lib/CodeGen/StrongPHIElimination.cpp

   1 //===- StrongPhiElimination.cpp - Eliminate PHI nodes by inserting copies -===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by Owen Anderson and is distributed under
   6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass eliminates machine instruction PHI nodes by inserting copy
  11 // instructions, using an intelligent copy-folding technique based on
  12 // dominator information.  This is technique is derived from:
  13 //
  14 //    Budimlic, et al. Fast copy coalescing and live-range identification.
  15 //    In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
  16 //    Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
  17 //    PLDI '02. ACM, New York, NY, 25-32.
  18 //    DOI= http://doi.acm.org/10.1145/512529.512534
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #define DEBUG_TYPE "strongphielim"
  23 #include "llvm/CodeGen/Passes.h"
  24 #include "llvm/CodeGen/BreakCriticalMachineEdge.h"
  25 #include "llvm/CodeGen/LiveVariables.h"
  26 #include "llvm/CodeGen/MachineDominators.h"
  27 #include "llvm/CodeGen/MachineFunctionPass.h"
  28 #include "llvm/CodeGen/MachineInstr.h"
  29 #include "llvm/CodeGen/SSARegMap.h"
  30 #include "llvm/Target/TargetInstrInfo.h"
  31 #include "llvm/Target/TargetMachine.h"
  32 #include "llvm/ADT/Statistic.h"
  33 #include "llvm/Support/Compiler.h"
  34 using namespace llvm;
  35
  36
  37 namespace {
  38   struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
  39     static char ID; // Pass identification, replacement for typeid
  40     StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
  41
  42     DenseMap<MachineBasicBlock*,
  43              SmallVector<std::pair<unsigned, unsigned>, 2> > Waiting;
  44
  45     bool runOnMachineFunction(MachineFunction &Fn);
  46
  47     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  48       AU.addPreserved<LiveVariables>();
  49       AU.addPreservedID(PHIEliminationID);
  50       AU.addRequired<MachineDominatorTree>();
  51       AU.addRequired<LiveVariables>();
  52       AU.setPreservesAll();
  53       MachineFunctionPass::getAnalysisUsage(AU);
  54     }
  55
  56     virtual void releaseMemory() {
  57       preorder.clear();
  58       maxpreorder.clear();
  59
  60       waiting.clear();
  61     }
  62
  63   private:
  64     struct DomForestNode {
  65     private:
  66       std::vector<DomForestNode*> children;
  67       unsigned reg;
  68
  69       void addChild(DomForestNode* DFN) { children.push_back(DFN); }
  70
  71     public:
  72       typedef std::vector<DomForestNode*>::iterator iterator;
  73
  74       DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
  75         if (parent)
  76           parent->addChild(this);
  77       }
  78
  79       ~DomForestNode() {
  80         for (iterator I = begin(), E = end(); I != E; ++I)
  81           delete *I;
  82       }
  83
  84       inline unsigned getReg() { return reg; }
  85
  86       inline DomForestNode::iterator begin() { return children.begin(); }
  87       inline DomForestNode::iterator end() { return children.end(); }
  88     };
  89
  90     DenseMap<MachineBasicBlock*, unsigned> preorder;
  91     DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
  92
  93     DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > waiting;
  94
  95
  96     void computeDFS(MachineFunction& MF);
  97     void processBlock(MachineBasicBlock* MBB);
  98
  99     std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
 100     void processPHIUnion(MachineInstr* Inst,
 101                          std::set<unsigned>& PHIUnion,
 102                          std::vector<StrongPHIElimination::DomForestNode*>& DF,
 103                          std::vector<std::pair<unsigned, unsigned> >& locals);
 104     void breakCriticalEdges(MachineFunction &Fn);
 105
 106   };
 107
 108   char StrongPHIElimination::ID = 0;
 109   RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
 110                   "Eliminate PHI nodes for register allocation, intelligently");
 111 }
 112
 113 const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
 114
 115 /// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
 116 /// of the given MachineFunction.  These numbers are then used in other parts
 117 /// of the PHI elimination process.
