//===- CSE.cpp - Simple and fast CSE pass ---------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2015 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See http://swift.org/LICENSE.txt for license information
// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
//
// This pass performs a simple dominator tree walk that eliminates trivially
// redundant instructions.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sil-cse"
#include "swift/SILPasses/Passes.h"
#include "swift/SIL/Dominance.h"
#include "swift/SIL/SILModule.h"
#include "swift/SIL/SILType.h"
#include "swift/SIL/SILValue.h"
#include "swift/SIL/SILVisitor.h"
#include "swift/SILPasses/Utils/Local.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/RecyclingAllocator.h"

STATISTIC(NumSimplify, "Number of instructions simplified or DCE'd");
STATISTIC(NumCSE,      "Number of instructions CSE'd");

using namespace swift;

//===----------------------------------------------------------------------===//
//                                Simple Value
//===----------------------------------------------------------------------===//

namespace {
  /// SimpleValue - Instances of this struct represent available values in the
  /// scoped hash table.
  struct SimpleValue {
    SILInstruction *Inst;

    SimpleValue(SILInstruction *I) : Inst(I) {
      assert((isSentinel() || canHandle(I)) && "Inst can't be handled!");
    }

    bool isSentinel() const {
      return Inst == llvm::DenseMapInfo<SILInstruction*>::getEmptyKey() ||
             Inst == llvm::DenseMapInfo<SILInstruction*>::getTombstoneKey();
    }

    static bool canHandle(SILInstruction *Inst) {
      switch(Inst->getKind()) {
      case ValueKind::FunctionRefInst:
      case ValueKind::BuiltinFunctionRefInst:
      case ValueKind::GlobalAddrInst:
      case ValueKind::IntegerLiteralInst:
      case ValueKind::FloatLiteralInst:
      case ValueKind::StringLiteralInst:
      case ValueKind::StructInst:
      case ValueKind::StructExtractInst:
      case ValueKind::StructElementAddrInst:
      case ValueKind::TupleInst:
      case ValueKind::TupleExtractInst:
      case ValueKind::TupleElementAddrInst:
      case ValueKind::MetatypeInst:
        return true;
      default:
        return false;
      }
    }
  };
} // end anonymous namespace

namespace llvm {
template<> struct DenseMapInfo<SimpleValue> {
  static inline SimpleValue getEmptyKey() {
    return DenseMapInfo<SILInstruction*>::getEmptyKey();
  }
  static inline SimpleValue getTombstoneKey() {
    return DenseMapInfo<SILInstruction*>::getTombstoneKey();
  }
  static unsigned getHashValue(SimpleValue Val);
  static bool isEqual(SimpleValue LHS, SimpleValue RHS);
};
} // end namespace llvm

namespace {
  class HashVisitor :
    public SILInstructionVisitor<HashVisitor, llvm::hash_code> {
    using hash_code = llvm::hash_code;
  public:
    hash_code visitValueBase(ValueBase *) {
      llvm_unreachable("No hash implemented for the given type");
    }

    hash_code visitFunctionRefInst(FunctionRefInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::FunctionRefInst),
                                X->getReferencedFunction());
    }

    hash_code visitBuiltinFunctionRefInst(BuiltinFunctionRefInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::BuiltinFunctionRefInst),
                                X->getName().get());
    }

    hash_code visitGlobalAddrInst(GlobalAddrInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::GlobalAddrInst),
                                X->getGlobal());
    }

    hash_code visitIntegerLiteralInst(IntegerLiteralInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::IntegerLiteralInst),
                                X->getType(),
                                X->getValue());
    }

    hash_code visitFloatLiteralInst(FloatLiteralInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::FloatLiteralInst),
                                X->getType(),
                                X->getBits());
    }

    hash_code visitStringLiteralInst(StringLiteralInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::StringLiteralInst),
                                X->getValue());
    }

    hash_code visitStructInst(StructInst *X) {
      // This is safe since we are hashing the operands using the actual pointer
      // values of the values being used by the operand.
      OperandValueArrayRef Operands(X->getAllOperands());
      return llvm::hash_combine(unsigned(ValueKind::StructInst),
                                X->getStructDecl(),
                                llvm::hash_combine_range(Operands.begin(),
                                                         Operands.end()));
    }

    hash_code visitStructExtractInst(StructExtractInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::StructExtractInst),
                                X->getStructDecl(),
                                X->getField(),
                                X->getOperand());
    }

