1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the interface to a pass that merges duplicate global
11 // constants together into a single constant that is shared. This is useful
12 // because some passes (ie TraceValues) insert a lot of string constants into
13 // the program, regardless of whether or not an existing string is available.
14 //
15 // Algorithm: ConstantMerge is designed to build up a map of available constants
16 // and eliminate duplicates when it is initialized.
17 //
18 //===----------------------------------------------------------------------===//
19
20 #include "llvm/Transforms/IPO.h"
21 #include "llvm/ADT/DenseMap.h"
22 #include "llvm/ADT/PointerIntPair.h"
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DataLayout.h"
27 #include "llvm/IR/DerivedTypes.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/IR/Operator.h"
30 #include "llvm/Pass.h"
31 using namespace llvm;
32
33 #define DEBUG_TYPE "constmerge"
34
35 STATISTIC(NumMerged, "Number of global constants merged");
36
37 namespace {
38 struct ConstantMerge : public ModulePass {
39 static char ID; // Pass identification, replacement for typeid
ConstantMerge__anonebb010780111::ConstantMerge40 ConstantMerge() : ModulePass(ID) {
41 initializeConstantMergePass(*PassRegistry::getPassRegistry());
42 }
43
44 // For this pass, process all of the globals in the module, eliminating
45 // duplicate constants.
46 bool runOnModule(Module &M) override;
47
48 // Return true iff we can determine the alignment of this global variable.
49 bool hasKnownAlignment(GlobalVariable *GV) const;
50
51 // Return the alignment of the global, including converting the default
52 // alignment to a concrete value.
53 unsigned getAlignment(GlobalVariable *GV) const;
54
55 const DataLayout *DL;
56 };
57 }
58
59 char ConstantMerge::ID = 0;
60 INITIALIZE_PASS(ConstantMerge, "constmerge",
61 "Merge Duplicate Global Constants", false, false)
62
createConstantMergePass()63 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
64
65
66
67 /// Find values that are marked as llvm.used.
FindUsedValues(GlobalVariable * LLVMUsed,SmallPtrSet<const GlobalValue *,8> & UsedValues)68 static void FindUsedValues(GlobalVariable *LLVMUsed,
69 SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
70 if (!LLVMUsed) return;
71 ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
72
73 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
74 Value *Operand = Inits->getOperand(i)->stripPointerCastsNoFollowAliases();
75 GlobalValue *GV = cast<GlobalValue>(Operand);
76 UsedValues.insert(GV);
77 }
78 }
79
80 // True if A is better than B.
IsBetterCanonical(const GlobalVariable & A,const GlobalVariable & B)81 static bool IsBetterCanonical(const GlobalVariable &A,
82 const GlobalVariable &B) {
83 if (!A.hasLocalLinkage() && B.hasLocalLinkage())
84 return true;
85
86 if (A.hasLocalLinkage() && !B.hasLocalLinkage())
87 return false;
88
89 return A.hasUnnamedAddr();
90 }
91
hasKnownAlignment(GlobalVariable * GV) const92 bool ConstantMerge::hasKnownAlignment(GlobalVariable *GV) const {
93 return DL || GV->getAlignment() != 0;
94 }
95
getAlignment(GlobalVariable * GV) const96 unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
97 unsigned Align = GV->getAlignment();
98 if (Align)
99 return Align;
100 if (DL)
101 return DL->getPreferredAlignment(GV);
102 return 0;
103 }
104
runOnModule(Module & M)105 bool ConstantMerge::runOnModule(Module &M) {
106 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
107 DL = DLP ? &DLP->getDataLayout() : nullptr;
108
109 // Find all the globals that are marked "used". These cannot be merged.
110 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
111 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
112 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
113
114 // Map unique <constants, has-unknown-alignment> pairs to globals. We don't
115 // want to merge globals of unknown alignment with those of explicit
116 // alignment. If we have DataLayout, we always know the alignment.
117 DenseMap<PointerIntPair<Constant*, 1, bool>, GlobalVariable*> CMap;
118
119 // Replacements - This vector contains a list of replacements to perform.
120 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
121
122 bool MadeChange = false;
123
124 // Iterate constant merging while we are still making progress. Merging two
125 // constants together may allow us to merge other constants together if the
126 // second level constants have initializers which point to the globals that
127 // were just merged.
128 while (1) {
129
130 // First: Find the canonical constants others will be merged with.
131 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
132 GVI != E; ) {
133 GlobalVariable *GV = GVI++;
134
135 // If this GV is dead, remove it.
136 GV->removeDeadConstantUsers();
137 if (GV->use_empty() && GV->hasLocalLinkage()) {
138 GV->eraseFromParent();
139 continue;
140 }
141
142 // Only process constants with initializers in the default address space.
143 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
144 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
145 // Don't touch values marked with attribute(used).
146 UsedGlobals.count(GV))
147 continue;
148
149 // This transformation is legal for weak ODR globals in the sense it
150 // doesn't change semantics, but we really don't want to perform it
151 // anyway; it's likely to pessimize code generation, and some tools
152 // (like the Darwin linker in cases involving CFString) don't expect it.
153 if (GV->isWeakForLinker())
154 continue;
155
156 Constant *Init = GV->getInitializer();
157
158 // Check to see if the initializer is already known.
159 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
160 GlobalVariable *&Slot = CMap[Pair];
161
162 // If this is the first constant we find or if the old one is local,
163 // replace with the current one. If the current is externally visible
164 // it cannot be replace, but can be the canonical constant we merge with.
165 if (!Slot || IsBetterCanonical(*GV, *Slot))
166 Slot = GV;
167 }
168
169 // Second: identify all globals that can be merged together, filling in
170 // the Replacements vector. We cannot do the replacement in this pass
171 // because doing so may cause initializers of other globals to be rewritten,
172 // invalidating the Constant* pointers in CMap.
173 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
174 GVI != E; ) {
175 GlobalVariable *GV = GVI++;
176
177 // Only process constants with initializers in the default address space.
178 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
179 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
180 // Don't touch values marked with attribute(used).
181 UsedGlobals.count(GV))
182 continue;
183
184 // We can only replace constant with local linkage.
185 if (!GV->hasLocalLinkage())
186 continue;
187
188 Constant *Init = GV->getInitializer();
189
190 // Check to see if the initializer is already known.
191 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
192 GlobalVariable *Slot = CMap[Pair];
193
194 if (!Slot || Slot == GV)
195 continue;
196
197 if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
198 continue;
199
200 if (!GV->hasUnnamedAddr())
201 Slot->setUnnamedAddr(false);
202
203 // Make all uses of the duplicate constant use the canonical version.
204 Replacements.push_back(std::make_pair(GV, Slot));
205 }
206
207 if (Replacements.empty())
208 return MadeChange;
209 CMap.clear();
210
211 // Now that we have figured out which replacements must be made, do them all
212 // now. This avoid invalidating the pointers in CMap, which are unneeded
213 // now.
214 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
215 // Bump the alignment if necessary.
216 if (Replacements[i].first->getAlignment() ||
217 Replacements[i].second->getAlignment()) {
218 Replacements[i].second->setAlignment(
219 std::max(getAlignment(Replacements[i].first),
220 getAlignment(Replacements[i].second)));
221 }
222
223 // Eliminate any uses of the dead global.
224 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
225
226 // Delete the global value from the module.
227 assert(Replacements[i].first->hasLocalLinkage() &&
228 "Refusing to delete an externally visible global variable.");
229 Replacements[i].first->eraseFromParent();
230 }
231
232 NumMerged += Replacements.size();
233 Replacements.clear();
234 }
235 }
236