llvm/lib/Analysis/MemoryDependenceAnalysis.cpp

//===- MemoryDependenceAnalysis.cpp - Mem Deps Implementation  --*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements an analysis that determines, for a given memory
// operation, what preceding memory operations it depends on.  It builds on 
// alias analysis information, and tries to provide a lazy, caching interface to
// a common kind of alias information query.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "memdep"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Function.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Target/TargetData.h"
using namespace llvm;

// Control the calculation of non-local dependencies by only examining the
// predecessors if the basic block has less than X amount (50 by default).
static cl::opt<int> 
PredLimit("nonlocaldep-threshold", cl::Hidden, cl::init(50),
          cl::desc("Control the calculation of non-local"
                   "dependencies (default = 50)"));           

STATISTIC(NumCacheNonlocal, "Number of cached non-local responses");
STATISTIC(NumUncacheNonlocal, "Number of uncached non-local responses");

char MemoryDependenceAnalysis::ID = 0;
  
// Register this pass...
static RegisterPass<MemoryDependenceAnalysis> X("memdep",
                                     "Memory Dependence Analysis", false, true);

/// verifyRemoved - Verify that the specified instruction does not occur
/// in our internal data structures.
void MemoryDependenceAnalysis::verifyRemoved(Instruction *D) const {
  for (LocalDepMapType::const_iterator I = LocalDeps.begin(),
       E = LocalDeps.end(); I != E; ++I) {
    assert(I->first != D && "Inst occurs in data structures");
    assert(I->second.getPointer() != D &&
           "Inst occurs in data structures");
  }

  for (NonLocalDepMapType::const_iterator I = NonLocalDeps.begin(),
       E = NonLocalDeps.end(); I != E; ++I) {
    assert(I->first != D && "Inst occurs in data structures");
    for (DenseMap<BasicBlock*, DepResultTy>::iterator II = I->second.begin(),
         EE = I->second.end(); II  != EE; ++II)
      assert(II->second.getPointer() != D && "Inst occurs in data structures");
  }

  for (ReverseDepMapType::const_iterator I = ReverseLocalDeps.begin(),
       E = ReverseLocalDeps.end(); I != E; ++I)
    for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(),
         EE = I->second.end(); II != EE; ++II)
      assert(*II != D && "Inst occurs in data structures");

  for (ReverseDepMapType::const_iterator I = ReverseNonLocalDeps.begin(),
       E = ReverseNonLocalDeps.end();
       I != E; ++I)
    for (SmallPtrSet<Instruction*, 4>::const_iterator II = I->second.begin(),
         EE = I->second.end(); II != EE; ++II)
      assert(*II != D && "Inst occurs in data structures");
}

/// getAnalysisUsage - Does not modify anything.  It uses Alias Analysis.
///
void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
  AU.setPreservesAll();
  AU.addRequiredTransitive<AliasAnalysis>();
  AU.addRequiredTransitive<TargetData>();
}

/// getCallSiteDependency - Private helper for finding the local dependencies
/// of a call site.
MemDepResult MemoryDependenceAnalysis::
getCallSiteDependency(CallSite C, BasicBlock::iterator ScanIt,
                      BasicBlock *BB) {
  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  TargetData &TD = getAnalysis<TargetData>();
  
  // Walk backwards through the block, looking for dependencies
  while (ScanIt != BB->begin()) {
    Instruction *Inst = --ScanIt;
    
    // If this inst is a memory op, get the pointer it accessed
    Value *Pointer = 0;
    uint64_t PointerSize = 0;
    if (StoreInst *S = dyn_cast<StoreInst>(Inst)) {
      Pointer = S->getPointerOperand();
      PointerSize = TD.getTypeStoreSize(S->getOperand(0)->getType());
    } else if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) {
      Pointer = AI;
      if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize()))
        PointerSize = C->getZExtValue() *
                      TD.getTypeStoreSize(AI->getAllocatedType());
      else
        PointerSize = ~0UL;
    } else if (VAArgInst *V = dyn_cast<VAArgInst>(Inst)) {
      Pointer = V->getOperand(0);
      PointerSize = TD.getTypeStoreSize(V->getType());
    } else if (FreeInst *F = dyn_cast<FreeInst>(Inst)) {
      Pointer = F->getPointerOperand();
      
