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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/CorrelatedValuePropagation.h"
     15 #include "llvm/Transforms/Scalar.h"
     16 #include "llvm/ADT/Statistic.h"
     17 #include "llvm/Analysis/GlobalsModRef.h"
     18 #include "llvm/Analysis/InstructionSimplify.h"
     19 #include "llvm/Analysis/LazyValueInfo.h"
     20 #include "llvm/IR/CFG.h"
     21 #include "llvm/IR/Constants.h"
     22 #include "llvm/IR/Function.h"
     23 #include "llvm/IR/Instructions.h"
     24 #include "llvm/IR/Module.h"
     25 #include "llvm/Pass.h"
     26 #include "llvm/Support/Debug.h"
     27 #include "llvm/Support/raw_ostream.h"
     28 #include "llvm/Transforms/Utils/Local.h"
     29 using namespace llvm;
     30 
     31 #define DEBUG_TYPE "correlated-value-propagation"
     32 
     33 STATISTIC(NumPhis,      "Number of phis propagated");
     34 STATISTIC(NumSelects,   "Number of selects propagated");
     35 STATISTIC(NumMemAccess, "Number of memory access targets propagated");
     36 STATISTIC(NumCmps,      "Number of comparisons propagated");
     37 STATISTIC(NumReturns,   "Number of return values propagated");
     38 STATISTIC(NumDeadCases, "Number of switch cases removed");
     39 STATISTIC(NumSDivs,     "Number of sdiv converted to udiv");
     40 STATISTIC(NumSRems,     "Number of srem converted to urem");
     41 
     42 namespace {
     43   class CorrelatedValuePropagation : public FunctionPass {
     44   public:
     45     static char ID;
     46     CorrelatedValuePropagation(): FunctionPass(ID) {
     47      initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
     48     }
     49 
     50     bool runOnFunction(Function &F) override;
     51 
     52     void getAnalysisUsage(AnalysisUsage &AU) const override {
     53       AU.addRequired<LazyValueInfoWrapperPass>();
     54       AU.addPreserved<GlobalsAAWrapperPass>();
     55     }
     56   };
     57 }
     58 
     59 char CorrelatedValuePropagation::ID = 0;
     60 INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
     61                 "Value Propagation", false, false)
     62 INITIALIZE_PASS_DEPENDENCY(LazyValueInfoWrapperPass)
     63 INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
     64                 "Value Propagation", false, false)
     65 
     66 // Public interface to the Value Propagation pass
     67 Pass *llvm::createCorrelatedValuePropagationPass() {
     68   return new CorrelatedValuePropagation();
     69 }
     70 
     71 static bool processSelect(SelectInst *S, LazyValueInfo *LVI) {
     72   if (S->getType()->isVectorTy()) return false;
     73   if (isa<Constant>(S->getOperand(0))) return false;
     74 
     75   Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S);
     76   if (!C) return false;
     77 
     78   ConstantInt *CI = dyn_cast<ConstantInt>(C);
     79   if (!CI) return false;
     80 
     81   Value *ReplaceWith = S->getOperand(1);
     82   Value *Other = S->getOperand(2);
     83   if (!CI->isOne()) std::swap(ReplaceWith, Other);
     84   if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
     85 
     86   S->replaceAllUsesWith(ReplaceWith);
     87   S->eraseFromParent();
     88 
     89   ++NumSelects;
     90 
     91   return true;
     92 }
     93 
     94 static bool processPHI(PHINode *P, LazyValueInfo *LVI) {
     95   bool Changed = false;
     96 
     97   BasicBlock *BB = P->getParent();
     98   for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
     99     Value *Incoming = P->getIncomingValue(i);
    100     if (isa<Constant>(Incoming)) continue;
    101 
    102     Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
    103 
    104     // Look if the incoming value is a select with a scalar condition for which
    105     // LVI can tells us the value. In that case replace the incoming value with
    106     // the appropriate value of the select. This often allows us to remove the
    107     // select later.
