lib/Transforms/Utils/LCSSA.cpp

   1 //===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file was developed by Owen Anderson and is distributed under the
   6 // University of Illinois Open Source License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass transforms loops by placing phi nodes at the end of the loops for
  11 // all values that are live across the loop boundary.  For example, it turns
  12 // the left into the right code:
  13 //
  14 // for (...)                for (...)
  15 //   if (c)                   if(c)
  16 //     X1 = ...                 X1 = ...
  17 //   else                     else
  18 //     X2 = ...                 X2 = ...
  19 //   X3 = phi(X1, X2)         X3 = phi(X1, X2)
  20 // ... = X3 + 4              X4 = phi(X3)
  21 //                           ... = X4 + 4
  22 //
  23 // This is still valid LLVM; the extra phi nodes are purely redundant, and will
  24 // be trivially eliminated by InstCombine.  The major benefit of this
  25 // transformation is that it makes many other loop optimizations, such as
  26 // LoopUnswitching, simpler.
  27 //
  28 //===----------------------------------------------------------------------===//
  29
  30 #include "llvm/Transforms/Scalar.h"
  31 #include "llvm/Constants.h"
  32 #include "llvm/Pass.h"
  33 #include "llvm/Function.h"
  34 #include "llvm/Instructions.h"
  35 #include "llvm/ADT/SetVector.h"
  36 #include "llvm/ADT/Statistic.h"
  37 #include "llvm/Analysis/Dominators.h"
  38 #include "llvm/Analysis/LoopInfo.h"
  39 #include "llvm/Support/CFG.h"
  40 #include <algorithm>
  41 #include <map>
  42
  43 using namespace llvm;
  44
  45 namespace {
  46   static Statistic<> NumLCSSA("lcssa",
  47                               "Number of live out of a loop variables");
  48
  49   struct LCSSA : public FunctionPass {
  50     // Cached analysis information for the current function.
  51     LoopInfo *LI;
  52     DominatorTree *DT;
  53     std::vector<BasicBlock*> LoopBlocks;
  54
  55     virtual bool runOnFunction(Function &F);
  56     bool visitSubloop(Loop* L);
  57     void ProcessInstruction(Instruction* Instr,
  58                             const std::vector<BasicBlock*>& exitBlocks);
  59
  60     /// This transformation requires natural loop information & requires that
  61     /// loop preheaders be inserted into the CFG.  It maintains both of these,
  62     /// as well as the CFG.  It also requires dominator information.
  63     ///
  64     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  65       AU.setPreservesCFG();
  66       AU.addRequiredID(LoopSimplifyID);
  67       AU.addPreservedID(LoopSimplifyID);
  68       AU.addRequired<LoopInfo>();
  69       AU.addRequired<DominatorTree>();
  70     }
  71   private:
  72     SetVector<Instruction*> getLoopValuesUsedOutsideLoop(Loop *L);
  73
  74     PHINode *GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
  75                               std::map<DominatorTree::Node*, PHINode*> &Phis);
  76
  77     /// inLoop - returns true if the given block is within the current loop
  78     const bool inLoop(BasicBlock* B) {
  79       return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
  80     }
  81   };
  82
  83   RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
  84 }
  85
  86 FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
  87 const PassInfo *llvm::LCSSAID = X.getPassInfo();
  88
  89 /// runOnFunction - Process all loops in the function, inner-most out.
  90 bool LCSSA::runOnFunction(Function &F) {
  91   bool changed = false;
  92
  93   LI = &getAnalysis<LoopInfo>();
  94   DT = &getAnalysis<DominatorTree>();
  95
  96   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
  97     changed |= visitSubloop(*I);
  98
  99   return changed;
 100 }
 101
 102 /// visitSubloop - Recursively process all subloops, and then process the given
 103 /// loop if it has live-out values.
 104 bool LCSSA::visitSubloop(Loop* L) {
 105   for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
 106     visitSubloop(*I);
 107
 108   // Speed up queries by creating a sorted list of blocks
 109   LoopBlocks.clear();
 110   LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
 111   std::sort(LoopBlocks.begin(), LoopBlocks.end());
 112
 113   SetVector<Instruction*> AffectedValues = getLoopValuesUsedOutsideLoop(L);
 114
 115   // If no values are affected, we can save a lot of work, since we know that
 116   // nothing will be changed.
 117   if (AffectedValues.empty())
 118     return false;
 119
 120   std::vector<BasicBlock*> exitBlocks;
 121   L->getExitBlocks(exitBlocks);
 122
 123
 124   // Iterate over all affected values for this loop and insert Phi nodes
 125   // for them in the appropriate exit blocks
 126
 127   for (SetVector<Instruction*>::iterator I = AffectedValues.begin(),
 128        E = AffectedValues.end(); I != E; ++I)
 129     ProcessInstruction(*I, exitBlocks);
 130
 131   assert(L->isLCSSAForm());
 132
 133   return true;
 134 }
 135
 136 /// processInstruction - Given a live-out instruction, insert LCSSA Phi nodes,
 137 /// eliminate all out-of-loop uses.
 138 void LCSSA::ProcessInstruction(Instruction *Instr,
 139                                const std::vector<BasicBlock*>& exitBlocks) {
 140   ++NumLCSSA; // We are applying the transformation
 141
 142   // Keep track of the blocks that have the value available already.