 118 void StrongPHIElimination::computeDFS(MachineFunction& MF) {
 119   SmallPtrSet<MachineDomTreeNode*, 8> frontier;
 120   SmallPtrSet<MachineDomTreeNode*, 8> visited;
 121
 122   unsigned time = 0;
 123
 124   MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
 125
 126   MachineDomTreeNode* node = DT.getRootNode();
 127
 128   std::vector<MachineDomTreeNode*> worklist;
 129   worklist.push_back(node);
 130
 131   while (!worklist.empty()) {
 132     MachineDomTreeNode* currNode = worklist.back();
 133
 134     if (!frontier.count(currNode)) {
 135       frontier.insert(currNode);
 136       ++time;
 137       preorder.insert(std::make_pair(currNode->getBlock(), time));
 138     }
 139
 140     bool inserted = false;
 141     for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
 142          I != E; ++I)
 143       if (!frontier.count(*I) && !visited.count(*I)) {
 144         worklist.push_back(*I);
 145         inserted = true;
 146         break;
 147       }
 148
 149     if (!inserted) {
 150       frontier.erase(currNode);
 151       visited.insert(currNode);
 152       maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
 153
 154       worklist.pop_back();
 155     }
 156   }
 157 }
 158
 159 /// PreorderSorter - a helper class that is used to sort registers
 160 /// according to the preorder number of their defining blocks
 161 class PreorderSorter {
 162 private:
 163   DenseMap<MachineBasicBlock*, unsigned>& preorder;
 164   LiveVariables& LV;
 165
 166 public:
 167   PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
 168                 LiveVariables& L) : preorder(p), LV(L) { }
 169
 170   bool operator()(unsigned A, unsigned B) {
 171     if (A == B)
 172       return false;
 173
 174     MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
 175     MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
 176
 177     if (preorder[ABlock] < preorder[BBlock])
 178       return true;
 179     else if (preorder[ABlock] > preorder[BBlock])
 180       return false;
 181
 182     assert(0 && "Error sorting by dominance!");
 183     return false;
 184   }
 185 };
 186
 187 /// computeDomForest - compute the subforest of the DomTree corresponding
 188 /// to the defining blocks of the registers in question
 189 std::vector<StrongPHIElimination::DomForestNode*>
 190 StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
 191   LiveVariables& LV = getAnalysis<LiveVariables>();
 192
 193   DomForestNode* VirtualRoot = new DomForestNode(0, 0);
 194   maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
 195
 196   std::vector<unsigned> worklist;
 197   worklist.reserve(regs.size());
 198   for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
 199        I != E; ++I)
 200     worklist.push_back(*I);
 201
 202   PreorderSorter PS(preorder, LV);
 203   std::sort(worklist.begin(), worklist.end(), PS);
 204
 205   DomForestNode* CurrentParent = VirtualRoot;
 206   std::vector<DomForestNode*> stack;
 207   stack.push_back(VirtualRoot);
 208
 209   for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
 210        I != E; ++I) {
 211     unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
 212     MachineBasicBlock* parentBlock =
 213       LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
 214
 215     while (pre > maxpreorder[parentBlock]) {
 216       stack.pop_back();
 217       CurrentParent = stack.back();
 218
 219       parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
 220     }
 221
 222     DomForestNode* child = new DomForestNode(*I, CurrentParent);
 223     stack.push_back(child);
 224     CurrentParent = child;
 225   }
 226
 227   std::vector<DomForestNode*> ret;
 228   ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
 229   return ret;
 230 }
 231
 232 /// isLiveIn - helper method that determines, from a VarInfo, if a register
 233 /// is live into a block
 234 bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
 235   if (V.AliveBlocks.test(MBB->getNumber()))
 236     return true;
 237
 238   if (V.DefInst->getParent() != MBB &&
 239       V.UsedBlocks.test(MBB->getNumber()))
 240     return true;
 241
 242   return false;
 243 }
 244
 245 /// isLiveOut - help method that determines, from a VarInfo, if a register is
 246 /// live out of a block.
 247 bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
 248   if (MBB == V.DefInst->getParent() ||
 249       V.UsedBlocks.test(MBB->getNumber())) {
 250     for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(),
 251          E = V.Kills.end(); I != E; ++I)
 252       if ((*I)->getParent() == MBB)
 253         return false;
 254
 255     return true;
 256   }
 257
 258   return false;
 259 }
 260
 261 /// processBlock - Eliminate PHIs in the given block
 262 void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
 263   LiveVariables& LV = getAnalysis<LiveVariables>();
 264
 265   // Holds names that have been added to a set in any PHI within this block
 266   // before the current one.
 267   std::set<unsigned> ProcessedNames;
 268
 269   MachineBasicBlock::iterator P = MBB->begin();
 270   while (P->getOpcode() == TargetInstrInfo::PHI) {
 271     LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());
 272
 273     unsigned DestReg = P->getOperand(0).getReg();
 274
 275     // Hold the names that are currently in the candidate set.
