1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
15 // Algorithm: ConstantMerge is designed to build up a map of available constants
16 // and eliminate duplicates when it is initialized.
18 //===----------------------------------------------------------------------===//
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/Pass.h"
33 STATISTIC(NumMerged, "Number of global constants merged");
36 struct ConstantMerge : public ModulePass {
37 static char ID; // Pass identification, replacement for typeid
38 ConstantMerge() : ModulePass(ID) {
39 initializeConstantMergePass(*PassRegistry::getPassRegistry());
42 // For this pass, process all of the globals in the module, eliminating
43 // duplicate constants.
44 bool runOnModule(Module &M);
46 // Return true iff we can determine the alignment of this global variable.
47 bool hasKnownAlignment(GlobalVariable *GV) const;
49 // Return the alignment of the global, including converting the default
50 // alignment to a concrete value.
51 unsigned getAlignment(GlobalVariable *GV) const;
57 char ConstantMerge::ID = 0;
58 INITIALIZE_PASS(ConstantMerge, "constmerge",
59 "Merge Duplicate Global Constants", false, false)
61 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
65 /// Find values that are marked as llvm.used.
66 static void FindUsedValues(GlobalVariable *LLVMUsed,
67 SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
68 if (LLVMUsed == 0) return;
69 ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
71 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
73 dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
74 UsedValues.insert(GV);
77 // True if A is better than B.
78 static bool IsBetterCannonical(const GlobalVariable &A,
79 const GlobalVariable &B) {
80 if (!A.hasLocalLinkage() && B.hasLocalLinkage())
83 if (A.hasLocalLinkage() && !B.hasLocalLinkage())
86 return A.hasUnnamedAddr();
89 bool ConstantMerge::hasKnownAlignment(GlobalVariable *GV) const {
90 return TD || GV->getAlignment() != 0;
93 unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
95 return TD->getPreferredAlignment(GV);
96 return GV->getAlignment();
99 bool ConstantMerge::runOnModule(Module &M) {
100 TD = getAnalysisIfAvailable<DataLayout>();
102 // Find all the globals that are marked "used". These cannot be merged.
103 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
104 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
105 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
107 // Map unique <constants, has-unknown-alignment> pairs to globals. We don't
108 // want to merge globals of unknown alignment with those of explicit
109 // alignment. If we have DataLayout, we always know the alignment.
110 DenseMap<PointerIntPair<Constant*, 1, bool>, GlobalVariable*> CMap;
112 // Replacements - This vector contains a list of replacements to perform.
113 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
115 bool MadeChange = false;
117 // Iterate constant merging while we are still making progress. Merging two
118 // constants together may allow us to merge other constants together if the
119 // second level constants have initializers which point to the globals that
123 // First: Find the canonical constants others will be merged with.
124 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
126 GlobalVariable *GV = GVI++;
128 // If this GV is dead, remove it.
129 GV->removeDeadConstantUsers();
130 if (GV->use_empty() && GV->hasLocalLinkage()) {
131 GV->eraseFromParent();
135 // Only process constants with initializers in the default address space.
136 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
137 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
138 // Don't touch values marked with attribute(used).
139 UsedGlobals.count(GV))
142 // This transformation is legal for weak ODR globals in the sense it
143 // doesn't change semantics, but we really don't want to perform it
144 // anyway; it's likely to pessimize code generation, and some tools
145 // (like the Darwin linker in cases involving CFString) don't expect it.
146 if (GV->isWeakForLinker())
149 Constant *Init = GV->getInitializer();
151 // Check to see if the initializer is already known.
152 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
153 GlobalVariable *&Slot = CMap[Pair];
155 // If this is the first constant we find or if the old one is local,
156 // replace with the current one. If the current is externally visible
157 // it cannot be replace, but can be the canonical constant we merge with.
158 if (Slot == 0 || IsBetterCannonical(*GV, *Slot))
162 // Second: identify all globals that can be merged together, filling in
163 // the Replacements vector. We cannot do the replacement in this pass
164 // because doing so may cause initializers of other globals to be rewritten,
165 // invalidating the Constant* pointers in CMap.
166 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
168 GlobalVariable *GV = GVI++;
170 // Only process constants with initializers in the default address space.
171 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
172 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
173 // Don't touch values marked with attribute(used).
174 UsedGlobals.count(GV))
177 // We can only replace constant with local linkage.
178 if (!GV->hasLocalLinkage())
181 Constant *Init = GV->getInitializer();
183 // Check to see if the initializer is already known.
184 PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
185 GlobalVariable *Slot = CMap[Pair];
187 if (!Slot || Slot == GV)
190 if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
193 if (!GV->hasUnnamedAddr())
194 Slot->setUnnamedAddr(false);
196 // Make all uses of the duplicate constant use the canonical version.
197 Replacements.push_back(std::make_pair(GV, Slot));
200 if (Replacements.empty())
204 // Now that we have figured out which replacements must be made, do them all
205 // now. This avoid invalidating the pointers in CMap, which are unneeded
207 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
208 // Bump the alignment if necessary.
209 if (Replacements[i].first->getAlignment() ||
210 Replacements[i].second->getAlignment()) {
211 Replacements[i].second->setAlignment(std::max(
212 Replacements[i].first->getAlignment(),
213 Replacements[i].second->getAlignment()));
216 // Eliminate any uses of the dead global.
217 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
219 // Delete the global value from the module.
220 assert(Replacements[i].first->hasLocalLinkage() &&
221 "Refusing to delete an externally visible global variable.");
222 Replacements[i].first->eraseFromParent();
225 NumMerged += Replacements.size();
226 Replacements.clear();