ConstantMerge.cpp revision d6e5cbc84226ad4a62e8847f1babecf86089d415 
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/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"
29using namespace llvm;
30
31STATISTIC(NumMerged, "Number of global constants merged");
32
33namespace {
34  struct ConstantMerge : public ModulePass {
35    static char ID; // Pass identification, replacement for typeid
36    ConstantMerge() : ModulePass(ID) {
37      initializeConstantMergePass(*PassRegistry::getPassRegistry());
38    }
39
40    // run - For this pass, process all of the globals in the module,
41    // eliminating duplicate constants.
42    //
43    bool runOnModule(Module &M);
44  };
45}
46
47char ConstantMerge::ID = 0;
48INITIALIZE_PASS(ConstantMerge, "constmerge",
49                "Merge Duplicate Global Constants", false, false)
50
51ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
52
53
54
55/// Find values that are marked as llvm.used.
56static 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;
61
62  for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
63    if (GlobalValue *GV =
64        dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
65      UsedValues.insert(GV);
66}
67
68// True if A is better than B.
69static bool IsBetterCannonical(const GlobalVariable &A,
70                               const GlobalVariable &B) {
71  if (!A.hasLocalLinkage() && B.hasLocalLinkage())
72    return true;
73
74  if (A.hasLocalLinkage() && !B.hasLocalLinkage())
75    return false;
76
77  return A.hasUnnamedAddr();
78}
79
80bool ConstantMerge::runOnModule(Module &M) {
81  // Find all the globals that are marked "used".  These cannot be merged.
82  SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
83  FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
84  FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
85
86  // Map unique constant/section pairs to globals.  We don't want to merge
87  // globals in different sections.
88  DenseMap<Constant*, GlobalVariable*> CMap;
89
90  // Replacements - This vector contains a list of replacements to perform.
91  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
92
93  bool MadeChange = false;
94
95  // Iterate constant merging while we are still making progress.  Merging two
96  // constants together may allow us to merge other constants together if the
97  // second level constants have initializers which point to the globals that
98  // were just merged.
99  while (1) {
100
101    // First: Find the canonical constants others will be merged with.
102    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
103         GVI != E; ) {
104      GlobalVariable *GV = GVI++;
105
106      // If this GV is dead, remove it.
107      GV->removeDeadConstantUsers();
108      if (GV->use_empty() && GV->hasLocalLinkage()) {
109        GV->eraseFromParent();
110        continue;
111      }
112
113      // Only process constants with initializers in the default address space.
114      if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
115          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
116          // Don't touch values marked with attribute(used).
117          UsedGlobals.count(GV))
118        continue;
119
120      Constant *Init = GV->getInitializer();
121
122      // Check to see if the initializer is already known.
123      GlobalVariable *&Slot = CMap[Init];
124
125      // If this is the first constant we find or if the old on is local,
126      // replace with the current one. It the current is externally visible
127      // it cannot be replace, but can be the canonical constant we merge with.
128      if (Slot == 0 || IsBetterCannonical(*GV, *Slot)) {
129        Slot = GV;
130      }
131    }
132
133    // Second: identify all globals that can be merged together, filling in
134    // the Replacements vector.  We cannot do the replacement in this pass
135    // because doing so may cause initializers of other globals to be rewritten,
136    // invalidating the Constant* pointers in CMap.
137    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
138         GVI != E; ) {
139      GlobalVariable *GV = GVI++;
140
141      // Only process constants with initializers in the default address space.
142      if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
143          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
144          // Don't touch values marked with attribute(used).
145          UsedGlobals.count(GV))
146        continue;
147
148      // We can only replace constant with local linkage.
149      if (!GV->hasLocalLinkage())
150        continue;
151
152      Constant *Init = GV->getInitializer();
153
154      // Check to see if the initializer is already known.
155      GlobalVariable *Slot = CMap[Init];
156
157      if (!Slot || Slot == GV)
158        continue;
159
160      if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
161        continue;
162
163      if (!GV->hasUnnamedAddr())
164        Slot->setUnnamedAddr(false);
165
166      // Make all uses of the duplicate constant use the canonical version.
167      Replacements.push_back(std::make_pair(GV, Slot));
168    }
169
170    if (Replacements.empty())
171      return MadeChange;
172    CMap.clear();
173
174    // Now that we have figured out which replacements must be made, do them all
175    // now.  This avoid invalidating the pointers in CMap, which are unneeded
176    // now.
177    for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
178      // Eliminate any uses of the dead global.
179      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
180
181      // Delete the global value from the module.
182      assert(Replacements[i].first->hasLocalLinkage() &&
183             "Refusing to delete an externally visible global variable.");
184      Replacements[i].first->eraseFromParent();
185    }
186
187    NumMerged += Replacements.size();
188    Replacements.clear();
189  }
190}
191