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#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"
31using namespace llvm;
32
33#define DEBUG_TYPE "constmerge"
34
35STATISTIC(NumMerged, "Number of global constants merged");
36
37namespace {
38  struct ConstantMerge : public ModulePass {
39    static char ID; // Pass identification, replacement for typeid
40    ConstantMerge() : ModulePass(ID) {
41      initializeConstantMergePass(*PassRegistry::getPassRegistry());
42    }
43
44    // For this pass, process all of the globals in the module, eliminating
45    // duplicate constants.
46    bool runOnModule(Module &M) override;
47
48    // Return true iff we can determine the alignment of this global variable.
49    bool hasKnownAlignment(GlobalVariable *GV) const;
50
51    // Return the alignment of the global, including converting the default
52    // alignment to a concrete value.
53    unsigned getAlignment(GlobalVariable *GV) const;
54
55  };
56}
57
58char ConstantMerge::ID = 0;
59INITIALIZE_PASS(ConstantMerge, "constmerge",
60                "Merge Duplicate Global Constants", false, false)
61
62ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
63
64
65
66/// Find values that are marked as llvm.used.
67static void FindUsedValues(GlobalVariable *LLVMUsed,
68                           SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
69  if (!LLVMUsed) return;
70  ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
71
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);
76  }
77}
78
79// True if A is better than B.
80static bool IsBetterCanonical(const GlobalVariable &A,
81                              const GlobalVariable &B) {
82  if (!A.hasLocalLinkage() && B.hasLocalLinkage())
83    return true;
84
85  if (A.hasLocalLinkage() && !B.hasLocalLinkage())
86    return false;
87
88  return A.hasUnnamedAddr();
89}
90
91unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
92  unsigned Align = GV->getAlignment();
93  if (Align)
94    return Align;
95  return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
96}
97
98bool ConstantMerge::runOnModule(Module &M) {
99
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);
104
105  // Map unique constants to globals.
106  DenseMap<Constant *, GlobalVariable *> CMap;
107
108  // Replacements - This vector contains a list of replacements to perform.
109  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
110
111  bool MadeChange = false;
112
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
116  // were just merged.
117  while (1) {
118
119    // First: Find the canonical constants others will be merged with.
120    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
121         GVI != E; ) {
122      GlobalVariable *GV = GVI++;
123
124      // If this GV is dead, remove it.
125      GV->removeDeadConstantUsers();
126      if (GV->use_empty() && GV->hasLocalLinkage()) {
127        GV->eraseFromParent();
128        continue;
129      }
130
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))
136        continue;
137
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())
143        continue;
144
145      Constant *Init = GV->getInitializer();
146
147      // Check to see if the initializer is already known.
148      GlobalVariable *&Slot = CMap[Init];
149
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))
154        Slot = GV;
155    }
156
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();
162         GVI != E; ) {
163      GlobalVariable *GV = GVI++;
164
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))
170        continue;
171
172      // We can only replace constant with local linkage.
173      if (!GV->hasLocalLinkage())
174        continue;
175
176      Constant *Init = GV->getInitializer();
177
178      // Check to see if the initializer is already known.
179      GlobalVariable *Slot = CMap[Init];
180
181      if (!Slot || Slot == GV)
182        continue;
183
184      if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
185        continue;
186
187      if (!GV->hasUnnamedAddr())
188        Slot->setUnnamedAddr(false);
189
190      // Make all uses of the duplicate constant use the canonical version.
191      Replacements.push_back(std::make_pair(GV, Slot));
192    }
193
194    if (Replacements.empty())
195      return MadeChange;
196    CMap.clear();
197
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
200    // now.
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)));
208      }
209
210      // Eliminate any uses of the dead global.
211      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
212
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();
217    }
218
219    NumMerged += Replacements.size();
220    Replacements.clear();
221  }
222}
223