      // FreeInsts erase the entire structure
      PointerSize = ~0UL;
    } else if (isa<CallInst>(Inst) || isa<InvokeInst>(Inst)) {
      if (AA.getModRefBehavior(CallSite::get(Inst)) ==
            AliasAnalysis::DoesNotAccessMemory)
        continue;
      return MemDepResult::get(Inst);
    } else
      continue;
    
    if (AA.getModRefInfo(C, Pointer, PointerSize) != AliasAnalysis::NoModRef)
      return MemDepResult::get(Inst);
  }
  
  // No dependence found.
  return MemDepResult::getNonLocal();
}

/// nonLocalHelper - Private helper used to calculate non-local dependencies
/// by doing DFS on the predecessors of a block to find its dependencies.
void MemoryDependenceAnalysis::nonLocalHelper(Instruction* query,
                                              BasicBlock* block,
                                     DenseMap<BasicBlock*, DepResultTy> &resp) {
  // Set of blocks that we've already visited in our DFS
  SmallPtrSet<BasicBlock*, 4> visited;
  // If we're updating a dirtied cache entry, we don't need to reprocess
  // already computed entries.
  for (DenseMap<BasicBlock*, DepResultTy>::iterator I = resp.begin(), 
       E = resp.end(); I != E; ++I)
    if (I->second.getInt() != Dirty)
      visited.insert(I->first);
  
  // Current stack of the DFS
  SmallVector<BasicBlock*, 4> stack;
  for (pred_iterator PI = pred_begin(block), PE = pred_end(block);
       PI != PE; ++PI)
    stack.push_back(*PI);
  
  // Do a basic DFS
  while (!stack.empty()) {
    BasicBlock* BB = stack.back();
    
    // If we've already visited this block, no need to revist
    if (visited.count(BB)) {
      stack.pop_back();
      continue;
    }
    
    // If we find a new block with a local dependency for query,
    // then we insert the new dependency and backtrack.
    if (BB != block) {
      visited.insert(BB);
      
      MemDepResult localDep = getDependencyFrom(query, BB->end(), BB);
      if (!localDep.isNonLocal()) {
        resp.insert(std::make_pair(BB, ConvFromResult(localDep)));
        stack.pop_back();
        continue;
      }
    // If we re-encounter the starting block, we still need to search it
    // because there might be a dependency in the starting block AFTER
    // the position of the query.  This is necessary to get loops right.
    } else if (BB == block) {
      visited.insert(BB);
      
      MemDepResult localDep = getDependencyFrom(query, BB->end(), BB);
      if (localDep.getInst() != query)
        resp.insert(std::make_pair(BB, ConvFromResult(localDep)));
      
      stack.pop_back();
      continue;
    }
    
    // If we didn't find anything, recurse on the precessors of this block
    // Only do this for blocks with a small number of predecessors.
    bool predOnStack = false;
    bool inserted = false;
    if (std::distance(pred_begin(BB), pred_end(BB)) <= PredLimit) { 
      for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
           PI != PE; ++PI)
        if (!visited.count(*PI)) {
          stack.push_back(*PI);
          inserted = true;
        } else
          predOnStack = true;
    }
    
    // If we inserted a new predecessor, then we'll come back to this block
    if (inserted)
      continue;
    // If we didn't insert because we have no predecessors, then this
    // query has no dependency at all.
    else if (!inserted && !predOnStack) {
      resp.insert(std::make_pair(BB, DepResultTy(0, None)));
    // If we didn't insert because our predecessors are already on the stack,
    // then we might still have a dependency, but it will be discovered during
    // backtracking.
    } else if (!inserted && predOnStack){
      resp.insert(std::make_pair(BB, DepResultTy(0, NonLocal)));
    }
    
    stack.pop_back();
  }
}

/// getNonLocalDependency - Fills the passed-in map with the non-local 
/// dependencies of the queries.  The map will contain NonLocal for
/// blocks between the query and its dependencies.
void MemoryDependenceAnalysis::getNonLocalDependency(Instruction* query,
                                    DenseMap<BasicBlock*, MemDepResult> &resp) {
  if (NonLocalDeps.count(query)) {
    DenseMap<BasicBlock*, DepResultTy> &cached = NonLocalDeps[query];
    NumCacheNonlocal++;
    
    SmallVector<BasicBlock*, 4> dirtied;
    for (DenseMap<BasicBlock*, DepResultTy>::iterator I = cached.begin(),
         E = cached.end(); I != E; ++I)
      if (I->second.getInt() == Dirty)
        dirtied.push_back(I->first);
    
    for (SmallVector<BasicBlock*, 4>::iterator I = dirtied.begin(),
         E = dirtied.end(); I != E; ++I) {
      MemDepResult localDep = getDependencyFrom(query, (*I)->end(), *I);
      if (!localDep.isNonLocal())
        cached[*I] = ConvFromResult(localDep);
      else {
        cached.erase(*I);
        nonLocalHelper(query, *I, cached);
      }
    }
    
