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/Module.h"
23 #include "llvm/Pass.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/Support/Compiler.h"
26 #include <map>
27 using namespace llvm;
28 
29 STATISTIC(NumMerged, "Number of global constants merged");
30 
31 namespace {
32   struct VISIBILITY_HIDDEN ConstantMerge : public ModulePass {
33     static char ID; // Pass identification, replacement for typeid
34     ConstantMerge() : ModulePass(&ID) {}
35 
36     // run - For this pass, process all of the globals in the module,
37     // eliminating duplicate constants.
38     //
39     bool runOnModule(Module &M);
40   };
41 }
42 
43 char ConstantMerge::ID = 0;
44 static RegisterPass<ConstantMerge>
45 X("constmerge", "Merge Duplicate Global Constants");
46 
47 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
48 
49 bool ConstantMerge::runOnModule(Module &M) {
50   // Map unique constant/section pairs to globals.  We don't want to merge
51   // globals in different sections.
52   std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
53 
54   // Replacements - This vector contains a list of replacements to perform.
55   std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
56 
57   bool MadeChange = false;
58 
59   // Iterate constant merging while we are still making progress.  Merging two
60   // constants together may allow us to merge other constants together if the
61   // second level constants have initializers which point to the globals that
62   // were just merged.
63   while (1) {
64     // First pass: identify all globals that can be merged together, filling in
65     // the Replacements vector.  We cannot do the replacement in this pass
66     // because doing so may cause initializers of other globals to be rewritten,
67     // invalidating the Constant* pointers in CMap.
68     //
69     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
70          GVI != E; ) {
71       GlobalVariable *GV = GVI++;
72 
73       // If this GV is dead, remove it.
74       GV->removeDeadConstantUsers();
75       if (GV->use_empty() && GV->hasLocalLinkage()) {
76         GV->eraseFromParent();
77         continue;
78       }
79 
80       // Only process constants with initializers.
81       if (GV->isConstant() && GV->hasDefinitiveInitializer()) {
82         Constant *Init = GV->getInitializer();
83 
84         // Check to see if the initializer is already known.
85         GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
86 
87         if (Slot == 0) {    // Nope, add it to the map.
88           Slot = GV;
89         } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
90           // Make all uses of the duplicate constant use the canonical version.
91           Replacements.push_back(std::make_pair(GV, Slot));
92         }
93       }
94     }
95 
96     if (Replacements.empty())
97       return MadeChange;
98     CMap.clear();
99 
100     // Now that we have figured out which replacements must be made, do them all
101     // now.  This avoid invalidating the pointers in CMap, which are unneeded
102     // now.
103     for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
104       // Eliminate any uses of the dead global...
105       Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
106 
107       // Delete the global value from the module...
108       M.getGlobalList().erase(Replacements[i].first);
109     }
110 
111     NumMerged += Replacements.size();
112     Replacements.clear();
113   }
114 }
115