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) {}
38 // run - For this pass, process all of the globals in the module,
39 // eliminating duplicate constants.
41 bool runOnModule(Module &M);
45 char ConstantMerge::ID = 0;
46 INITIALIZE_PASS(ConstantMerge, "constmerge",
47 "Merge Duplicate Global Constants", false, false);
49 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
53 /// Find values that are marked as llvm.used.
54 static void FindUsedValues(GlobalVariable *LLVMUsed,
55 SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
56 if (LLVMUsed == 0) return;
57 ConstantArray *Inits = dyn_cast<ConstantArray>(LLVMUsed->getInitializer());
58 if (Inits == 0) return;
60 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
62 dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
63 UsedValues.insert(GV);
66 bool ConstantMerge::runOnModule(Module &M) {
67 // Find all the globals that are marked "used". These cannot be merged.
68 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
69 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
70 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
72 // Map unique constant/section pairs to globals. We don't want to merge
73 // globals in different sections.
74 DenseMap<Constant*, GlobalVariable*> CMap;
76 // Replacements - This vector contains a list of replacements to perform.
77 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
79 bool MadeChange = false;
81 // Iterate constant merging while we are still making progress. Merging two
82 // constants together may allow us to merge other constants together if the
83 // second level constants have initializers which point to the globals that
86 // First pass: identify all globals that can be merged together, filling in
87 // the Replacements vector. We cannot do the replacement in this pass
88 // because doing so may cause initializers of other globals to be rewritten,
89 // invalidating the Constant* pointers in CMap.
91 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
93 GlobalVariable *GV = GVI++;
95 // If this GV is dead, remove it.
96 GV->removeDeadConstantUsers();
97 if (GV->use_empty() && GV->hasLocalLinkage()) {
98 GV->eraseFromParent();
102 // Only process constants with initializers in the default addres space.
103 if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
104 GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty() ||
105 // Don't touch values marked with attribute(used).
106 UsedGlobals.count(GV))
111 Constant *Init = GV->getInitializer();
113 // Check to see if the initializer is already known.
114 GlobalVariable *&Slot = CMap[Init];
116 if (Slot == 0) { // Nope, add it to the map.
118 } else if (GV->hasLocalLinkage()) { // Yup, this is a duplicate!
119 // Make all uses of the duplicate constant use the canonical version.
120 Replacements.push_back(std::make_pair(GV, Slot));
124 if (Replacements.empty())
128 // Now that we have figured out which replacements must be made, do them all
129 // now. This avoid invalidating the pointers in CMap, which are unneeded
131 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
132 // Eliminate any uses of the dead global.
133 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
135 // Delete the global value from the module.
136 Replacements[i].first->eraseFromParent();
139 NumMerged += Replacements.size();
140 Replacements.clear();