    hash_code visitStructElementAddrInst(StructElementAddrInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::StructElementAddrInst),
                                X->getStructDecl(),
                                X->getField(),
                                X->getOperand());
    }

    hash_code visitTupleInst(TupleInst *X) {
      OperandValueArrayRef Operands(X->getAllOperands());
      return llvm::hash_combine(unsigned(ValueKind::TupleInst),
                                X->getTupleType(),
                                llvm::hash_combine_range(Operands.begin(),
                                                         Operands.end()));
    }

    hash_code visitTupleExtractInst(TupleExtractInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::TupleExtractInst),
                                X->getTupleType(),
                                X->getFieldNo(),
                                X->getOperand());
    }

    hash_code visitTupleElementAddrInst(TupleElementAddrInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::TupleElementAddrInst),
                                X->getTupleType(),
                                X->getFieldNo(),
                                X->getOperand());
    }

    hash_code visitMetatypeInst(MetatypeInst *X) {
      return llvm::hash_combine(unsigned(ValueKind::MetatypeInst),
                                X->getType());
    }

  };
} // end anonymous namespace

unsigned llvm::DenseMapInfo<SimpleValue>::getHashValue(SimpleValue Val) {
  return HashVisitor().visit(Val.Inst);
}

bool llvm::DenseMapInfo<SimpleValue>::isEqual(SimpleValue LHS,
                                              SimpleValue RHS) {
  SILInstruction *LHSI = LHS.Inst, *RHSI = RHS.Inst;
  if (LHS.isSentinel() || RHS.isSentinel())
    return LHSI == RHSI;

  return LHSI->getKind() == RHSI->getKind() && LHSI->isIdenticalTo(RHSI);
}

//===----------------------------------------------------------------------===//
//                               CSE Interface
//===----------------------------------------------------------------------===//

namespace {

/// CSE - This pass does a simple depth-first walk over the dominator tree,
/// eliminating trivially redundant instructions and using simplifyInstruction
/// to canonicalize things as it goes. It is intended to be fast and catch
/// obvious cases so that SILCombine and other passes are more effective.
class CSE {
public:
  SILModule &Module;

  typedef llvm::ScopedHashTableVal<SimpleValue, ValueBase*> SimpleValueHTType;
  typedef llvm::RecyclingAllocator<llvm::BumpPtrAllocator,
                                   SimpleValueHTType> AllocatorTy;
  typedef llvm::ScopedHashTable<SimpleValue, ValueBase*,
                                llvm::DenseMapInfo<SimpleValue>,
                                AllocatorTy> ScopedHTType;

  /// AvailableValues - This scoped hash table contains the current values of
  /// all of our simple scalar expressions.  As we walk down the domtree, we
  /// look to see if instructions are in this: if so, we replace them with what
  /// we find, otherwise we insert them so that dominated values can succeed in
  /// their lookup.
  ScopedHTType *AvailableValues;

  explicit CSE(SILModule &M) : Module(M) { }

  bool runOnFunction(SILFunction &F);

private:

  // NodeScope - almost a POD, but needs to call the constructors for the
  // scoped hash tables so that a new scope gets pushed on. These are RAII so
  // that the scope gets popped when the NodeScope is destroyed.
  class NodeScope {
   public:
    NodeScope(ScopedHTType *availableValues)
      : Scope(*availableValues) {
    }

   private:
    NodeScope(const NodeScope&) = delete;
    void operator=(const NodeScope&) = delete;

    ScopedHTType::ScopeTy Scope;
  };

  // StackNode - contains all the needed information to create a stack for doing
  // a depth first traversal of the tree. This includes scopes for values and
  // loads as well as the generation. There is a child iterator so that the
  // children do not need to be store spearately.
  class StackNode {
   public:
    StackNode(ScopedHTType *availableValues,
              DominanceInfoNode *n,
              DominanceInfoNode::iterator child,
              DominanceInfoNode::iterator end) :
      Node(n), ChildIter(child), EndIter(end),
      Scopes(availableValues),
      Processed(false) {}

    // Accessors.
    DominanceInfoNode *node() { return Node; }
    DominanceInfoNode::iterator childIter() { return ChildIter; }
    DominanceInfoNode *nextChild() {
      DominanceInfoNode *child = *ChildIter;
      ++ChildIter;
      return child;
    }
    DominanceInfoNode::iterator end() { return EndIter; }
    bool isProcessed() { return Processed; }
    void process() { Processed = true; }

   private:
    StackNode(const StackNode&) = delete;
    void operator=(const StackNode&) = delete;

