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      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 #include "llvm/Transforms/Scalar.h"
     15 #include "llvm/ADT/Statistic.h"
     16 #include "llvm/Analysis/InstructionSimplify.h"
     17 #include "llvm/Analysis/LazyValueInfo.h"
     18 #include "llvm/IR/CFG.h"
     19 #include "llvm/IR/Constants.h"
     20 #include "llvm/IR/Function.h"
     21 #include "llvm/IR/Instructions.h"
     22 #include "llvm/IR/Module.h"
     23 #include "llvm/Pass.h"
     24 #include "llvm/Support/Debug.h"
     25 #include "llvm/Support/raw_ostream.h"
     26 #include "llvm/Transforms/Utils/Local.h"
     27 using namespace llvm;
     28 
     29 #define DEBUG_TYPE "correlated-value-propagation"
     30 
     31 STATISTIC(NumPhis,      "Number of phis propagated");
     32 STATISTIC(NumSelects,   "Number of selects propagated");
     33 STATISTIC(NumMemAccess, "Number of memory access targets propagated");
     34 STATISTIC(NumCmps,      "Number of comparisons propagated");
     35 STATISTIC(NumDeadCases, "Number of switch cases removed");
     36 
     37 namespace {
     38   class CorrelatedValuePropagation : public FunctionPass {
     39     LazyValueInfo *LVI;
     40 
     41     bool processSelect(SelectInst *SI);
     42     bool processPHI(PHINode *P);
     43     bool processMemAccess(Instruction *I);
     44     bool processCmp(CmpInst *C);
     45     bool processSwitch(SwitchInst *SI);
     46 
     47   public:
     48     static char ID;
     49     CorrelatedValuePropagation(): FunctionPass(ID) {
     50      initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
     51     }
     52 
     53     bool runOnFunction(Function &F) override;
     54 
     55     void getAnalysisUsage(AnalysisUsage &AU) const override {
     56       AU.addRequired<LazyValueInfo>();
     57     }
     58   };
     59 }
     60 
     61 char CorrelatedValuePropagation::ID = 0;
     62 INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
     63                 "Value Propagation", false, false)
     64 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
     65 INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
     66                 "Value Propagation", false, false)
     67 
     68 // Public interface to the Value Propagation pass
     69 Pass *llvm::createCorrelatedValuePropagationPass() {
     70   return new CorrelatedValuePropagation();
     71 }
     72 
     73 bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
     74   if (S->getType()->isVectorTy()) return false;
     75   if (isa<Constant>(S->getOperand(0))) return false;
     76 
     77   Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S);
     78   if (!C) return false;
     79 
     80   ConstantInt *CI = dyn_cast<ConstantInt>(C);
     81   if (!CI) return false;
     82 
     83   Value *ReplaceWith = S->getOperand(1);
     84   Value *Other = S->getOperand(2);
     85   if (!CI->isOne()) std::swap(ReplaceWith, Other);
     86   if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
     87 
     88   S->replaceAllUsesWith(ReplaceWith);
     89   S->eraseFromParent();
     90 
     91   ++NumSelects;
     92 
     93   return true;
     94 }
     95 
     96 bool CorrelatedValuePropagation::processPHI(PHINode *P) {
     97   bool Changed = false;
     98 
     99   BasicBlock *BB = P->getParent();
    100   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
    101     Value *Incoming = P->getIncomingValue(i);
    102     if (isa<Constant>(Incoming)) continue;
    103 
    104     Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
    105 
    106     // Look if the incoming value is a select with a constant but LVI tells us
    107     // that the incoming value can never be that constant. In that case replace
    108     // the incoming value with the other value of the select. This often allows
    109     // us to remove the select later.
    110     if (!V) {
    111       SelectInst *SI = dyn_cast<SelectInst>(Incoming);
    112       if (!SI) continue;
    113 
    114       Constant *C = dyn_cast<Constant>(SI->getFalseValue());
    115       if (!C) continue;
    116 
    117       if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
    118                                   P->getIncomingBlock(i), BB, P) !=
    119           LazyValueInfo::False)
    120         continue;
    121 
    122       DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
    123       V = SI->getTrueValue();
    124     }
    125 
    126     P->setIncomingValue(i, V);
    127     Changed = true;
    128   }
    129 
    130   // FIXME: Provide TLI, DT, AT to SimplifyInstruction.
    131   const DataLayout &DL = BB->getModule()->getDataLayout();
    132   if (Value *V = SimplifyInstruction(P, DL)) {
    133     P->replaceAllUsesWith(V);
    134     P->eraseFromParent();
    135     Changed = true;
    136   }
    137 
    138   if (Changed)
    139     ++NumPhis;
    140 
    141   return Changed;
    142 }
    143 
    144 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
    145   Value *Pointer = nullptr;
    146   if (LoadInst *L = dyn_cast<LoadInst>(I))
    147     Pointer = L->getPointerOperand();
    148   else
    149     Pointer = cast<StoreInst>(I)->getPointerOperand();
    150 
    151   if (isa<Constant>(Pointer)) return false;
    152 
    153   Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
    154   if (!C) return false;
    155 
    156   ++NumMemAccess;
    157   I->replaceUsesOfWith(Pointer, C);
    158   return true;
    159 }
    160 
    161 /// processCmp - If the value of this comparison could be determined locally,
    162 /// constant propagation would already have figured it out.  Instead, walk
    163 /// the predecessors and statically evaluate the comparison based on information
    164 /// available on that edge.  If a given static evaluation is true on ALL
    165 /// incoming edges, then it's true universally and we can simplify the compare.
