path: root/lib/Transforms/Scalar/SCCP.cpp

//===- SCCP.cpp - Sparse Conditional Constant Propagation -----------------===//
// 
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// 
//===----------------------------------------------------------------------===//
//
// This file implements sparse conditional constant propagation and merging:
//
// Specifically, this:
//   * Assumes values are constant unless proven otherwise
//   * Assumes BasicBlocks are dead unless proven otherwise
//   * Proves values to be constant, and replaces them with constants
//   * Proves conditional branches to be unconditional
//
// Notice that:
//   * This pass has a habit of making definitions be dead.  It is a good idea
//     to to run a DCE pass sometime after running this pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "sccp"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/InstVisitor.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/hash_map"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
#include <set>
using namespace llvm;

// LatticeVal class - This class represents the different lattice values that an
// instruction may occupy.  It is a simple class with value semantics.
//
namespace {

class LatticeVal {
  enum { 
    undefined,           // This instruction has no known value
    constant,            // This instruction has a constant value
    overdefined          // This instruction has an unknown value
  } LatticeValue;        // The current lattice position
  Constant *ConstantVal; // If Constant value, the current value
public:
  inline LatticeVal() : LatticeValue(undefined), ConstantVal(0) {}

  // markOverdefined - Return true if this is a new status to be in...
  inline bool markOverdefined() {
    if (LatticeValue != overdefined) {
      LatticeValue = overdefined;
      return true;
    }
    return false;
  }

  // markConstant - Return true if this is a new status for us...
  inline bool markConstant(Constant *V) {
    if (LatticeValue != constant) {
      LatticeValue = constant;
      ConstantVal = V;
      return true;
    } else {
      assert(ConstantVal == V && "Marking constant with different value");
    }
    return false;
  }

  inline bool isUndefined()   const { return LatticeValue == undefined; }
  inline bool isConstant()    const { return LatticeValue == constant; }
  inline bool isOverdefined() const { return LatticeValue == overdefined; }

  inline Constant *getConstant() const {
    assert(isConstant() && "Cannot get the constant of a non-constant!");
    return ConstantVal;
  }
};

} // end anonymous namespace


//===----------------------------------------------------------------------===//
//
/// SCCPSolver - This class is a general purpose solver for Sparse Conditional
/// Constant Propagation.
///
class SCCPSolver : public InstVisitor<SCCPSolver> {
  std::set<BasicBlock*>     BBExecutable;// The basic blocks that are executable
  hash_map<Value*, LatticeVal> ValueState;  // The state each value is in...

  /// TrackedFunctionRetVals - If we are tracking arguments into and the return
  /// value out of a function, it will have an entry in this map, indicating
  /// what the known return value for the function is.
  hash_map<Function*, LatticeVal> TrackedFunctionRetVals;

  // The reason for two worklists is that overdefined is the lowest state
  // on the lattice, and moving things to overdefined as fast as possible
  // makes SCCP converge much faster.
  // By having a separate worklist, we accomplish this because everything
  // possibly overdefined will become overdefined at the soonest possible
  // point.
  std::vector<Value*> Over