    // Update the reverse non-local dependency cache.
    for (DenseMap<BasicBlock*, DepResultTy>::iterator I = cached.begin(),
         E = cached.end(); I != E; ++I) {
      if (Instruction *Inst = I->second.getPointer())
        ReverseNonLocalDeps[Inst].insert(query);
      resp[I->first] = ConvToResult(I->second);
    }
    
    return;
  }
  
  NumUncacheNonlocal++;
  
  // If not, go ahead and search for non-local deps.
  DenseMap<BasicBlock*, DepResultTy> &cached = NonLocalDeps[query];
  nonLocalHelper(query, query->getParent(), cached);

  // Update the non-local dependency cache
  for (DenseMap<BasicBlock*, DepResultTy>::iterator I = cached.begin(),
       E = cached.end(); I != E; ++I) {
    // FIXME: Merge with the code above!
    if (Instruction *Inst = I->second.getPointer())
      ReverseNonLocalDeps[Inst].insert(query);
    resp[I->first] = ConvToResult(I->second);
  }
}

/// getDependency - Return the instruction on which a memory operation
/// depends.  The local parameter indicates if the query should only
/// evaluate dependencies within the same basic block.
MemDepResult MemoryDependenceAnalysis::
getDependencyFrom(Instruction *QueryInst, BasicBlock::iterator ScanIt, 
                  BasicBlock *BB) {
  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  TargetData &TD = getAnalysis<TargetData>();
  
  // Get the pointer value for which dependence will be determined
  Value *MemPtr = 0;
  uint64_t MemSize = 0;
  bool MemVolatile = false;
  
  if (StoreInst* S = dyn_cast<StoreInst>(QueryInst)) {
    MemPtr = S->getPointerOperand();
    MemSize = TD.getTypeStoreSize(S->getOperand(0)->getType());
    MemVolatile = S->isVolatile();
  } else if (LoadInst* L = dyn_cast<LoadInst>(QueryInst)) {
    MemPtr = L->getPointerOperand();
    MemSize = TD.getTypeStoreSize(L->getType());
    MemVolatile = L->isVolatile();
  } else if (VAArgInst* V = dyn_cast<VAArgInst>(QueryInst)) {
    MemPtr = V->getOperand(0);
    MemSize = TD.getTypeStoreSize(V->getType());
  } else if (FreeInst* F = dyn_cast<FreeInst>(QueryInst)) {
    MemPtr = F->getPointerOperand();
    // FreeInsts erase the entire structure, not just a field.
    MemSize = ~0UL;
  } else if (isa<CallInst>(QueryInst) || isa<InvokeInst>(QueryInst))
    return getCallSiteDependency(CallSite::get(QueryInst), ScanIt, BB);
  else  // Non-memory instructions depend on nothing.
    return MemDepResult::getNone();
  
  // Walk backwards through the basic block, looking for dependencies
  while (ScanIt != BB->begin()) {
    Instruction *Inst = --ScanIt;

    // If the access is volatile and this is a volatile load/store, return a
    // dependence.
    if (MemVolatile &&
        ((isa<LoadInst>(Inst) && cast<LoadInst>(Inst)->isVolatile()) ||
         (isa<StoreInst>(Inst) && cast<StoreInst>(Inst)->isVolatile())))
      return MemDepResult::get(Inst);

    // MemDep is broken w.r.t. loads: it says that two loads of the same pointer
    // depend on each other.  :(
    // FIXME: ELIMINATE THIS!
    if (LoadInst *L = dyn_cast<LoadInst>(Inst)) {
      Value *Pointer = L->getPointerOperand();
      uint64_t PointerSize = TD.getTypeStoreSize(L->getType());
      
      // If we found a pointer, check if it could be the same as our pointer
      AliasAnalysis::AliasResult R =
        AA.alias(Pointer, PointerSize, MemPtr, MemSize);
      
      if (R == AliasAnalysis::NoAlias)
        continue;
      
      // May-alias loads don't depend on each other without a dependence.
      if (isa<LoadInst>(QueryInst) && R == AliasAnalysis::MayAlias)
        continue;
      return MemDepResult::get(Inst);
    }
    
