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 #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"
33 #define DEBUG_TYPE "constmerge"
35 STATISTIC(NumMerged, "Number of global constants merged");
38 struct ConstantMerge : public ModulePass {
39 static char ID; // Pass identification, replacement for typeid
40 ConstantMerge() : ModulePass(ID) {
41 initializeConstantMergePass(*PassRegistry::getPassRegistry());
44 // For this pass, process all of the globals in the module, eliminating
45 // duplicate constants.
46 bool runOnModule(Module &M) override;
48 // Return true iff we can determine the alignment of this global variable.
49 bool hasKnownAlignment(GlobalVariable *GV) const;
51 // Return the alignment of the global, including converting the default
52 // alignment to a concrete value.
53 unsigned getAlignment(GlobalVariable *GV) const;
58 char ConstantMerge::ID = 0;
59 INITIALIZE_PASS(ConstantMerge, "constmerge",
60 "Merge Duplicate Global Constants", false, false)
62 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
66 /// Find values that are marked as llvm.used.
67 static void FindUsedValues(GlobalVariable *LLVMUsed,
68 SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
69 if (!LLVMUsed) return;
70 ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
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);
79 // True if A is better than B.
80 static bool IsBetterCanonical(const GlobalVariable &A,
81 const GlobalVariable &B) {
82 if (!A.hasLocalLinkage() && B.hasLocalLinkage())
85 if (A.hasLocalLinkage() && !B.hasLocalLinkage())
88 return A.hasUnnamedAddr();
91 unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
92 unsigned Align = GV->getAlignment();
95 return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
98 bool ConstantMerge::runOnModule(Module &M) {
100 // Find all the globals that are marked "used". These cannot be merged.
101 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
102 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
103 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
105 // Map unique constants to globals.
106 DenseMap<Constant *, GlobalVariable *> CMap;
108 // Replacements - This vector contains a list of replacements to perform.
109 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
111 bool MadeChange = false;
113 // Iterate constant merging while we are still making progress. Merging two
114 // constants together may allow us to merge other constants together if the
115 // second level constants have initializers which point to the globals that
119 // First: Find the canonical constants others will be merged with.
120 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
122 GlobalVariable *GV = &*GVI++;
124 // If this GV is dead, remove it.
125 GV->removeDeadConstantUsers();
126 if (GV->use_empty() && GV->hasLocalLinkage()) {
127 GV->eraseFromParent();
131 // Only process constants with initializers in the default address space.
132 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
133 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
134 // Don't touch values marked with attribute(used).
135 UsedGlobals.count(GV))
138 // This transformation is legal for weak ODR globals in the sense it
139 // doesn't change semantics, but we really don't want to perform it
140 // anyway; it's likely to pessimize code generation, and some tools
141 // (like the Darwin linker in cases involving CFString) don't expect it.
142 if (GV->isWeakForLinker())
145 Constant *Init = GV->getInitializer();
147 // Check to see if the initializer is already known.
148 GlobalVariable *&Slot = CMap[Init];
150 // If this is the first constant we find or if the old one is local,
151 // replace with the current one. If the current is externally visible
152 // it cannot be replace, but can be the canonical constant we merge with.
153 if (!Slot || IsBetterCanonical(*GV, *Slot))
157 // Second: identify all globals that can be merged together, filling in
158 // the Replacements vector. We cannot do the replacement in this pass
159 // because doing so may cause initializers of other globals to be rewritten,
160 // invalidating the Constant* pointers in CMap.
161 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
163 GlobalVariable *GV = &*GVI++;
165 // Only process constants with initializers in the default address space.
166 if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
167 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
168 // Don't touch values marked with attribute(used).
169 UsedGlobals.count(GV))
172 // We can only replace constant with local linkage.
173 if (!GV->hasLocalLinkage())
176 Constant *Init = GV->getInitializer();
178 // Check to see if the initializer is already known.
179 GlobalVariable *Slot = CMap[Init];
181 if (!Slot || Slot == GV)
184 if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
187 if (!GV->hasUnnamedAddr())
188 Slot->setUnnamedAddr(false);
190 // Make all uses of the duplicate constant use the canonical version.
191 Replacements.push_back(std::make_pair(GV, Slot));
194 if (Replacements.empty())
198 // Now that we have figured out which replacements must be made, do them all
199 // now. This avoid invalidating the pointers in CMap, which are unneeded
201 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
202 // Bump the alignment if necessary.
203 if (Replacements[i].first->getAlignment() ||
204 Replacements[i].second->getAlignment()) {
205 Replacements[i].second->setAlignment(
206 std::max(getAlignment(Replacements[i].first),
207 getAlignment(Replacements[i].second)));
210 // Eliminate any uses of the dead global.
211 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
213 // Delete the global value from the module.
214 assert(Replacements[i].first->hasLocalLinkage() &&
215 "Refusing to delete an externally visible global variable.");
216 Replacements[i].first->eraseFromParent();
219 NumMerged += Replacements.size();
220 Replacements.clear();