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 is distributed under the University of Illinois Open Source
   6 // 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 #define DEBUG_TYPE "lcssa"
  31 #include "llvm/Transforms/Scalar.h"
  32 #include "llvm/Constants.h"
  33 #include "llvm/Pass.h"
  34 #include "llvm/Function.h"
  35 #include "llvm/Instructions.h"
  36 #include "llvm/Analysis/Dominators.h"
  37 #include "llvm/Analysis/LoopPass.h"
  38 #include "llvm/Analysis/ScalarEvolution.h"
  39 #include "llvm/Transforms/Utils/SSAUpdater.h"
  40 #include "llvm/ADT/Statistic.h"
  41 #include "llvm/ADT/STLExtras.h"
  42 #include "llvm/Support/PredIteratorCache.h"
  43 using namespace llvm;
  44
  45 STATISTIC(NumLCSSA, "Number of live out of a loop variables");
  46
  47 namespace {
  48   struct LCSSA : public LoopPass {
  49     static char ID; // Pass identification, replacement for typeid
  50     LCSSA() : LoopPass(ID) {
  51       initializeLCSSAPass(*PassRegistry::getPassRegistry());
  52     }
  53
  54     // Cached analysis information for the current function.
  55     DominatorTree *DT;
  56     std::vector<BasicBlock*> LoopBlocks;
  57     PredIteratorCache PredCache;
  58     Loop *L;
  59
  60     virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
  61
  62     /// This transformation requires natural loop information & requires that
  63     /// loop preheaders be inserted into the CFG.  It maintains both of these,
  64     /// as well as the CFG.  It also requires dominator information.
  65     ///
  66     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  67       AU.setPreservesCFG();
  68
  69       AU.addRequired<DominatorTree>();
  70       AU.addRequired<LoopInfo>();
  71       AU.addPreservedID(LoopSimplifyID);
  72       AU.addPreserved<ScalarEvolution>();
  73     }
  74   private:
  75     bool ProcessInstruction(Instruction *Inst,
  76                             const SmallVectorImpl<BasicBlock*> &ExitBlocks);
  77
  78     /// verifyAnalysis() - Verify loop nest.
  79     virtual void verifyAnalysis() const {
  80       // Check the special guarantees that LCSSA makes.
  81       assert(L->isLCSSAForm(*DT) && "LCSSA form not preserved!");
  82     }
  83
  84     /// inLoop - returns true if the given block is within the current loop
  85     bool inLoop(BasicBlock *B) const {
  86       return std::binary_search(LoopBlocks.begin(), LoopBlocks.end(), B);
  87     }
  88   };
  89 }
  90
  91 char LCSSA::ID = 0;
  92 INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
  93 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  94 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
  95 INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
  96
  97 Pass *llvm::createLCSSAPass() { return new LCSSA(); }
  98 char &llvm::LCSSAID = LCSSA::ID;
  99
 100
 101 /// BlockDominatesAnExit - Return true if the specified block dominates at least
 102 /// one of the blocks in the specified list.
 103 static bool BlockDominatesAnExit(BasicBlock *BB,
 104                                  const SmallVectorImpl<BasicBlock*> &ExitBlocks,
 105                                  DominatorTree *DT) {
 106   DomTreeNode *DomNode = DT->getNode(BB);
 107   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
 108     if (DT->dominates(DomNode, DT->getNode(ExitBlocks[i])))
 109       return true;
 110
 111   return false;
 112 }
 113
 114
 115 /// runOnFunction - Process all loops in the function, inner-most out.
 116 bool LCSSA::runOnLoop(Loop *TheLoop, LPPassManager &LPM) {
 117   L = TheLoop;
 118
 119   DT = &getAnalysis<DominatorTree>();
 120
 121   // Get the set of exiting blocks.
 122   SmallVector<BasicBlock*, 8> ExitBlocks;
 123   L->getExitBlocks(ExitBlocks);
 124
 125   if (ExitBlocks.empty())
 126     return false;
 127
 128   // Speed up queries by creating a sorted vector of blocks.
 129   LoopBlocks.clear();
 130   LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
 131   array_pod_sort(LoopBlocks.begin(), LoopBlocks.end());
 132
 133   // Look at all the instructions in the loop, checking to see if they have uses
 134   // outside the loop.  If so, rewrite those uses.
 135   bool MadeChange = false;
 136
 137   for (Loop::block_iterator BBI = L->block_begin(), E = L->block_end();
 138        BBI != E; ++BBI) {
 139     BasicBlock *BB = *BBI;
 140
 141     // For large loops, avoid use-scanning by using dominance information:  In
 142     // particular, if a block does not dominate any of the loop exits, then none
 143     // of the values defined in the block could be used outside the loop.
 144     if (!BlockDominatesAnExit(BB, ExitBlocks, DT))
 145       continue;
 146
 147     for (BasicBlock::iterator I = BB->begin(), E = BB->end();
 148          I != E; ++I) {
 149       // Reject two common cases fast: instructions with no uses (like stores)
 150       // and instructions with one use that is in the same block as this.