    108     if (!V) {
    109       SelectInst *SI = dyn_cast<SelectInst>(Incoming);
    110       if (!SI) continue;
    111 
    112       Value *Condition = SI->getCondition();
    113       if (!Condition->getType()->isVectorTy()) {
    114         if (Constant *C = LVI->getConstantOnEdge(
    115                 Condition, P->getIncomingBlock(i), BB, P)) {
    116           if (C->isOneValue()) {
    117             V = SI->getTrueValue();
    118           } else if (C->isZeroValue()) {
    119             V = SI->getFalseValue();
    120           }
    121           // Once LVI learns to handle vector types, we could also add support
    122           // for vector type constants that are not all zeroes or all ones.
    123         }
    124       }
    125 
    126       // Look if the select has a constant but LVI tells us that the incoming
    127       // value can never be that constant. In that case replace the incoming
    128       // value with the other value of the select. This often allows us to
    129       // remove the select later.
    130       if (!V) {
    131         Constant *C = dyn_cast<Constant>(SI->getFalseValue());
    132         if (!C) continue;
    133 
    134         if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
    135               P->getIncomingBlock(i), BB, P) !=
    136             LazyValueInfo::False)
    137           continue;
    138         V = SI->getTrueValue();
    139       }
    140 
    141       DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
    142     }
    143 
    144     P->setIncomingValue(i, V);
    145     Changed = true;
    146   }
    147 
    148   // FIXME: Provide TLI, DT, AT to SimplifyInstruction.
    149   const DataLayout &DL = BB->getModule()->getDataLayout();
    150   if (Value *V = SimplifyInstruction(P, DL)) {
    151     P->replaceAllUsesWith(V);
    152     P->eraseFromParent();
    153     Changed = true;
    154   }
    155 
    156   if (Changed)
    157     ++NumPhis;
    158 
    159   return Changed;
    160 }
    161 
    162 static bool processMemAccess(Instruction *I, LazyValueInfo *LVI) {
    163   Value *Pointer = nullptr;
    164   if (LoadInst *L = dyn_cast<LoadInst>(I))
    165     Pointer = L->getPointerOperand();
    166   else
    167     Pointer = cast<StoreInst>(I)->getPointerOperand();
    168 
    169   if (isa<Constant>(Pointer)) return false;
    170 
    171   Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
    172   if (!C) return false;
    173 
    174   ++NumMemAccess;
    175   I->replaceUsesOfWith(Pointer, C);
    176   return true;
    177 }
    178 
    179 /// See if LazyValueInfo's ability to exploit edge conditions or range
    180 /// information is sufficient to prove this comparison. Even for local
    181 /// conditions, this can sometimes prove conditions instcombine can't by
    182 /// exploiting range information.
    183 static bool processCmp(CmpInst *C, LazyValueInfo *LVI) {
    184   Value *Op0 = C->getOperand(0);
    185   Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
    186   if (!Op1) return false;
    187 
    188   // As a policy choice, we choose not to waste compile time on anything where
    189   // the comparison is testing local values.  While LVI can sometimes reason
    190   // about such cases, it's not its primary purpose.  We do make sure to do
    191   // the block local query for uses from terminator instructions, but that's
    192   // handled in the code for each terminator.
    193   auto *I = dyn_cast<Instruction>(Op0);
    194   if (I && I->getParent() == C->getParent())
    195     return false;
    196 
    197   LazyValueInfo::Tristate Result =
    198     LVI->getPredicateAt(C->getPredicate(), Op0, Op1, C);
    199   if (Result == LazyValueInfo::Unknown) return false;
    200 
    201   ++NumCmps;
    202   if (Result == LazyValueInfo::True)
    203     C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
    204   else
    205     C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
    206   C->eraseFromParent();
    207 
    208   return true;
    209 }
    210 
    211 /// Simplify a switch instruction by removing cases which can never fire. If the
    212 /// uselessness of a case could be determined locally then constant propagation
    213 /// would already have figured it out. Instead, walk the predecessors and
    214 /// statically evaluate cases based on information available on that edge. Cases
    215 /// that cannot fire no matter what the incoming edge can safely be removed. If
    216 /// a case fires on every incoming edge then the entire switch can be removed
    217 /// and replaced with a branch to the case destination.
