lib/Transforms/Utils/PromoteMemoryToRegister.cpp

   1 //===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===//
   2 //
   3 // This pass is used to promote memory references to be register references.  A
   4 // simple example of the transformation performed by this pass is:
   5 //
   6 //        FROM CODE                           TO CODE
   7 //   %X = alloca int, uint 1                 ret int 42
   8 //   store int 42, int *%X
   9 //   %Y = load int* %X
  10 //   ret int %Y
  11 //
  12 // To do this transformation, a simple analysis is done to ensure it is safe.
  13 // Currently this just loops over all alloca instructions, looking for
  14 // instructions that are only used in simple load and stores.
  15 //
  16 // After this, the code is transformed by...something magical :)
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #include "llvm/Transforms/Scalar/PromoteMemoryToRegister.h"
  21 #include "llvm/Analysis/Dominators.h"
  22 #include "llvm/iMemory.h"
  23 #include "llvm/iPHINode.h"
  24 #include "llvm/iTerminators.h"
  25 #include "llvm/Pass.h"
  26 #include "llvm/Function.h"
  27 #include "llvm/BasicBlock.h"
  28 #include "llvm/ConstantVals.h"
  29
  30 using namespace std;
  31
  32 namespace {
  33
  34 //instance of the promoter -- to keep all the local function data.
  35 // gets re-created for each function processed
  36 class PromoteInstance
  37 {
  38         protected:
  39         vector<AllocaInst*>                     Allocas;   // the alloca instruction..
  40         map<Instruction *, int>                 AllocaLookup; //reverse mapping of above
  41
  42         vector<vector<BasicBlock *> >           WriteSets; // index corresponds to Allocas
  43         vector<vector<BasicBlock *> >           PhiNodes;  // index corresponds to Allocas
  44         vector<vector<Value *> >                CurrentValue; //the current value stack
  45
  46         //list of instructions to remove at end of pass :)
  47         vector<Instruction *> killlist;
  48
  49         set<BasicBlock *>                       visited;        //the basic blocks we've already visited
  50         map<BasicBlock *, vector<PHINode *> >   new_phinodes;   //the phinodes we're adding
  51
  52
  53         void traverse(BasicBlock *f, BasicBlock * predecessor);
  54         bool PromoteFunction(Function *F, DominanceFrontier &DF);
  55         bool queuePhiNode(BasicBlock *bb, int alloca_index);
  56         void findSafeAllocas(Function *M);
  57         bool didchange;
  58         public:
  59         // I do this so that I can force the deconstruction of the local variables
  60         PromoteInstance(Function *F, DominanceFrontier &DF)
  61         {
  62                 didchange=PromoteFunction(F, DF);
  63         }
  64         //This returns whether the pass changes anything
  65         operator bool () { return didchange; }
  66 };
  67
  68 }  // end of anonymous namespace
  69
  70 // findSafeAllocas - Find allocas that are safe to promote
  71 //
  72 void PromoteInstance::findSafeAllocas(Function *F)
  73 {
  74   BasicBlock *BB = F->getEntryNode();  // Get the entry node for the function
  75
  76   // Look at all instructions in the entry node
  77   for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
  78     if (AllocaInst *AI = dyn_cast<AllocaInst>(*I))       // Is it an alloca?
  79       if (!AI->isArrayAllocation()) {
  80         bool isSafe = true;
  81         for (Value::use_iterator UI = AI->use_begin(), UE = AI->use_end();
  82              UI != UE; ++UI) {   // Loop over all of the uses of the alloca
  83
  84           // Only allow nonindexed memory access instructions...
  85           if (MemAccessInst *MAI = dyn_cast<MemAccessInst>(*UI)) {
  86             if (MAI->hasIndices()) {  // indexed?
  87               // Allow the access if there is only one index and the index is zero.
  88               if (*MAI->idx_begin() != ConstantUInt::get(Type::UIntTy, 0) ||
  89                   MAI->idx_begin()+1 != MAI->idx_end()) {
  90                 isSafe = false; break;
  91               }
  92             }
  93           } else {
  94             isSafe = false; break;   // Not a load or store?
  95           }
  96         }
  97         if (isSafe)              // If all checks pass, add alloca to safe list
  98           {
  99             AllocaLookup[AI]=Allocas.size();
 100             Allocas.push_back(AI);
 101           }
 102       }
 103 }
 104
 105
 106
 107 bool PromoteInstance::PromoteFunction(Function *F, DominanceFrontier & DF) {
 108         // Calculate the set of safe allocas
 109         findSafeAllocas(F);
 110
 111         // Add each alloca to the killlist
 112         // note: killlist is destroyed MOST recently added to least recently.
