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/Constants.h"
23 #include "llvm/DerivedTypes.h"
24 #include "llvm/Module.h"
25 #include "llvm/Pass.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include "llvm/ADT/Statistic.h"
31 STATISTIC(NumMerged, "Number of global constants merged");
34 struct ConstantMerge : public ModulePass {
35 static char ID; // Pass identification, replacement for typeid
36 ConstantMerge() : ModulePass(ID) {
37 initializeConstantMergePass(*PassRegistry::getPassRegistry());
40 // run - For this pass, process all of the globals in the module,
41 // eliminating duplicate constants.
43 bool runOnModule(Module &M);
47 char ConstantMerge::ID = 0;
48 INITIALIZE_PASS(ConstantMerge, "constmerge",
49 "Merge Duplicate Global Constants", false, false)
51 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
55 /// Find values that are marked as llvm.used.
56 static void FindUsedValues(GlobalVariable *LLVMUsed,
57 SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
58 if (LLVMUsed == 0) return;
59 ConstantArray *Inits = dyn_cast<ConstantArray>(LLVMUsed->getInitializer());
60 if (Inits == 0) return;
62 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
64 dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
65 UsedValues.insert(GV);
68 bool ConstantMerge::runOnModule(Module &M) {
69 // Find all the globals that are marked "used". These cannot be merged.
70 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
71 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
72 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
74 // Map unique constant/section pairs to globals. We don't want to merge
75 // globals in different sections.
76 DenseMap<Constant*, GlobalVariable*> CMap;
78 // Replacements - This vector contains a list of replacements to perform.
79 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
81 bool MadeChange = false;
83 // Iterate constant merging while we are still making progress. Merging two
84 // constants together may allow us to merge other constants together if the
85 // second level constants have initializers which point to the globals that
88 // First pass: identify all globals that can be merged together, filling in
89 // the Replacements vector. We cannot do the replacement in this pass
90 // because doing so may cause initializers of other globals to be rewritten,
91 // invalidating the Constant* pointers in CMap.
93 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
95 GlobalVariable *GV = GVI++;
97 // If this GV is dead, remove it.
98 GV->removeDeadConstantUsers();
99 if (GV->use_empty() && GV->hasLocalLinkage()) {
100 GV->eraseFromParent();
104 // Only process constants with initializers in the default addres space.
105 if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
106 GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty() ||
107 // Don't touch values marked with attribute(used).
108 UsedGlobals.count(GV))
113 Constant *Init = GV->getInitializer();
115 // Check to see if the initializer is already known.
116 GlobalVariable *&Slot = CMap[Init];
118 if (Slot == 0) { // Nope, add it to the map.
120 } else if (GV->hasLocalLinkage()) { // Yup, this is a duplicate!
121 // Make all uses of the duplicate constant use the canonical version.
122 Replacements.push_back(std::make_pair(GV, Slot));
126 if (Replacements.empty())
130 // Now that we have figured out which replacements must be made, do them all
131 // now. This avoid invalidating the pointers in CMap, which are unneeded
133 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
134 // Eliminate any uses of the dead global.
135 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
137 // Delete the global value from the module.
138 Replacements[i].first->eraseFromParent();
141 NumMerged += Replacements.size();
142 Replacements.clear();