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 #define DEBUG_TYPE "constmerge"
21 #include "llvm/Transforms/IPO.h"
22 #include "llvm/ADT/DenseMap.h"
23 #include "llvm/ADT/PointerIntPair.h"
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/DataLayout.h"
28 #include "llvm/IR/DerivedTypes.h"
29 #include "llvm/IR/Module.h"
30 #include "llvm/IR/Operator.h"
31 #include "llvm/Pass.h"
32 using namespace llvm;
33
34 STATISTIC(NumMerged, "Number of global constants merged");
35
36 namespace {
37 struct ConstantMerge : public ModulePass {
38 static char ID; // Pass identification, replacement for typeid
ConstantMerge__anoneb2c38ab0111::ConstantMerge39 ConstantMerge() : ModulePass(ID) {
40 initializeConstantMergePass(*PassRegistry::getPassRegistry());
41 }
42
43 // For this pass, process all of the globals in the module, eliminating
44 // duplicate constants.
45 bool runOnModule(Module &M);
46
47 // Return true iff we can determine the alignment of this global variable.
48 bool hasKnownAlignment(GlobalVariable *GV) const;
49
50 // Return the alignment of the global, including converting the default
51 // alignment to a concrete value.
52 unsigned getAlignment(GlobalVariable *GV) const;
53
54 const DataLayout *TD;
55 };
56 }
57
58 char ConstantMerge::ID = 0;
59 INITIALIZE_PASS(ConstantMerge, "constmerge",
60 "Merge Duplicate Global Constants", false, false)
61
createConstantMergePass()62 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
63
64
65
66 /// Find values that are marked as llvm.used.
FindUsedValues(GlobalVariable * LLVMUsed,SmallPtrSet<const GlobalValue *,8> & UsedValues)67 static void FindUsedValues(GlobalVariable *LLVMUsed,
68 SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
69 if (LLVMUsed == 0) return;
70 ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
71
72 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
73 Value *Operand = Inits->getOperand(i)->stripPointerCastsNoFollowAliases();
74 GlobalValue *GV = cast<GlobalValue>(Operand);
75 UsedValues.insert(GV);
76 }
77 }
78
79 // True if A is better than B.
IsBetterCannonical(const GlobalVariable & A,const GlobalVariable & B)80 static bool IsBetterCannonical(const GlobalVariable &A,
81 const GlobalVariable &B) {
82 if (!A.hasLocalLinkage() && B.hasLocalLinkage())
83 return true;
84
85 if (A.hasLocalLinkage() && !B.hasLocalLinkage())
86 return false;
87
88 return A.hasUnnamedAddr();
89 }
90
hasKnownAlignment(GlobalVariable * GV) const91 bool ConstantMerge::hasKnownAlignment(GlobalVariable *GV) const {
92 return TD || GV->getAlignment() != 0;
93 }
94
getAlignment(GlobalVariable * GV) const95 unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
96 if (TD)
97 return TD->getPreferredAlignment(GV);
98 return GV->getAlignment();
99 }
100
runOnModule(Module & M)101 bool ConstantMerge::runOnModule(Module &M) {
102 TD = getAnalysisIfAvailable<DataLayout>();
103
104 // Find all the globals that are marked "used". These cannot be merged.
105 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
106 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
107 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
108
109 // Map unique <constants, has-unknown-alignment> pairs to globals. We don't
110 // want to merge globals of unknown alignment with those of explicit
111 // alignment. If we have DataLayout, we always know the alignment.
112 DenseMap<PointerIntPair<Constant*, 1, bool>, GlobalVariable*> CMap;
113
114 // Replacements - This vector contains a list of replacements to perform.
115 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
116
117 bool MadeChange = false;
118
119 // Iterate constant merging while we are still making progress. Merging two
120 // constants together may allow us to merge other constants together if the
121 // second level constants have initializers which point to the globals that
122 // were just merged.
123 while (1) {
124
125 // First: Find the canonical constants others will be merged with.
126 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
127 GVI != E; ) {
128 GlobalVariable *GV = GVI++;
129
130 // If this GV is dead, remove it.
131 GV->removeDeadConstantUsers();
132 if (GV->use_empty() && GV->hasLocalLinkage()) {
133 GV->eraseFromParent();
134 continue;
135 }
136
137 // Only process constants with initializers in the default address space.
138 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
139 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
140 // Don't touch values marked with attribute(used).
141 UsedGlobals.count(GV))
142 continue;
143
144 // This transformation is legal for weak ODR globals in the sense it
145 // doesn't change semantics, but we really don't want to perform it
146 // anyway; it's likely to pessimize code generation, and some tools
147 // (like the Darwin linker in cases involving CFString) don't expect it.
148 if (GV->isWeakForLinker())
149 continue;
150
151 Constant *Init = GV->getInitializer();
152
153 // Check to see if the initializer is already known.
154 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
155 GlobalVariable *&Slot = CMap[Pair];
156
157 // If this is the first constant we find or if the old one is local,
158 // replace with the current one. If the current is externally visible
159 // it cannot be replace, but can be the canonical constant we merge with.
160 if (Slot == 0 || IsBetterCannonical(*GV, *Slot))
161 Slot = GV;
162 }
163
164 // Second: identify all globals that can be merged together, filling in
165 // the Replacements vector. We cannot do the replacement in this pass
166 // because doing so may cause initializers of other globals to be rewritten,
167 // invalidating the Constant* pointers in CMap.
168 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
169 GVI != E; ) {
170 GlobalVariable *GV = GVI++;
171
172 // Only process constants with initializers in the default address space.
173 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
174 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
175 // Don't touch values marked with attribute(used).
176 UsedGlobals.count(GV))
177 continue;
178
179 // We can only replace constant with local linkage.
180 if (!GV->hasLocalLinkage())
181 continue;
182
183 Constant *Init = GV->getInitializer();
184
185 // Check to see if the initializer is already known.
186 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
187 GlobalVariable *Slot = CMap[Pair];
188
189 if (!Slot || Slot == GV)
190 continue;
191
192 if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
193 continue;
194
195 if (!GV->hasUnnamedAddr())
196 Slot->setUnnamedAddr(false);
197
198 // Make all uses of the duplicate constant use the canonical version.
199 Replacements.push_back(std::make_pair(GV, Slot));
200 }
201
202 if (Replacements.empty())
203 return MadeChange;
204 CMap.clear();
205
206 // Now that we have figured out which replacements must be made, do them all
207 // now. This avoid invalidating the pointers in CMap, which are unneeded
208 // now.
209 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
210 // Bump the alignment if necessary.
211 if (Replacements[i].first->getAlignment() ||
212 Replacements[i].second->getAlignment()) {
213 Replacements[i].second->setAlignment(std::max(
214 Replacements[i].first->getAlignment(),
215 Replacements[i].second->getAlignment()));
216 }
217
218 // Eliminate any uses of the dead global.
219 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
220
221 // Delete the global value from the module.
222 assert(Replacements[i].first->hasLocalLinkage() &&
223 "Refusing to delete an externally visible global variable.");
224 Replacements[i].first->eraseFromParent();
225 }
226
227 NumMerged += Replacements.size();
228 Replacements.clear();
229 }
230 }
231