    218 static bool processSwitch(SwitchInst *SI, LazyValueInfo *LVI) {
    219   Value *Cond = SI->getCondition();
    220   BasicBlock *BB = SI->getParent();
    221 
    222   // If the condition was defined in same block as the switch then LazyValueInfo
    223   // currently won't say anything useful about it, though in theory it could.
    224   if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
    225     return false;
    226 
    227   // If the switch is unreachable then trying to improve it is a waste of time.
    228   pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
    229   if (PB == PE) return false;
    230 
    231   // Analyse each switch case in turn.  This is done in reverse order so that
    232   // removing a case doesn't cause trouble for the iteration.
    233   bool Changed = false;
    234   for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
    235        ) {
    236     ConstantInt *Case = CI.getCaseValue();
    237 
    238     // Check to see if the switch condition is equal to/not equal to the case
    239     // value on every incoming edge, equal/not equal being the same each time.
    240     LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
    241     for (pred_iterator PI = PB; PI != PE; ++PI) {
    242       // Is the switch condition equal to the case value?
    243       LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
    244                                                               Cond, Case, *PI,
    245                                                               BB, SI);
    246       // Give up on this case if nothing is known.
    247       if (Value == LazyValueInfo::Unknown) {
    248         State = LazyValueInfo::Unknown;
    249         break;
    250       }
    251 
    252       // If this was the first edge to be visited, record that all other edges
    253       // need to give the same result.
    254       if (PI == PB) {
    255         State = Value;
    256         continue;
    257       }
    258 
    259       // If this case is known to fire for some edges and known not to fire for
    260       // others then there is nothing we can do - give up.
    261       if (Value != State) {
    262         State = LazyValueInfo::Unknown;
    263         break;
    264       }
    265     }
    266 
    267     if (State == LazyValueInfo::False) {
    268       // This case never fires - remove it.
    269       CI.getCaseSuccessor()->removePredecessor(BB);
    270       SI->removeCase(CI); // Does not invalidate the iterator.
    271 
    272       // The condition can be modified by removePredecessor's PHI simplification
    273       // logic.
    274       Cond = SI->getCondition();
    275 
    276       ++NumDeadCases;
    277       Changed = true;
    278     } else if (State == LazyValueInfo::True) {
    279       // This case always fires.  Arrange for the switch to be turned into an
    280       // unconditional branch by replacing the switch condition with the case
    281       // value.
    282       SI->setCondition(Case);
    283       NumDeadCases += SI->getNumCases();
    284       Changed = true;
    285       break;
    286     }
    287   }
    288 
    289   if (Changed)
    290     // If the switch has been simplified to the point where it can be replaced
    291     // by a branch then do so now.
    292     ConstantFoldTerminator(BB);
    293 
    294   return Changed;
    295 }
    296 
    297 /// Infer nonnull attributes for the arguments at the specified callsite.
    298 static bool processCallSite(CallSite CS, LazyValueInfo *LVI) {
    299   SmallVector<unsigned, 4> Indices;
    300   unsigned ArgNo = 0;
    301 
    302   for (Value *V : CS.args()) {
    303     PointerType *Type = dyn_cast<PointerType>(V->getType());
    304     // Try to mark pointer typed parameters as non-null.  We skip the
    305     // relatively expensive analysis for constants which are obviously either
    306     // null or non-null to start with.
