lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

   1 //===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===//
   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 file implements the Correlated Value Propagation pass.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #define DEBUG_TYPE "correlated-value-propagation"
  15 #include "llvm/Transforms/Scalar.h"
  16 #include "llvm/Function.h"
  17 #include "llvm/Instructions.h"
  18 #include "llvm/Pass.h"
  19 #include "llvm/Analysis/LazyValueInfo.h"
  20 #include "llvm/Support/CFG.h"
  21 #include "llvm/Transforms/Utils/Local.h"
  22 #include "llvm/ADT/DepthFirstIterator.h"
  23 #include "llvm/ADT/Statistic.h"
  24 using namespace llvm;
  25
  26 STATISTIC(NumPhis,      "Number of phis propagated");
  27 STATISTIC(NumSelects,   "Number of selects propagated");
  28 STATISTIC(NumMemAccess, "Number of memory access targets propagated");
  29 STATISTIC(NumCmps,      "Number of comparisons propagated");
  30
  31 namespace {
  32   class CorrelatedValuePropagation : public FunctionPass {
  33     LazyValueInfo *LVI;
  34
  35     bool processSelect(SelectInst *SI);
  36     bool processPHI(PHINode *P);
  37     bool processMemAccess(Instruction *I);
  38     bool processCmp(CmpInst *C);
  39
  40   public:
  41     static char ID;
  42     CorrelatedValuePropagation(): FunctionPass(ID) {
  43      initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
  44     }
  45
  46     bool runOnFunction(Function &F);
  47
  48     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  49       AU.addRequired<LazyValueInfo>();
  50     }
  51   };
  52 }
  53
  54 char CorrelatedValuePropagation::ID = 0;
  55 INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
  56                 "Value Propagation", false, false)
  57 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
  58 INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
  59                 "Value Propagation", false, false)
  60
  61 // Public interface to the Value Propagation pass
  62 Pass *llvm::createCorrelatedValuePropagationPass() {
  63   return new CorrelatedValuePropagation();
  64 }
  65
  66 bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
  67   if (S->getType()->isVectorTy()) return false;
  68   if (isa<Constant>(S->getOperand(0))) return false;
  69
  70   Constant *C = LVI->getConstant(S->getOperand(0), S->getParent());
  71   if (!C) return false;
  72
  73   ConstantInt *CI = dyn_cast<ConstantInt>(C);
  74   if (!CI) return false;
  75
  76   S->replaceAllUsesWith(S->getOperand(CI->isOne() ? 1 : 2));
  77   S->eraseFromParent();
  78
  79   ++NumSelects;
  80
  81   return true;
  82 }
  83
  84 bool CorrelatedValuePropagation::processPHI(PHINode *P) {
  85   bool Changed = false;
  86
  87   BasicBlock *BB = P->getParent();
  88   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
  89     Value *Incoming = P->getIncomingValue(i);
  90     if (isa<Constant>(Incoming)) continue;
  91
  92     Constant *C = LVI->getConstantOnEdge(P->getIncomingValue(i),
  93                                          P->getIncomingBlock(i),
  94                                          BB);
  95     if (!C) continue;
  96
  97     P->setIncomingValue(i, C);
  98     Changed = true;
  99   }
 100
 101   if (Value *ConstVal = P->hasConstantValue()) {
 102     P->replaceAllUsesWith(ConstVal);
 103     P->eraseFromParent();
 104     Changed = true;
 105   }
 106
 107   ++NumPhis;
 108
 109   return Changed;
 110 }
 111
 112 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
 113   Value *Pointer = 0;
 114   if (LoadInst *L = dyn_cast<LoadInst>(I))
 115     Pointer = L->getPointerOperand();
 116   else
 117     Pointer = cast<StoreInst>(I)->getPointerOperand();
 118
 119   if (isa<Constant>(Pointer)) return false;
 120
 121   Constant *C = LVI->getConstant(Pointer, I->getParent());
 122   if (!C) return false;
 123
 124   ++NumMemAccess;
 125   I->replaceUsesOfWith(Pointer, C);
 126   return true;
 127 }
 128
 129 /// processCmp - If the value of this comparison could be determined locally,
 130 /// constant propagation would already have figured it out.  Instead, walk
 131 /// the predecessors and statically evaluate the comparison based on information
 132 /// available on that edge.  If a given static evaluation is true on ALL
 133 /// incoming edges, then it's true universally and we can simplify the compare.
 134 bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
 135   Value *Op0 = C->getOperand(0);
 136   if (isa<Instruction>(Op0) &&
 137       cast<Instruction>(Op0)->getParent() == C->getParent())
 138     return false;
 139
 140   Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
 141   if (!Op1) return false;
 142
 143   pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
 144   if (PI == PE) return false;
 145
 146   LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
 147                                     C->getOperand(0), Op1, *PI, C->getParent());
 148   if (Result == LazyValueInfo::Unknown) return false;
 149
 150   ++PI;
 151   while (PI != PE) {
 152     LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
 153                                     C->getOperand(0), Op1, *PI, C->getParent());
 154     if (Res != Result) return false;
 155     ++PI;
 156   }
 157
 158   ++NumCmps;
 159
 160   if (Result == LazyValueInfo::True)
 161     C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
 162   else
 163     C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
 164
 165   C->eraseFromParent();
 166
 167   return true;
 168 }
 169
 170 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
 171   LVI = &getAnalysis<LazyValueInfo>();
 172
 173   bool FnChanged = false;
 174
 175   // Perform a depth-first walk of the CFG so that we don't waste time
 176   // optimizing unreachable blocks.
 177   for (df_iterator<BasicBlock*> FI = df_begin(&F.getEntryBlock()),
 178        FE = df_end(&F.getEntryBlock()); FI != FE; ++FI) {
 179     bool BBChanged = false;
 180     for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
 181       Instruction *II = BI++;
 182       switch (II->getOpcode()) {
 183       case Instruction::Select:
 184         BBChanged |= processSelect(cast<SelectInst>(II));
 185         break;
 186       case Instruction::PHI:
 187         BBChanged |= processPHI(cast<PHINode>(II));
 188         break;
 189       case Instruction::ICmp:
 190       case Instruction::FCmp:
 191         BBChanged |= processCmp(cast<CmpInst>(II));
 192         break;
 193       case Instruction::Load:
 194       case Instruction::Store:
 195         BBChanged |= processMemAccess(II);
 196         break;
 197       }
 198     }
 199
 200     // Propagating correlated values might leave cruft around.
 201     // Try to clean it up before we continue.
 202     if (BBChanged)
 203       SimplifyInstructionsInBlock(*FI);
 204
 205     FnChanged |= BBChanged;
 206   }
 207
 208   return FnChanged;
 209 }