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DataStructureOpt.cpp

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00001 //===- DataStructureOpt.cpp - Data Structure Analysis Based Optimizations -===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This pass uses DSA to a series of simple optimizations, like marking
00011 // unwritten global variables 'constant'.
00012 //
00013 //===----------------------------------------------------------------------===//
00014 
00015 #include "llvm/Analysis/DataStructure/DataStructure.h"
00016 #include "llvm/Analysis/DataStructure/DSGraph.h"
00017 #include "llvm/Module.h"
00018 #include "llvm/Constant.h"
00019 #include "llvm/ADT/Statistic.h"
00020 using namespace llvm;
00021 
00022 namespace {
00023   Statistic<>
00024   NumGlobalsConstanted("ds-opt", "Number of globals marked constant");
00025   Statistic<>
00026   NumGlobalsIsolated("ds-opt", "Number of globals with references dropped");
00027 
00028   class DSOpt : public ModulePass {
00029     TDDataStructures *TD;
00030   public:
00031     bool runOnModule(Module &M) {
00032       TD = &getAnalysis<TDDataStructures>();
00033       bool Changed = OptimizeGlobals(M);
00034       return Changed;
00035     }
00036 
00037     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00038       AU.addRequired<TDDataStructures>();      // Uses TD Datastructures
00039       AU.addPreserved<LocalDataStructures>();  // Preserves local...
00040       AU.addPreserved<TDDataStructures>();     // Preserves bu...
00041       AU.addPreserved<BUDataStructures>();     // Preserves td...
00042     }
00043 
00044   private:
00045     bool OptimizeGlobals(Module &M);
00046   };
00047 
00048   RegisterOpt<DSOpt> X("ds-opt", "DSA-based simple optimizations");
00049 }
00050 
00051 /// OptimizeGlobals - This method uses information taken from DSA to optimize
00052 /// global variables.
00053 ///
00054 bool DSOpt::OptimizeGlobals(Module &M) {
00055   DSGraph &GG = TD->getGlobalsGraph();
00056   const DSGraph::ScalarMapTy &SM = GG.getScalarMap();
00057   bool Changed = false;
00058 
00059   for (Module::giterator I = M.gbegin(), E = M.gend(); I != E; ++I)
00060     if (!I->isExternal()) { // Loop over all of the non-external globals...
00061       // Look up the node corresponding to this global, if it exists.
00062       DSNode *GNode = 0;
00063       DSGraph::ScalarMapTy::const_iterator SMI = SM.find(I);
00064       if (SMI != SM.end()) GNode = SMI->second.getNode();
00065     
00066       if (GNode == 0 && I->hasInternalLinkage()) {
00067         // If there is no entry in the scalar map for this global, it was never
00068         // referenced in the program.  If it has internal linkage, that means we
00069         // can delete it.  We don't ACTUALLY want to delete the global, just
00070         // remove anything that references the global: later passes will take
00071         // care of nuking it.
00072         if (!I->use_empty()) {
00073           I->replaceAllUsesWith(Constant::getNullValue((Type*)I->getType()));
00074           ++NumGlobalsIsolated;
00075         }
00076       } else if (GNode && GNode->isComplete()) {
00077 
00078         // If the node has not been read or written, and it is not externally
00079         // visible, kill any references to it so it can be DCE'd.
00080         if (!GNode->isModified() && !GNode->isRead() &&I->hasInternalLinkage()){
00081           if (!I->use_empty()) {
00082             I->replaceAllUsesWith(Constant::getNullValue((Type*)I->getType()));
00083             ++NumGlobalsIsolated;
00084           }
00085         }
00086 
00087         // We expect that there will almost always be a node for this global.
00088         // If there is, and the node doesn't have the M bit set, we can set the
00089         // 'constant' bit on the global.
00090         if (!GNode->isModified() && !I->isConstant()) {
00091           I->setConstant(true);
00092           ++NumGlobalsConstanted;
00093           Changed = true;
00094         }
00095       }
00096     }
00097   return Changed;
00098 }