    307     if (Type && !CS.paramHasAttr(ArgNo + 1, Attribute::NonNull) &&
    308         !isa<Constant>(V) &&
    309         LVI->getPredicateAt(ICmpInst::ICMP_EQ, V,
    310                             ConstantPointerNull::get(Type),
    311                             CS.getInstruction()) == LazyValueInfo::False)
    312       Indices.push_back(ArgNo + 1);
    313     ArgNo++;
    314   }
    315 
    316   assert(ArgNo == CS.arg_size() && "sanity check");
    317 
    318   if (Indices.empty())
    319     return false;
    320 
    321   AttributeSet AS = CS.getAttributes();
    322   LLVMContext &Ctx = CS.getInstruction()->getContext();
    323   AS = AS.addAttribute(Ctx, Indices, Attribute::get(Ctx, Attribute::NonNull));
    324   CS.setAttributes(AS);
    325 
    326   return true;
    327 }
    328 
    329 // Helper function to rewrite srem and sdiv. As a policy choice, we choose not
    330 // to waste compile time on anything where the operands are local defs.  While
    331 // LVI can sometimes reason about such cases, it's not its primary purpose.
    332 static bool hasLocalDefs(BinaryOperator *SDI) {
    333   for (Value *O : SDI->operands()) {
    334     auto *I = dyn_cast<Instruction>(O);
    335     if (I && I->getParent() == SDI->getParent())
    336       return true;
    337   }
    338   return false;
    339 }
    340 
    341 static bool hasPositiveOperands(BinaryOperator *SDI, LazyValueInfo *LVI) {
    342   Constant *Zero = ConstantInt::get(SDI->getType(), 0);
    343   for (Value *O : SDI->operands()) {
    344     auto Result = LVI->getPredicateAt(ICmpInst::ICMP_SGE, O, Zero, SDI);
    345     if (Result != LazyValueInfo::True)
    346       return false;
    347   }
    348   return true;
    349 }
    350 
    351 static bool processSRem(BinaryOperator *SDI, LazyValueInfo *LVI) {
    352   if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI) ||
    353       !hasPositiveOperands(SDI, LVI))
    354     return false;
    355 
    356   ++NumSRems;
    357   auto *BO = BinaryOperator::CreateURem(SDI->getOperand(0), SDI->getOperand(1),
    358                                         SDI->getName(), SDI);
    359   SDI->replaceAllUsesWith(BO);
    360   SDI->eraseFromParent();
    361   return true;
    362 }
    363 
    364 /// See if LazyValueInfo's ability to exploit edge conditions or range
    365 /// information is sufficient to prove the both operands of this SDiv are
    366 /// positive.  If this is the case, replace the SDiv with a UDiv. Even for local
    367 /// conditions, this can sometimes prove conditions instcombine can't by
    368 /// exploiting range information.
    369 static bool processSDiv(BinaryOperator *SDI, LazyValueInfo *LVI) {
    370   if (SDI->getType()->isVectorTy() || hasLocalDefs(SDI) ||
    371       !hasPositiveOperands(SDI, LVI))
    372     return false;
    373 
    374   ++NumSDivs;
    375   auto *BO = BinaryOperator::CreateUDiv(SDI->getOperand(0), SDI->getOperand(1),
    376                                         SDI->getName(), SDI);
    377   BO->setIsExact(SDI->isExact());
    378   SDI->replaceAllUsesWith(BO);
    379   SDI->eraseFromParent();
    380 
    381   return true;
    382 }
    383 
    384 static Constant *getConstantAt(Value *V, Instruction *At, LazyValueInfo *LVI) {
    385   if (Constant *C = LVI->getConstant(V, At->getParent(), At))
    386     return C;
    387 
    388   // TODO: The following really should be sunk inside LVI's core algorithm, or
    389   // at least the outer shims around such.
    390   auto *C = dyn_cast<CmpInst>(V);
    391   if (!C) return nullptr;
    392 
    393   Value *Op0 = C->getOperand(0);
    394   Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
    395   if (!Op1) return nullptr;
    396 
    397   LazyValueInfo::Tristate Result =
    398     LVI->getPredicateAt(C->getPredicate(), Op0, Op1, At);
    399   if (Result == LazyValueInfo::Unknown)
    400     return nullptr;
    401 
    402   return (Result == LazyValueInfo::True) ?
