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"
29 using namespace llvm;
30 
31 STATISTIC(NumMerged, "Number of global constants merged");
32 
33 namespace {
34   struct ConstantMerge : public ModulePass {
35     static char ID; // Pass identification, replacement for typeid
36     ConstantMerge() : ModulePass(ID) {}
37 
38     // run - For this pass, process all of the globals in the module,
39     // eliminating duplicate constants.
40     //
41     bool runOnModule(Module &M);
42   };
43 }
44 
45 char ConstantMerge::ID = 0;
46 INITIALIZE_PASS(ConstantMerge, "constmerge",
47                 "Merge Duplicate Global Constants", false, false);
48 
49 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
50 
51 
52 
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;
59 
60   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
61     if (GlobalValue *GV =
62         dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
63       UsedValues.insert(GV);
64 }
65 
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);
71 
72   // Map unique constant/section pairs to globals.  We don't want to merge
73   // globals in different sections.
74   DenseMap<Constant*, GlobalVariable*> CMap;
75 
76   // Replacements - This vector contains a list of replacements to perform.
77   SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
78 
79   bool MadeChange = false;
80 
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
84   // were just merged.
85   while (1) {
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.
90     //
91     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
92          GVI != E; ) {
93       GlobalVariable *GV = GVI++;
94 
95       // If this GV is dead, remove it.
96       GV->removeDeadConstantUsers();
97       if (GV->use_empty() && GV->hasLocalLinkage()) {
98         GV->eraseFromParent();
99         continue;
100       }
101 
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))
107         continue;
108 
109 
110 
111       Constant *Init = GV->getInitializer();
112 
113       // Check to see if the initializer is already known.
114       GlobalVariable *&Slot = CMap[Init];
115 
116       if (Slot == 0) {    // Nope, add it to the map.
117         Slot = GV;
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));
121       }
122     }
123 
124     if (Replacements.empty())
125       return MadeChange;
126     CMap.clear();
127 
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
130     // now.
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);
134 
135       // Delete the global value from the module.
136       Replacements[i].first->eraseFromParent();
137     }
138 
139     NumMerged += Replacements.size();
140     Replacements.clear();
141   }
142 }
143