 113         killlist.assign(Allocas.begin(), Allocas.end());
 114
 115         // Calculate the set of write-locations for each alloca.
 116         // this is analogous to counting the number of 'redefinitions' of each variable.
 117         for (unsigned i = 0; i<Allocas.size(); ++i)
 118         {
 119                 AllocaInst * AI = Allocas[i];
 120                 WriteSets.push_back(std::vector<BasicBlock *>()); //add a new set
 121                 for (Value::use_iterator U = AI->use_begin();U!=AI->use_end();++U)
 122                 {
 123                         if (MemAccessInst *MAI = dyn_cast<StoreInst>(*U)) {
 124                                 WriteSets[i].push_back(MAI->getParent()); // jot down the basic-block it came from
 125                         }
 126                 }
 127         }
 128
 129         // Compute the locations where PhiNodes need to be inserted
 130         // look at the dominance frontier of EACH basic-block we have a write in
 131         PhiNodes.resize(Allocas.size());
 132         for (unsigned i = 0; i<Allocas.size(); ++i)
 133         {
 134                 for (unsigned j = 0; j<WriteSets[i].size(); j++)
 135                 {
 136                         //look up the DF for this write, add it to PhiNodes
 137                         DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
 138                         DominanceFrontier::DomSetType     s = (*it).second;
 139                         for (DominanceFrontier::DomSetType::iterator p = s.begin();p!=s.end(); ++p)
 140                         {
 141                                 if (queuePhiNode(*p, i))
 142                                   PhiNodes[i].push_back(*p);
 143                         }
 144                 }
 145                 // perform iterative step
 146                 for (unsigned k = 0; k<PhiNodes[i].size(); k++)
 147                 {
 148                         DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
 149                         DominanceFrontier::DomSetType     s = it->second;
 150                         for (DominanceFrontier::DomSetType::iterator p = s.begin(); p!=s.end(); ++p)
 151                         {
 152                                 if (queuePhiNode(*p,i))
 153                                 PhiNodes[i].push_back(*p);
 154                         }
 155                 }
 156         }
 157
 158         // Walks all basic blocks in the function
 159         // performing the SSA rename algorithm
 160         // and inserting the phi nodes we marked as necessary
 161         BasicBlock * f = F->front(); //get root basic-block
 162
 163         CurrentValue.push_back(vector<Value *>(Allocas.size()));
 164
 165         traverse(f, NULL);  // there is no predecessor of the root node
 166
 167
 168         // ** REMOVE EVERYTHING IN THE KILL-LIST **
 169         // we need to kill 'uses' before root values
 170         // so we should probably run through in reverse
 171         for (vector<Instruction *>::reverse_iterator i = killlist.rbegin(); i!=killlist.rend(); ++i)
 172         {
 173                 Instruction * r = *i;
 174                 BasicBlock * o = r->getParent();
 175                 //now go find..
 176
 177                 BasicBlock::InstListType & l = o->getInstList();
 178                 o->getInstList().remove(r);
 179                 delete r;
 180         }
 181
 182         return !Allocas.empty();
 183 }
 184
 185
 186
 187 void PromoteInstance::traverse(BasicBlock *f, BasicBlock * predecessor)
 188 {
 189         vector<Value *> * tos = &CurrentValue.back(); //look at top-
 190
 191         //if this is a BB needing a phi node, lookup/create the phinode for
 192         // each variable we need phinodes for.
 193         map<BasicBlock *, vector<PHINode *> >::iterator nd = new_phinodes.find(f);
 194         if (nd!=new_phinodes.end())
 195         {
 196                 for (unsigned k = 0; k!=nd->second.size(); ++k)
 197                 if (nd->second[k])
 198                 {
 199                         //at this point we can assume that the array has phi nodes.. let's
 200                         // add the incoming data
 201                         if ((*tos)[k])
 202                         nd->second[k]->addIncoming((*tos)[k],predecessor);
 203                         //also note that the active variable IS designated by the phi node
 204                         (*tos)[k] = nd->second[k];
 205                 }
 206         }
 207
 208         //don't revisit nodes
 209         if (visited.find(f)!=visited.end())
 210         return;
 211         //mark as visited
 212         visited.insert(f);
 213
 214         BasicBlock::iterator i = f->begin();
 215         //keep track of the value of each variable we're watching.. how?
 216         while(i!=f->end())
 217         {
 218                 Instruction * inst = *i; //get the instruction
 219                 //is this a write/read?
