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/GlobalVariable.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.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/SSAUpdater.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/IRBuilder.h"
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(uint32_t o = ~2U) : opcode(o) { }

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

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

    uint32_t nextValueNumber;

    Expression create_expression(Instruction* I);
    Expression create_extractvalue_expression(ExtractValueInst* EI);
    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 (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;
}

Expression ValueTable::create_extractvalue_expression(ExtractValueInst *EI) {
  assert(EI != 0 && "Not an ExtractValueInst?");
  Expression e;
  e.type = EI->getType();
  e.opcode = 0;

  IntrinsicInst *I = dyn_cast<IntrinsicInst>(EI->getAggregateOperand());
  if (I != 0 && EI->getNumIndices() == 1 && *EI->idx_begin() == 0 ) {
    // EI might be an extract from one of our recognised intrinsics. If it
    // is we'll synthesize a semantically equivalent expression instead on
    // an extract value expression.
    switch (I->getIntrinsicID()) {
      case Intrinsic::uadd_with_overflow:
        e.opcode = Instruction::Add;
        break;
      case Intrinsic::usub_with_overflow:
        e.opcode = Instruction::Sub;
        break;
      case Intrinsic::umul_with_overflow:
        e.opcode = Instruction::Mul;
        break;
      default:
        break;
    }

    if (e.opcode != 0) {
      // Intrinsic recognized. Grab its args to finish building the expression.
      assert(I->getNumArgOperands() == 2 &&
             "Expect two args for recognised intrinsics.");
      e.varargs.push_back(lookup_or_add(I->getArgOperand(0)));
      e.varargs.push_back(lookup_or_add(I->getArgOperand(1)));
      return e;
    }
  }

  // Not a recognised intrinsic. Fall back to producing an extract value
  // expression.