 151       if (I->use_empty() ||
 152           (I->hasOneUse() && I->use_back()->getParent() == BB &&
 153            !isa<PHINode>(I->use_back())))
 154         continue;
 155
 156       MadeChange |= ProcessInstruction(I, ExitBlocks);
 157     }
 158   }
 159
 160   assert(L->isLCSSAForm(*DT));
 161   PredCache.clear();
 162
 163   return MadeChange;
 164 }
 165
 166 /// isExitBlock - Return true if the specified block is in the list.
 167 static bool isExitBlock(BasicBlock *BB,
 168                         const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
 169   for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
 170     if (ExitBlocks[i] == BB)
 171       return true;
 172   return false;
 173 }
 174
 175 /// ProcessInstruction - Given an instruction in the loop, check to see if it
 176 /// has any uses that are outside the current loop.  If so, insert LCSSA PHI
 177 /// nodes and rewrite the uses.
 178 bool LCSSA::ProcessInstruction(Instruction *Inst,
 179                                const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
 180   SmallVector<Use*, 16> UsesToRewrite;
 181
 182   BasicBlock *InstBB = Inst->getParent();
 183
 184   for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
 185        UI != E; ++UI) {
 186     User *U = *UI;
 187     BasicBlock *UserBB = cast<Instruction>(U)->getParent();
 188     if (PHINode *PN = dyn_cast<PHINode>(U))
 189       UserBB = PN->getIncomingBlock(UI);
 190
 191     if (InstBB != UserBB && !inLoop(UserBB))
 192       UsesToRewrite.push_back(&UI.getUse());
 193   }
 194
 195   // If there are no uses outside the loop, exit with no change.
 196   if (UsesToRewrite.empty()) return false;
 197
 198   ++NumLCSSA; // We are applying the transformation
 199
 200   // Invoke instructions are special in that their result value is not available
 201   // along their unwind edge. The code below tests to see whether DomBB dominates
 202   // the value, so adjust DomBB to the normal destination block, which is
 203   // effectively where the value is first usable.
 204   BasicBlock *DomBB = Inst->getParent();
 205   if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
 206     DomBB = Inv->getNormalDest();
 207
 208   DomTreeNode *DomNode = DT->getNode(DomBB);
 209
 210   SmallVector<PHINode*, 16> AddedPHIs;
 211
 212   SSAUpdater SSAUpdate;
 213   SSAUpdate.Initialize(Inst->getType(), Inst->getName());
 214
 215   // Insert the LCSSA phi's into all of the exit blocks dominated by the
 216   // value, and add them to the Phi's map.
 217   for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
 218       BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
 219     BasicBlock *ExitBB = *BBI;
 220     if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue;
 221
 222     // If we already inserted something for this BB, don't reprocess it.
 223     if (SSAUpdate.HasValueForBlock(ExitBB)) continue;
 224
 225     PHINode *PN = PHINode::Create(Inst->getType(),
 226                                   PredCache.GetNumPreds(ExitBB),
 227                                   Inst->getName()+".lcssa",
 228                                   ExitBB->begin());
 229
 230     // Add inputs from inside the loop for this PHI.
 231     for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
 232       PN->addIncoming(Inst, *PI);
 233
 234       // If the exit block has a predecessor not within the loop, arrange for
 235       // the incoming value use corresponding to that predecessor to be
 236       // rewritten in terms of a different LCSSA PHI.
 237       if (!inLoop(*PI))
 238         UsesToRewrite.push_back(
 239           &PN->getOperandUse(
 240             PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1)));
 241     }
 242
 243     AddedPHIs.push_back(PN);
 244
 245     // Remember that this phi makes the value alive in this block.
 246     SSAUpdate.AddAvailableValue(ExitBB, PN);
 247   }
 248
 249   // Rewrite all uses outside the loop in terms of the new PHIs we just
 250   // inserted.
 251   for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
 252     // If this use is in an exit block, rewrite to use the newly inserted PHI.
 253     // This is required for correctness because SSAUpdate doesn't handle uses in
 254     // the same block.  It assumes the PHI we inserted is at the end of the
 255     // block.
 256     Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
 257     BasicBlock *UserBB = User->getParent();
 258     if (PHINode *PN = dyn_cast<PHINode>(User))
 259       UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
 260
 261     if (isa<PHINode>(UserBB->begin()) &&
 262         isExitBlock(UserBB, ExitBlocks)) {
 263       UsesToRewrite[i]->set(UserBB->begin());
 264       continue;
 265     }
 266
 267     // Otherwise, do full PHI insertion.
 268     SSAUpdate.RewriteUse(*UsesToRewrite[i]);
 269   }
 270
 271   // Remove PHI nodes that did not have any uses rewritten.
 272   for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
 273     if (AddedPHIs[i]->use_empty())
 274       AddedPHIs[i]->eraseFromParent();
 275   }
 276
 277   return true;
 278 }
 279