    166 bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
    167   Value *Op0 = C->getOperand(0);
    168   if (isa<Instruction>(Op0) &&
    169       cast<Instruction>(Op0)->getParent() == C->getParent())
    170     return false;
    171 
    172   Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
    173   if (!Op1) return false;
    174 
    175   pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
    176   if (PI == PE) return false;
    177 
    178   LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
    179                                     C->getOperand(0), Op1, *PI,
    180                                     C->getParent(), C);
    181   if (Result == LazyValueInfo::Unknown) return false;
    182 
    183   ++PI;
    184   while (PI != PE) {
    185     LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
    186                                     C->getOperand(0), Op1, *PI,
    187                                     C->getParent(), C);
    188     if (Res != Result) return false;
    189     ++PI;
    190   }
    191 
    192   ++NumCmps;
    193 
    194   if (Result == LazyValueInfo::True)
    195     C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
    196   else
    197     C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
    198 
    199   C->eraseFromParent();
    200 
    201   return true;
    202 }
    203 
    204 /// processSwitch - Simplify a switch instruction by removing cases which can
    205 /// never fire.  If the uselessness of a case could be determined locally then
    206 /// constant propagation would already have figured it out.  Instead, walk the
    207 /// predecessors and statically evaluate cases based on information available
    208 /// on that edge.  Cases that cannot fire no matter what the incoming edge can
    209 /// safely be removed.  If a case fires on every incoming edge then the entire
    210 /// switch can be removed and replaced with a branch to the case destination.
    211 bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
    212   Value *Cond = SI->getCondition();
    213   BasicBlock *BB = SI->getParent();
    214 
    215   // If the condition was defined in same block as the switch then LazyValueInfo
    216   // currently won't say anything useful about it, though in theory it could.
    217   if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
    218     return false;
    219 
    220   // If the switch is unreachable then trying to improve it is a waste of time.
    221   pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
    222   if (PB == PE) return false;
    223 
    224   // Analyse each switch case in turn.  This is done in reverse order so that
    225   // removing a case doesn't cause trouble for the iteration.
    226   bool Changed = false;
    227   for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
    228        ) {
    229     ConstantInt *Case = CI.getCaseValue();
    230 
    231     // Check to see if the switch condition is equal to/not equal to the case
    232     // value on every incoming edge, equal/not equal being the same each time.
    233     LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
    234     for (pred_iterator PI = PB; PI != PE; ++PI) {
    235       // Is the switch condition equal to the case value?
    236       LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
    237                                                               Cond, Case, *PI,
    238                                                               BB, SI);
    239       // Give up on this case if nothing is known.
    240       if (Value == LazyValueInfo::Unknown) {
    241         State = LazyValueInfo::Unknown;
    242         break;
    243       }
    244 
    245       // If this was the first edge to be visited, record that all other edges
    246       // need to give the same result.
    247       if (PI == PB) {
    248         State = Value;
    249         continue;
    250       }
    251 
    252       // If this case is known to fire for some edges and known not to fire for
    253       // others then there is nothing we can do - give up.
    254       if (Value != State) {
    255         State = LazyValueInfo::Unknown;
    256         break;
    257       }
    258     }
    259 
    260     if (State == LazyValueInfo::False) {
    261       // This case never fires - remove it.
    262       CI.getCaseSuccessor()->removePredecessor(BB);
    263       SI->removeCase(CI); // Does not invalidate the iterator.
    264 
    265       // The condition can be modified by removePredecessor's PHI simplification
    266       // logic.
    267       Cond = SI->getCondition();
    268 
    269       ++NumDeadCases;
    270       Changed = true;
    271     } else if (State == LazyValueInfo::True) {
    272       // This case always fires.  Arrange for the switch to be turned into an
    273       // unconditional branch by replacing the switch condition with the case
    274       // value.
    275       SI->setCondition(Case);
    276       NumDeadCases += SI->getNumCases();
    277       Changed = true;
    278       break;
    279     }
    280   }
    281 
    282   if (Changed)
    283     // If the switch has been simplified to the point where it can be replaced
    284     // by a branch then do so now.
    285     ConstantFoldTerminator(BB);
    286 
    287   return Changed;
    288 }
    289 
    290 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
    291   if (skipOptnoneFunction(F))
    292     return false;
    293 
    294   LVI = &getAnalysis<LazyValueInfo>();
    295 
    296   bool FnChanged = false;
    297 
    298   for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
    299     bool BBChanged = false;
    300     for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
    301       Instruction *II = BI++;
    302       switch (II->getOpcode()) {
    303       case Instruction::Select:
    304         BBChanged |= processSelect(cast<SelectInst>(II));
    305         break;
    306       case Instruction::PHI:
    307         BBChanged |= processPHI(cast<PHINode>(II));
    308         break;
    309       case Instruction::ICmp:
    310       case Instruction::FCmp:
    311         BBChanged |= processCmp(cast<CmpInst>(II));
    312         break;
    313       case Instruction::Load:
    314       case Instruction::Store:
    315         BBChanged |= processMemAccess(II);
    316         break;
    317       }
    318     }
    319 
    320     Instruction *Term = FI->getTerminator();
    321     switch (Term->getOpcode()) {
    322     case Instruction::Switch:
    323       BBChanged |= processSwitch(cast<SwitchInst>(Term));
    324       break;
    325     }
    326 
    327     FnChanged |= BBChanged;
    328   }
    329 
    330   return FnChanged;
    331 }
    332