
200     // because doing so may cause initializers of other globals to be rewritten,
201     // invalidating the Constant* pointers in CMap.
202     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
203          GVI != E; ) {
204       GlobalVariable *GV = &*GVI++;
205 
206       if (isUnmergeableGlobal(GV, UsedGlobals))
207         continue;
208 
209       // We can only replace constant with local linkage.
210       if (!GV->hasLocalLinkage())
211         continue;
212 
213       Constant *Init = GV->getInitializer();
214 
215       // Check to see if the initializer is already known.
216       auto Found = CMap.find(Init);
217       if (Found == CMap.end())
218         continue;
219 
220       GlobalVariable *Slot = Found->second;
221       if (Slot == GV)
222         continue;
223 
224       if (makeMergeable(GV, Slot) == CanMerge::No)
225         continue;
226 
227       // Make all uses of the duplicate constant use the canonical version.
228       LLVM_DEBUG(dbgs() << "Will replace: @" << GV->getName() << " -> @"
229                         << Slot->getName() << "\n");
230       SameContentReplacements.push_back(std::make_pair(GV, Slot));
231     }
232 
233     // Now that we have figured out which replacements must be made, do them all
234     // now.  This avoid invalidating the pointers in CMap, which are unneeded
235     // now.
236     for (unsigned i = 0, e = SameContentReplacements.size(); i != e; ++i) {
237       GlobalVariable *Old = SameContentReplacements[i].first;
238       GlobalVariable *New = SameContentReplacements[i].second;
239       replace(M, Old, New);
240       ++ChangesMade;
241       ++NumIdenticalMerged;
242     }
243 
244     if (ChangesMade == OldChangesMade)
245       break;
246     OldChangesMade = ChangesMade;
247 
248     SameContentReplacements.clear();
249     CMap.clear();
250   }
251 
252   return ChangesMade;
253 }
254 
255 PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
256   if (!mergeConstants(M))
257     return PreservedAnalyses::all();
258   return PreservedAnalyses::none();
259 }
260 
261 namespace {
262 
263 struct ConstantMergeLegacyPass : public ModulePass {
264   static char ID; // Pass identification, replacement for typeid
265 
266   ConstantMergeLegacyPass() : ModulePass(ID) {
267     initializeConstantMergeLegacyPassPass(*PassRegistry::getPassRegistry());
268   }
269 
270   // For this pass, process all of the globals in the module, eliminating
271   // duplicate constants.
272   bool runOnModule(Module &M) override {
273     if (skipModule(M))
274       return false;
275     return mergeConstants(M);
276   }
277 };
278 
279 } // end anonymous namespace
280 
281 char ConstantMergeLegacyPass::ID = 0;
282 
283 INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
284                 "Merge Duplicate Global Constants", false, false)
285 
286 ModulePass *llvm::createConstantMergePass() {
287   return new ConstantMergeLegacyPass();
288 }
289