1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
2 //
3 // This file defines the interface to a pass that merges duplicate global
4 // constants together into a single constant that is shared.  This is useful
5 // because some passes (ie TraceValues) insert a lot of string constants into
6 // the program, regardless of whether or not an existing string is available.
7 //
8 // Algorithm: ConstantMerge is designed to build up a map of available constants
9 // and eliminate duplicates when it is initialized.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/Transforms/IPO.h"
14 #include "llvm/Module.h"
15 #include "llvm/Pass.h"
16 #include "Support/Statistic.h"
17 
18 namespace {
19   Statistic<> NumMerged("constmerge", "Number of global constants merged");
20 
21   struct ConstantMerge : public Pass {
22     // run - For this pass, process all of the globals in the module,
23     // eliminating duplicate constants.
24     //
25     bool run(Module &M);
26   };
27 
28   RegisterOpt<ConstantMerge> X("constmerge","Merge Duplicate Global Constants");
29 }
30 
31 Pass *createConstantMergePass() { return new ConstantMerge(); }
32 
33 
34 bool ConstantMerge::run(Module &M) {
35   std::map<Constant*, GlobalVariable*> CMap;
36   bool MadeChanges = false;
37 
38   for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
39     // Only process constants with initializers
40     if (GV->isConstant() && GV->hasInitializer()) {
41       Constant *Init = GV->getInitializer();
42 
43       // Check to see if the initializer is already known...
44       std::map<Constant*, GlobalVariable*>::iterator I = CMap.find(Init);
45 
46       if (I == CMap.end()) {    // Nope, add it to the map
47         CMap.insert(I, std::make_pair(Init, GV));
48       } else {                  // Yup, this is a duplicate!
49         // Make all uses of the duplicate constant use the cannonical version...
50         GV->replaceAllUsesWith(I->second);
51 
52         // Delete the global value from the module... and back up iterator to
53         // not skip the next global...
54         GV = --M.getGlobalList().erase(GV);
55 
56         ++NumMerged;
57         MadeChanges = true;
58       }
59     }
60 
61   return MadeChanges;
62 }
63