    // FIXME: This claims that an access depends on the allocation.  This may
    // make sense, but is dubious at best.  It would be better to fix GVN to
    // handle a 'None' Query.
    if (AllocationInst *AI = dyn_cast<AllocationInst>(Inst)) {
      Value *Pointer = AI;
      uint64_t PointerSize;
      if (ConstantInt *C = dyn_cast<ConstantInt>(AI->getArraySize()))
        PointerSize = C->getZExtValue() * 
          TD.getTypeStoreSize(AI->getAllocatedType());
      else
        PointerSize = ~0UL;
      
      AliasAnalysis::AliasResult R =
        AA.alias(Pointer, PointerSize, MemPtr, MemSize);
      
      if (R == AliasAnalysis::NoAlias)
        continue;
      return MemDepResult::get(Inst);
    }
      
    
    // See if this instruction mod/ref's the pointer.
    AliasAnalysis::ModRefResult MRR = AA.getModRefInfo(Inst, MemPtr, MemSize);

    if (MRR == AliasAnalysis::NoModRef)
      continue;
    
    // Loads don't depend on read-only instructions.
    if (isa<LoadInst>(QueryInst) && MRR == AliasAnalysis::Ref)
      continue;
    
    // Otherwise, there is a dependence.
    return MemDepResult::get(Inst);
  }
  
  // If we found nothing, return the non-local flag.
  return MemDepResult::getNonLocal();
}

/// getDependency - Return the instruction on which a memory operation
/// depends.
MemDepResult MemoryDependenceAnalysis::getDependency(Instruction *QueryInst) {
  Instruction *ScanPos = QueryInst;
  
  // Check for a cached result
  DepResultTy &LocalCache = LocalDeps[QueryInst];
  
  // If the cached entry is non-dirty, just return it.
  if (LocalCache.getInt() != Dirty)
    return ConvToResult(LocalCache);
    
  // Otherwise, if we have a dirty entry, we know we can start the scan at that
  // instruction, which may save us some work.
  if (Instruction *Inst = LocalCache.getPointer())
    ScanPos = Inst;
  
  // Do the scan.
  MemDepResult Res = 
    getDependencyFrom(QueryInst, ScanPos, QueryInst->getParent());  
  
  // Remember the result!
  // FIXME: Don't convert back and forth!  Make a shared helper function.
  LocalCache = ConvFromResult(Res);
  if (Instruction *I = Res.getInst())
    ReverseLocalDeps[I].insert(QueryInst);
  
  return Res;
}


/// dropInstruction - Remove an instruction from the analysis, making 
/// absolutely conservative assumptions when updating the cache.  This is
/// useful, for example when an instruction is changed rather than removed.
void MemoryDependenceAnalysis::dropInstruction(Instruction* drop) {
  LocalDepMapType::iterator depGraphEntry = LocalDeps.find(drop);
  if (depGraphEntry != LocalDeps.end())
    if (Instruction *Inst = depGraphEntry->second.getPointer())
      ReverseLocalDeps[Inst].erase(drop);
  
  // Drop dependency information for things that depended on this instr
  SmallPtrSet<Instruction*, 4>& set = ReverseLocalDeps[drop];
  for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end();
       I != E; ++I)
    LocalDeps.erase(*I);
  
  LocalDeps.erase(drop);
  ReverseLocalDeps.erase(drop);
  
  for (DenseMap<BasicBlock*, DepResultTy>::iterator DI =
         NonLocalDeps[drop].begin(), DE = NonLocalDeps[drop].end();
       DI != DE; ++DI)
    if (Instruction *Inst = DI->second.getPointer())
      ReverseNonLocalDeps[Inst].erase(drop);
  
  if (ReverseNonLocalDeps.count(drop)) {
    SmallPtrSet<Instruction*, 4>& set =
      ReverseNonLocalDeps[drop];
    for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end();
         I != E; ++I)
      for (DenseMap<BasicBlock*, DepResultTy>::iterator DI =
           NonLocalDeps[*I].begin(), DE = NonLocalDeps[*I].end();
           DI != DE; ++DI)
        if (DI->second == DepResultTy(drop, Normal))
          // FIXME: Why not remember the old insertion point??
          DI->second = DepResultTy(0, Dirty);
  }
  