    403     ConstantInt::getTrue(C->getContext()) :
    404     ConstantInt::getFalse(C->getContext());
    405 }
    406 
    407 static bool runImpl(Function &F, LazyValueInfo *LVI) {
    408   bool FnChanged = false;
    409 
    410   for (BasicBlock &BB : F) {
    411     bool BBChanged = false;
    412     for (BasicBlock::iterator BI = BB.begin(), BE = BB.end(); BI != BE;) {
    413       Instruction *II = &*BI++;
    414       switch (II->getOpcode()) {
    415       case Instruction::Select:
    416         BBChanged |= processSelect(cast<SelectInst>(II), LVI);
    417         break;
    418       case Instruction::PHI:
    419         BBChanged |= processPHI(cast<PHINode>(II), LVI);
    420         break;
    421       case Instruction::ICmp:
    422       case Instruction::FCmp:
    423         BBChanged |= processCmp(cast<CmpInst>(II), LVI);
    424         break;
    425       case Instruction::Load:
    426       case Instruction::Store:
    427         BBChanged |= processMemAccess(II, LVI);
    428         break;
    429       case Instruction::Call:
    430       case Instruction::Invoke:
    431         BBChanged |= processCallSite(CallSite(II), LVI);
    432         break;
    433       case Instruction::SRem:
    434         BBChanged |= processSRem(cast<BinaryOperator>(II), LVI);
    435         break;
    436       case Instruction::SDiv:
    437         BBChanged |= processSDiv(cast<BinaryOperator>(II), LVI);
    438         break;
    439       }
    440     }
    441 
    442     Instruction *Term = BB.getTerminator();
    443     switch (Term->getOpcode()) {
    444     case Instruction::Switch:
    445       BBChanged |= processSwitch(cast<SwitchInst>(Term), LVI);
    446       break;
    447     case Instruction::Ret: {
    448       auto *RI = cast<ReturnInst>(Term);
    449       // Try to determine the return value if we can.  This is mainly here to
    450       // simplify the writing of unit tests, but also helps to enable IPO by
    451       // constant folding the return values of callees.
    452       auto *RetVal = RI->getReturnValue();
    453       if (!RetVal) break; // handle "ret void"
    454       if (isa<Constant>(RetVal)) break; // nothing to do
    455       if (auto *C = getConstantAt(RetVal, RI, LVI)) {
    456         ++NumReturns;
    457         RI->replaceUsesOfWith(RetVal, C);
    458         BBChanged = true;
    459       }
    460     }
    461     };
    462 
    463     FnChanged |= BBChanged;
    464   }
    465 
    466   return FnChanged;
    467 }
    468 
    469 bool CorrelatedValuePropagation::runOnFunction(Function &F) {
    470   if (skipFunction(F))
    471     return false;
    472 
    473   LazyValueInfo *LVI = &getAnalysis<LazyValueInfoWrapperPass>().getLVI();
    474   return runImpl(F, LVI);
    475 }
    476 
    477 PreservedAnalyses
    478 CorrelatedValuePropagationPass::run(Function &F, FunctionAnalysisManager &AM) {
    479 
    480   LazyValueInfo *LVI = &AM.getResult<LazyValueAnalysis>(F);
    481   bool Changed = runImpl(F, LVI);
    482 
    483   // FIXME: We need to invalidate LVI to avoid PR28400. Is there a better
    484   // solution?
    485   AM.invalidate<LazyValueAnalysis>(F);
    486 
    487   if (!Changed)
    488     return PreservedAnalyses::all();
    489   PreservedAnalyses PA;
    490   PA.preserve<GlobalsAA>();
    491   return PA;
    492 }
    493