 143   std::map<DominatorTree::Node*, PHINode*> Phis;
 144
 145   DominatorTree::Node *InstrNode = DT->getNode(Instr->getParent());
 146
 147   // Insert the LCSSA phi's into the exit blocks (dominated by the value), and
 148   // add them to the Phi's map.
 149   for (std::vector<BasicBlock*>::const_iterator BBI = exitBlocks.begin(),
 150       BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
 151     BasicBlock *BB = *BBI;
 152     DominatorTree::Node *ExitBBNode = DT->getNode(BB);
 153     PHINode *&Phi = Phis[ExitBBNode];
 154     if (!Phi && InstrNode->dominates(ExitBBNode)) {
 155       Phi = new PHINode(Instr->getType(), Instr->getName()+".lcssa",
 156                         BB->begin());
 157       Phi->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
 158
 159       // Add inputs from inside the loop for this PHI.
 160       for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
 161         Phi->addIncoming(Instr, *PI);
 162
 163       // Remember that this phi makes the value alive in this block.
 164       Phis[ExitBBNode] = Phi;
 165     }
 166   }
 167
 168
 169   // Record all uses of Instr outside the loop.  We need to rewrite these.  The
 170   // LCSSA phis won't be included because they use the value in the loop.
 171   for (Value::use_iterator UI = Instr->use_begin(), E = Instr->use_end();
 172        UI != E;) {
 173     BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
 174     if (PHINode *P = dyn_cast<PHINode>(*UI)) {
 175       unsigned OperandNo = UI.getOperandNo();
 176       UserBB = P->getIncomingBlock(OperandNo/2);
 177     }
 178
 179     // If the user is in the loop, don't rewrite it!
 180     if (UserBB == Instr->getParent() || inLoop(UserBB)) {
 181       ++UI;
 182       continue;
 183     }
 184
 185     // Otherwise, patch up uses of the value with the appropriate LCSSA Phi,
 186     // inserting PHI nodes into join points where needed.
 187     Value *Val = GetValueForBlock(DT->getNode(UserBB), Instr, Phis);
 188
 189     // Preincrement the iterator to avoid invalidating it when we change the
 190     // value.
 191     Use &U = UI.getUse();
 192     ++UI;
 193     U.set(Val);
 194   }
 195 }
 196
 197 /// getLoopValuesUsedOutsideLoop - Return any values defined in the loop that
 198 /// are used by instructions outside of it.
 199 SetVector<Instruction*> LCSSA::getLoopValuesUsedOutsideLoop(Loop *L) {
 200
 201   // FIXME: For large loops, we may be able to avoid a lot of use-scanning
 202   // by using dominance information.  In particular, if a block does not
 203   // dominate any of the loop exits, then none of the values defined in the
 204   // block could be used outside the loop.
 205
 206   SetVector<Instruction*> AffectedValues;
 207   for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
 208        BB != E; ++BB) {
 209     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
 210       for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
 211            ++UI) {
 212         BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
 213         if (PHINode* p = dyn_cast<PHINode>(*UI)) {
 214           unsigned OperandNo = UI.getOperandNo();
 215           UserBB = p->getIncomingBlock(OperandNo/2);
 216         }
 217
 218         if (*BB != UserBB && !inLoop(UserBB)) {
 219           AffectedValues.insert(I);
 220           break;
 221         }
 222       }
 223   }
 224   return AffectedValues;
 225 }
 226
 227 /// GetValueForBlock - Get the value to use within the specified basic block.
 228 /// available values are in Phis.
 229 PHINode *LCSSA::GetValueForBlock(DominatorTree::Node *BB, Instruction *OrigInst,
 230                                std::map<DominatorTree::Node*, PHINode*> &Phis) {
 231   // If we have already computed this value, return the previously computed val.
 232   PHINode *&V = Phis[BB];
 233   if (V) return V;
 234
 235   DominatorTree::Node *IDom = BB->getIDom();
 236
 237   // Otherwise, there are two cases: we either have to insert a PHI node or we
 238   // don't.  We need to insert a PHI node if this block is not dominated by one
 239   // of the exit nodes from the loop (the loop could have multiple exits, and
 240   // though the value defined *inside* the loop dominated all its uses, each
 241   // exit by itself may not dominate all the uses).
 242   //
 243   // The simplest way to check for this condition is by checking to see if the
 244   // idom is in the loop.  If so, we *know* that none of the exit blocks
 245   // dominate this block.  Note that we *know* that the block defining the
 246   // original instruction is in the idom chain, because if it weren't, then the
 247   // original value didn't dominate this use.
 248   if (!inLoop(IDom->getBlock())) {
 249     // Idom is not in the loop, we must still be "below" the exit block and must
 250     // be fully dominated by the value live in the idom.
 251     return V = GetValueForBlock(IDom, OrigInst, Phis);
 252   }
 253
 254   BasicBlock *BBN = BB->getBlock();
 255
 256   // Otherwise, the idom is the loop, so we need to insert a PHI node.  Do so
 257   // now, then get values to fill in the incoming values for the PHI.
 258   V = new PHINode(OrigInst->getType(), OrigInst->getName()+".lcssa",
 259                   BBN->begin());
 260   V->reserveOperandSpace(std::distance(pred_begin(BBN), pred_end(BBN)));
 261
 262   // Fill in the incoming values for the block.
 263   for (pred_iterator PI = pred_begin(BBN), E = pred_end(BBN); PI != E; ++PI)
 264     V->addIncoming(GetValueForBlock(DT->getNode(*PI), OrigInst, Phis), *PI);
 265   return V;
 266 }
 267