  ReverseNonLocalDeps.erase(drop);
  NonLocalDeps.erase(drop);
}

/// removeInstruction - Remove an instruction from the dependence analysis,
/// updating the dependence of instructions that previously depended on it.
/// This method attempts to keep the cache coherent using the reverse map.
void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) {
  // Walk through the Non-local dependencies, removing this one as the value
  // for any cached queries.
  for (DenseMap<BasicBlock*, DepResultTy>::iterator DI =
       NonLocalDeps[RemInst].begin(), DE = NonLocalDeps[RemInst].end();
       DI != DE; ++DI)
    if (Instruction *Inst = DI->second.getPointer())
      ReverseNonLocalDeps[Inst].erase(RemInst);

  // Shortly after this, we will look for things that depend on RemInst.  In
  // order to update these, we'll need a new dependency to base them on.  We
  // could completely delete any entries that depend on this, but it is better
  // to make a more accurate approximation where possible.  Compute that better
  // approximation if we can.
  DepResultTy NewDependency;
  
  // If we have a cached local dependence query for this instruction, remove it.
  //
  LocalDepMapType::iterator LocalDepEntry = LocalDeps.find(RemInst);
  if (LocalDepEntry != LocalDeps.end()) {
    DepResultTy LocalDep = LocalDepEntry->second;
    
    // Remove this local dependency info.
    LocalDeps.erase(LocalDepEntry);
    
    // Remove us from DepInst's reverse set now that the local dep info is gone.
    if (Instruction *Inst = LocalDep.getPointer())
      ReverseLocalDeps[Inst].erase(RemInst);

    // If we have unconfirmed info, don't trust it.
    if (LocalDep.getInt() != Dirty) {
      // If we have a confirmed non-local flag, use it.
      if (LocalDep.getInt() == NonLocal || LocalDep.getInt() == None) {
        // The only time this dependency is confirmed is if it is non-local.
        NewDependency = LocalDep;
      } else {
        // If we have dep info for RemInst, set them to it.
        Instruction *NDI = next(BasicBlock::iterator(LocalDep.getPointer()));
        if (NDI != RemInst) // Don't use RemInst for the new dependency!
          NewDependency = DepResultTy(NDI, Dirty);
      }
    }
  }
  
  // If we don't already have a local dependency answer for this instruction,
  // use the immediate successor of RemInst.  We use the successor because
  // getDependence starts by checking the immediate predecessor of what is in
  // the cache.
  if (NewDependency == DepResultTy(0, Dirty))
    NewDependency = DepResultTy(next(BasicBlock::iterator(RemInst)), Dirty);
  
  // Loop over all of the things that depend on the instruction we're removing.
  // 
  ReverseDepMapType::iterator ReverseDepIt = ReverseLocalDeps.find(RemInst);
  if (ReverseDepIt != ReverseLocalDeps.end()) {
    SmallPtrSet<Instruction*, 4> &ReverseDeps = ReverseDepIt->second;
    for (SmallPtrSet<Instruction*, 4>::iterator I = ReverseDeps.begin(),
         E = ReverseDeps.end(); I != E; ++I) {
      Instruction *InstDependingOnRemInst = *I;
      
      // If we thought the instruction depended on itself (possible for
      // unconfirmed dependencies) ignore the update.
      if (InstDependingOnRemInst == RemInst) continue;
      
      // Insert the new dependencies.
      LocalDeps[InstDependingOnRemInst] = NewDependency;
      
      // If our NewDependency is an instruction, make sure to remember that new
      // things depend on it.
      if (Instruction *Inst = NewDependency.getPointer())
        ReverseLocalDeps[Inst].insert(InstDependingOnRemInst);
    }
    ReverseLocalDeps.erase(RemInst);
  }
  
  ReverseDepIt = ReverseNonLocalDeps.find(RemInst);
  if (ReverseDepIt != ReverseNonLocalDeps.end()) {
    SmallPtrSet<Instruction*, 4>& set = ReverseDepIt->second;
    for (SmallPtrSet<Instruction*, 4>::iterator I = set.begin(), E = set.end();
         I != E; ++I)
      for (DenseMap<BasicBlock*, DepResultTy>::iterator
           DI = NonLocalDeps[*I].begin(), DE = NonLocalDeps[*I].end();
           DI != DE; ++DI)
        if (DI->second == DepResultTy(RemInst, Normal))
          // FIXME: Why not remember the old insertion point??
          DI->second = DepResultTy(0, Dirty);
    ReverseNonLocalDeps.erase(ReverseDepIt);
  }
  
  NonLocalDeps.erase(RemInst);

  getAnalysis<AliasAnalysis>().deleteValue(RemInst);
  
  DEBUG(verifyRemoved(RemInst));
}