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