GVN.cpp

//===- GVN.cpp - Eliminate redundant values and loads ---------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs global value numbering to eliminate fully redundant
// instructions.  It also performs simple dead load elimination.
//
// Note that this pass does the value numbering itself; it does not use the
// ValueNumbering analysis passes.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "gvn"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Function.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/PHITransAddr.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/IRBuilder.h"
#include <list>
using namespace llvm;

STATISTIC(NumGVNInstr,  "Number of instructions deleted");
STATISTIC(NumGVNLoad,   "Number of loads deleted");
STATISTIC(NumGVNPRE,    "Number of instructions PRE'd");
STATISTIC(NumGVNBlocks, "Number of blocks merged");
STATISTIC(NumPRELoad,   "Number of loads PRE'd");

static cl::opt<bool> EnablePRE("enable-pre",
                               cl::init(true), cl::Hidden);
static cl::opt<bool> EnableLoadPRE("enable-load-pre", cl::init(true));

//===----------------------------------------------------------------------===//
//                         ValueTable Class
//===----------------------------------------------------------------------===//

/// This class holds the mapping between values and value numbers.  It is used
/// as an efficient mechanism to determine the expression-wise equivalence of
/// two values.
namespace {
  struct Expression {
    uint32_t opcode;
    const Type* type;
    SmallVector<uint32_t, 4> varargs;

    Expression() { }
    Expression(uint32_t o) : opcode(o) { }

    bool operator==(const Expression &other) const {
      if (opcode != other.opcode)
        return false;
      else if (opcode == ~0U || opcode == ~1U)
        return true;
      else if (type != other.type)
        return false;
      else if (varargs != other.varargs)
        return false;
      return true;
    }
  };

  class ValueTable {
    private:
      DenseMap<Value*, uint32_t> valueNumbering;
      DenseMap<Expression, uint32_t> expressionNumbering;
      AliasAnalysis* AA;
      MemoryDependenceAnalysis* MD;
      DominatorTree* DT;

      uint32_t nextValueNumber;

      Expression create_expression(Instruction* I);
      uint32_t lookup_or_add_call(CallInst* C);
    public:
      ValueTable() : nextValueNumber(1) { }
      uint32_t lookup_or_add(Value *V);
      uint32_t lookup(Value *V) const;
      void add(Value *V, uint32_t num);
      void clear();
      void erase(Value *v);
      void setAliasAnalysis(AliasAnalysis* A) { AA = A; }
      AliasAnalysis *getAliasAnalysis() const { return AA; }
      void setMemDep(MemoryDependenceAnalysis* M) { MD = M; }
      void setDomTree(DominatorTree* D) { DT = D; }
      uint32_t getNextUnusedValueNumber() { return nextValueNumber; }
      void verifyRemoved(const Value *) const;
  };
}

namespace llvm {
template <> struct DenseMapInfo<Expression> {
  static inline Expression getEmptyKey() {
    return ~0U;
  }

  static inline Expression getTombstoneKey() {
    return ~1U;
  }

  static unsigned getHashValue(const Expression e) {
    unsigned hash = e.opcode;

    hash = ((unsigned)((uintptr_t)e.type >> 4) ^
            (unsigned)((uintptr_t)e.type >> 9));

    for (SmallVector<uint32_t, 4>::const_iterator I = e.varargs.begin(),
         E = e.varargs.end(); I != E; ++I)
      hash = *I + hash * 37;
    
    return hash;
  }
  static bool isEqual(const Expression &LHS, const Expression &RHS) {
    return LHS == RHS;
  }
};

}

//===----------------------------------------------------------------------===//
//                     ValueTable Internal Functions
//===----------------------------------------------------------------------===//


Expression ValueTable::create_expression(Instruction *I) {
  Expression e;
  e.type = I->getType();
  e.opcode = I->getOpcode();
  for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end();
       OI != OE; ++OI)
    e.varargs.push_back(lookup_or_add(*OI));
  
  if (CmpInst *C = dyn_cast<CmpInst>(I))
    e.opcode = (C->getOpcode() << 8) | C->getPredicate();
  else if (ExtractValueInst *E = dyn_cast<ExtractValueInst>(I)) {
    for (ExtractValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
         II != IE; ++II)
      e.varargs.push_back(*II);
  } else if (InsertValueInst *E = dyn_cast<InsertValueInst>(I)) {
    for (InsertValueInst::idx_iterator II = E->idx_begin(), IE = E->idx_end();
         II != IE; ++II)
      e.varargs.push_back(*II);
  }
  
  return e;
}

//===----------------------------------------------------------------------===//
//                     ValueTable External Functions
//===----------------------------------------------------------------------===//

/// add - Insert a value into the table with a specified value number.
void ValueTable::add(Value *V, uint32_t num) {
  valueNumbering.insert(std::make_pair(V, num));
}

uint32_t ValueTable::lookup_or_add_call(CallInst* C) {
  if (AA->doesNotAccessMemory(C)) {
    Expression exp = create_expression(C);
    uint32_t& e = expressionNumbering[exp];
    if (!e) e = nextValueNumber++;
    valueNumbering[C] = e;
    return e;
  } else if (AA->onlyReadsMemory(C)) {
    Expression exp = create_expression(C);
    uint32_t& e = expressionNumbering[exp];
    if (!e) {
      e = nextValueNumber++;