 276     std::set<unsigned> PHIUnion;
 277     std::set<MachineBasicBlock*> UnionedBlocks;
 278
 279     for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
 280       unsigned SrcReg = P->getOperand(i-1).getReg();
 281       LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
 282
 283       // Check for trivial interferences
 284       if (isLiveIn(SrcInfo, P->getParent()) ||
 285           isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
 286           ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
 287             isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
 288           ProcessedNames.count(SrcReg) ||
 289           UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
 290
 291         // add a copy from a_i to p in Waiting[From[a_i]]
 292         MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
 293         Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
 294       } else {
 295         PHIUnion.insert(SrcReg);
 296         UnionedBlocks.insert(SrcInfo.DefInst->getParent());
 297       }
 298     }
 299
 300     std::vector<StrongPHIElimination::DomForestNode*> DF =
 301                                                      computeDomForest(PHIUnion);
 302
 303     // Walk DomForest to resolve interferences
 304     std::vector<std::pair<unsigned, unsigned> > localInterferences;
 305     processPHIUnion(P, PHIUnion, DF, localInterferences);
 306
 307     // FIXME: Check for local interferences
 308
 309     ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
 310     ++P;
 311   }
 312 }
 313
 314 /// processPHIUnion - Take a set of candidate registers to be coallesced when
 315 /// decomposing the PHI instruction.  Use the DominanceForest to remove the ones
 316 /// that are known to interfere, and flag others that need to be checked for
 317 /// local interferences.
 318 void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
 319                                            std::set<unsigned>& PHIUnion,
 320                         std::vector<StrongPHIElimination::DomForestNode*>& DF,
 321                         std::vector<std::pair<unsigned, unsigned> >& locals) {
 322
 323   std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
 324   SmallPtrSet<DomForestNode*, 4> visited;
 325
 326   LiveVariables& LV = getAnalysis<LiveVariables>();
 327   unsigned DestReg = Inst->getOperand(0).getReg();
 328
 329   // DF walk on the DomForest
 330   while (!worklist.empty()) {
 331     DomForestNode* DFNode = worklist.back();
 332
 333     LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
 334     visited.insert(DFNode);
 335
 336     bool inserted = false;
 337     for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
 338          CI != CE; ++CI) {
 339       DomForestNode* child = *CI;
 340       LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
 341
 342       if (isLiveOut(Info, CInfo.DefInst->getParent())) {
 343         // Insert copies for parent
 344         for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
 345           if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
 346             unsigned SrcReg = DFNode->getReg();
 347             MachineBasicBlock* From = Inst->getOperand(i).getMBB();
 348
 349             Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
 350             PHIUnion.erase(SrcReg);
 351           }
 352         }
 353       } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
 354                  Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
 355         // Add (p, c) to possible local interferences
 356         locals.push_back(std::make_pair(DFNode->getReg(), child->getReg()));
 357       }
 358
 359       if (!visited.count(child)) {
 360         worklist.push_back(child);
 361         inserted = true;
 362       }
 363     }
 364
 365     if (!inserted) worklist.pop_back();
 366   }
 367 }
 368
 369 /// breakCriticalEdges - Break critical edges coming into blocks with PHI
 370 /// nodes, preserving dominator and livevariable info.
 371 void StrongPHIElimination::breakCriticalEdges(MachineFunction &Fn) {
 372   typedef std::pair<MachineBasicBlock*, MachineBasicBlock*> MBB_pair;
 373
 374   MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
 375   LiveVariables& LV = getAnalysis<LiveVariables>();
 376
 377   // Find critical edges
 378   std::vector<MBB_pair> criticals;
 379   for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
 380     if (!I->empty() &&
 381         I->begin()->getOpcode() == TargetInstrInfo::PHI &&
 382         I->pred_size() > 1)
 383       for (MachineBasicBlock::pred_iterator PI = I->pred_begin(),
 384            PE = I->pred_end(); PI != PE; ++PI)
 385         if ((*PI)->succ_size() > 1)
 386           criticals.push_back(std::make_pair(*PI, I));
 387
 388   for (std::vector<MBB_pair>::iterator I = criticals.begin(),
 389        E = criticals.end(); I != E; ++I) {
 390     // Split the edge
 391     MachineBasicBlock* new_bb = SplitCriticalMachineEdge(I->first, I->second);
 392
 393     // Update dominators
 394     MDT.splitBlock(I->first);
 395
 396     // Update livevariables
 397     for (unsigned var = 1024; var < Fn.getSSARegMap()->getLastVirtReg(); ++var)
 398       if (isLiveOut(LV.getVarInfo(var), I->first))
 399         LV.getVarInfo(var).AliveBlocks.set(new_bb->getNumber());
 400   }
 401 }
 402
 403 bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
 404   breakCriticalEdges(Fn);
 405   computeDFS(Fn);
 406
 407   for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
 408     if (!I->empty() &&
 409         I->begin()->getOpcode() == TargetInstrInfo::PHI)
 410       processBlock(I);
 411
 412   return false;
 413 }