ConstantMerge.cpp revision a65d6a686e6ad865c61aec70c5bdfb30bf6f5b22 
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
68bool 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);
73
74  // Map unique constant/section pairs to globals.  We don't want to merge
75  // globals in different sections.
76  DenseMap<Constant*, GlobalVariable*> CMap;
77
78  // Replacements - This vector contains a list of replacements to perform.
79  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
80
81  bool MadeChange = false;
82
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
86  // were just merged.
87  while (1) {
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.
92    //
93    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
94         GVI != E; ) {
95      GlobalVariable *GV = GVI++;
96
97      // If this GV is dead, remove it.
98      GV->removeDeadConstantUsers();
99      if (GV->use_empty() && GV->hasLocalLinkage()) {
100        GV->eraseFromParent();
101        continue;
102      }
103
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))
109        continue;
110
111
112
113      Constant *Init = GV->getInitializer();
114
115      // Check to see if the initializer is already known.
116      GlobalVariable *&Slot = CMap[Init];
117
118      if (Slot == 0) {    // Nope, add it to the map.
119        Slot = GV;
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));
123      }
124    }
125
126    if (Replacements.empty())
127      return MadeChange;
128    CMap.clear();
129
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
132    // now.
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);
136
137      // Delete the global value from the module.
138      Replacements[i].first->eraseFromParent();
139    }
140
141    NumMerged += Replacements.size();
142    Replacements.clear();
143  }
144}
145