 220                 if (LoadInst * LI = dyn_cast<LoadInst>(inst))
 221                 {
 222                         // This is a bit weird...
 223                         Value * ptr = LI->getPointerOperand(); //of type value
 224                         if (AllocaInst * srcinstr = dyn_cast<AllocaInst>(ptr))
 225                         {
 226                                 map<Instruction *, int>::iterator ai = AllocaLookup.find(srcinstr);
 227                                 if (ai!=AllocaLookup.end())
 228                                 {
 229                                         if (Value *r = (*tos)[ai->second])
 230                                         {
 231                                                 //walk the use list of this load and replace
 232                                                 // all uses with r
 233                                                 LI->replaceAllUsesWith(r);
 234                                                 //now delete the instruction.. somehow..
 235                                                 killlist.push_back((Instruction *)LI);
 236                                         }
 237                                 }
 238                         }
 239                 }
 240                 else if (StoreInst * SI = dyn_cast<StoreInst>(inst))
 241                 {
 242                         // delete this instruction and mark the name as the
 243                         // current holder of the value
 244                         Value * ptr =  SI->getPointerOperand(); //of type value
 245                         if (Instruction * srcinstr = dyn_cast<Instruction>(ptr))
 246                         {
 247                                 map<Instruction *, int>::iterator ai = AllocaLookup.find(srcinstr);
 248                                 if (ai!=AllocaLookup.end())
 249                                 {
 250                                         //what value were we writing?
 251                                         Value * writeval = SI->getOperand(0);
 252                                         //write down...
 253                                         (*tos)[ai->second] = writeval;
 254                                         //now delete it.. somehow?
 255                                         killlist.push_back((Instruction *)SI);
 256                                 }
 257                         }
 258
 259                 }
 260                 else if (TerminatorInst * TI = dyn_cast<TerminatorInst>(inst))
 261                 {
 262                         // Recurse across our sucessors
 263                         for (unsigned i = 0; i!=TI->getNumSuccessors(); i++)
 264                         {
 265                                 CurrentValue.push_back(CurrentValue.back());
 266                                 traverse(TI->getSuccessor(i),f); //this node IS the predecessor
 267                                 CurrentValue.pop_back();
 268                         }
 269                 }
 270                 i++;
 271         }
 272 }
 273
 274 // queues a phi-node to be added to a basic-block for a specific Alloca
 275 // returns true  if there wasn't already a phi-node for that variable
 276
 277
 278 bool PromoteInstance::queuePhiNode(BasicBlock *bb, int i /*the alloca*/)
 279 {
 280         map<BasicBlock *, vector<PHINode *> >::iterator nd;
 281         //look up the basic-block in question
 282         nd = new_phinodes.find(bb);
 283         //if the basic-block has no phi-nodes added, or at least none
 284         //for the i'th alloca. then add.
 285         if (nd==new_phinodes.end() || nd->second[i]==NULL)
 286         {
 287                 //we're not added any phi nodes to this basicblock yet
 288                 // create the phi-node array.
 289                 if (nd==new_phinodes.end())
 290                 {
 291                         new_phinodes[bb] = vector<PHINode *>(Allocas.size());
 292                         nd = new_phinodes.find(bb);
 293                 }
 294
 295                 //find the type the alloca returns
 296                 const PointerType * pt = Allocas[i]->getType();
 297                 //create a phi-node using the DEREFERENCED type
 298                 PHINode * ph = new PHINode(pt->getElementType(), Allocas[i]->getName()+".mem2reg");
 299                 nd->second[i] = ph;
 300                 //add the phi-node to the basic-block
 301                 bb->getInstList().push_front(ph);
 302                 return true;
 303         }
 304         return false;
 305 }
 306
 307
 308 namespace {
 309   struct PromotePass : public FunctionPass {
 310
 311     // runOnFunction - To run this pass, first we calculate the alloca
 312     // instructions that are safe for promotion, then we promote each one.
 313     //
 314     virtual bool runOnFunction(Function *F) {
 315       return (bool)PromoteInstance(F, getAnalysis<DominanceFrontier>());
 316     }
 317
 318
 319     // getAnalysisUsage - We need dominance frontiers
 320     //
 321     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
 322       AU.addRequired(DominanceFrontier::ID);
 323     }
 324   };
 325 }
 326
 327
 328 // createPromoteMemoryToRegister - Provide an entry point to create this pass.
 329 //
 330 Pass *createPromoteMemoryToRegister() {
 331         return new PromotePass();
 332 }
 333
 334