    // Members.
    DominanceInfoNode *Node;
    DominanceInfoNode::iterator ChildIter;
    DominanceInfoNode::iterator EndIter;
    NodeScope Scopes;
    bool Processed;
  };

  bool processNode(DominanceInfoNode *Node);
};
}  // end anonymous namespace

//===----------------------------------------------------------------------===//
//                             CSE Implementation
//===----------------------------------------------------------------------===//

bool CSE::runOnFunction(SILFunction &F) {
  std::vector<StackNode *> nodesToProcess;

  DominanceInfo DT(&F);

  // Tables that the pass uses when walking the domtree.
  ScopedHTType AVTable;
  AvailableValues = &AVTable;

  bool Changed = false;

  // Process the root node.
  nodesToProcess.push_back(
    new StackNode(AvailableValues, DT.getRootNode(),
                  DT.getRootNode()->begin(),
                  DT.getRootNode()->end()));

  // Process the stack.
  while (!nodesToProcess.empty()) {
    // Grab the first item off the stack. Set the current generation, remove
    // the node from the stack, and process it.
    StackNode *NodeToProcess = nodesToProcess.back();

    // Check if the node needs to be processed.
    if (!NodeToProcess->isProcessed()) {
      // Process the node.
      Changed |= processNode(NodeToProcess->node());
      NodeToProcess->process();

    } else if (NodeToProcess->childIter() != NodeToProcess->end()) {
      // Push the next child onto the stack.
      DominanceInfoNode *child = NodeToProcess->nextChild();
      nodesToProcess.push_back(
          new StackNode(AvailableValues, child, child->begin(), child->end()));
    } else {
      // It has been processed, and there are no more children to process,
      // so delete it and pop it off the stack.
      delete NodeToProcess;
      nodesToProcess.pop_back();
    }
  } // while (!nodes...)

  return Changed;
}

bool CSE::processNode(DominanceInfoNode *Node) {
  SILBasicBlock *BB = Node->getBlock();
  bool Changed = false;

  // See if any instructions in the block can be eliminated.  If so, do it.  If
  // not, add them to AvailableValues.
  for (SILBasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
    SILInstruction *Inst = I++;

    DEBUG(llvm::dbgs() << "SILCSE VISITING: " << *Inst << "\n");

    // Dead instructions should just be removed.
    if (isInstructionTriviallyDead(Inst)) {
      DEBUG(llvm::dbgs() << "SILCSE DCE: " << *Inst << '\n');
      Inst->eraseFromParent();
      Changed = true;
      ++NumSimplify;
      continue;
    }

    // If the instruction can be simplified (e.g. X+0 = X) then replace it with
    // its simpler value.
    if (SILValue V = simplifyInstruction(Inst)) {
      DEBUG(llvm::dbgs() << "SILCSE SIMPLIFY: " << *Inst << "  to: " << *V
            << '\n');
      SILValue(Inst, 0).replaceAllUsesWith(V);
      Inst->eraseFromParent();
      Changed = true;
      ++NumSimplify;
      continue;
    }

    // If this is not a simple instruction that we can value number, skip it.
    if (!SimpleValue::canHandle(Inst))
      continue;

    // Now that we know we have an instruction we understand see if the
    // instruction has an available value.  If so, use it.
    if (ValueBase *V = AvailableValues->lookup(Inst)) {
      DEBUG(llvm::dbgs() << "SILCSE CSE: " << *Inst << "  to: " << *V
            << '\n');
      Inst->replaceAllUsesWith(V);
      Inst->eraseFromParent();
      Changed = true;
      ++NumCSE;
      continue;
    }

    // Otherwise, just remember that this value is available.
    AvailableValues->insert(Inst, Inst);
    DEBUG(llvm::dbgs() << "SILCSE Adding to value table: " << *Inst
          << " -> " << *Inst << "\n");
  }

  return Changed;
}

//===----------------------------------------------------------------------===//
//                              Top Level Driver
//===----------------------------------------------------------------------===//

void swift::performSILCSE(SILModule *M) {
  CSE C(*M);
  for (SILFunction &F : *M) {
    // If F is just a declaration and not a definition, skip it since it has no
    // BB's to process.
    if (F.empty())
      continue;

    C.runOnFunction(F);
  }
}