lib/Transforms/IPO/ConstantMerge.cpp

//===- ConstantMerge.cpp - Merge duplicate global constants -----------------=//
//
// This file defines the interface to a pass that merges duplicate global
// constants together into a single constant that is shared.  This is useful
// because some passes (ie TraceValues) insert a lot of string constants into
// the program, regardless of whether or not they duplicate an existing string.
//
// Algorithm: ConstantMerge is designed to build up a map of available constants
// and elminate duplicates when it is initialized.
//
// The DynamicConstantMerge method is a superset of the ConstantMerge algorithm
// that checks for each method to see if constants have been added to the
// constant pool since it was last run... if so, it processes them.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/ConstantMerge.h"
#include "llvm/GlobalVariable.h"

// mergeDuplicateConstants - Workhorse for the pass.  This eliminates duplicate
// constants, starting at global ConstantNo, and adds vars to the map if they
// are new and unique.
//
static inline 
bool mergeDuplicateConstants(Module *M, unsigned &ConstantNo,
                             map<ConstPoolVal*, GlobalVariable*> &CMap) {
  Module::GlobalListType &GList = M->getGlobalList();
  if (GList.size() <= ConstantNo) return false;   // No new constants
  bool MadeChanges = false;
  
  for (; ConstantNo < GList.size(); ++ConstantNo) {
    GlobalVariable *GV = GList[ConstantNo];
    if (GV->isConstant()) {  // Only process constants
      assert(GV->hasInitializer() && "Globals constants must have inits!");
      ConstPoolVal *Init = GV->getInitializer();

      // Check to see if the initializer is already known...
      map<ConstPoolVal*, GlobalVariable*>::iterator I = CMap.find(Init);

      if (I == CMap.end()) {    // Nope, add it to the map
        CMap.insert(make_pair(Init, GV));
      } else {                  // Yup, this is a duplicate!
        // Make all uses of the duplicate constant use the cannonical version...
        GV->replaceAllUsesWith(I->second);

        // Remove and delete the global value from the module...
        delete GList.remove(GList.begin()+ConstantNo);

        --ConstantNo;  // Don't skip the next constant.
        MadeChanges = true;
      }
    }
  }
  return MadeChanges;
}


// mergeDuplicateConstants - Static accessor for clients that don't want to
// deal with passes.
//
bool ConstantMerge::mergeDuplicateConstants(Module *M) {
  map<ConstPoolVal*, GlobalVariable*> Constants;
  unsigned LastConstantSeen = 0;
  return ::mergeDuplicateConstants(M, LastConstantSeen, Constants);
}


// doPassInitialization - For this pass, process all of the globals in the
// module, eliminating duplicate constants.
//
bool ConstantMerge::doPassInitialization(Module *M) {
  return ::mergeDuplicateConstants(M, LastConstantSeen, Constants);
}

// doPerMethodWork - Check to see if any globals have been added to the 
// global list for the module.  If so, eliminate them.
//
bool DynamicConstantMerge::doPerMethodWork(Method *M) {
  return ::mergeDuplicateConstants(M->getParent(), LastConstantSeen, Constants);
}