path: root/lib/Analysis/ConstantFolding.cpp

//===-- ConstantFolding.cpp - Fold instructions into constants ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines routines for folding instructions into constants.
//
// Also, to supplement the basic VMCore ConstantExpr simplifications,
// this file defines some additional folding routines that can make use of
// TargetData information. These functions cannot go in VMCore due to library
// dependency issues.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/LLVMContext.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringMap.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
#include <cerrno>
#include <cmath>
using namespace llvm;

//===----------------------------------------------------------------------===//
// Constant Folding internal helper functions
//===----------------------------------------------------------------------===//

/// IsConstantOffsetFromGlobal - If this constant is actually a constant offset
/// from a global, return the global and the constant.  Because of
/// constantexprs, this function is recursive.
static bool IsConstantOffsetFromGlobal(Constant *C, GlobalValue *&GV,
                                       int64_t &Offset, const TargetData &TD) {
  // Trivial case, constant is the global.
  if ((GV = dyn_cast<GlobalValue>(C))) {
    Offset = 0;
    return true;
  }
  
  // Otherwise, if this isn't a constant expr, bail out.
  ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
  if (!CE) return false;
  
  // Look through ptr->int and ptr->ptr casts.
  if (CE->getOpcode() == Instruction::PtrToInt ||
      CE->getOpcode() == Instruction::BitCast)
    return IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD);
  
  // i32* getelementptr ([5 x i32]* @a, i32 0, i32 5)    
  if (CE->getOpcode() == Instruction::GetElementPtr) {
    // Cannot compute this if the element type of the pointer is missing size
    // info.
    if (!cast<PointerType>(CE->getOperand(0)->getType())
                 ->getElementType()->isSized())
      return false;
    
    // If the base isn't a global+constant, we aren't either.
    if (!IsConstantOffsetFromGlobal(CE->getOperand(0), GV, Offset, TD))
      return false;
    
    // Otherwise, add any offset that our operands provide.
    gep_type_iterator GTI = gep_type_begin(CE);
    for (User::const_op_iterator i = CE->op_begin() + 1, e = CE->op_end();
         i != e; ++i, ++GTI) {
      ConstantInt *CI = dyn_cast<ConstantInt>(*i);
      if (!CI) return false;  // Index isn't a simple constant?
      if (CI->getZExtValue() == 0) continue;  // Not adding anything.
      
      if (const StructType *ST = dyn_cast<StructType>(*GTI)) {
        // N = N + Offset
        Offset += TD.getStructLayout(ST)->getElementOffset(CI->getZExtValue());
      } else {
        const SequentialType *SQT = cast<SequentialType>(*GTI);
        Offset += TD.getTypeAllocSize(SQT->getElementType())*CI->getSExtValue();
      }
    }
    return true;
  }
  
  return false;
}

/// ConstantFoldLoadFromConstPtr - Return the value that a load from C would
/// produce if it is constant and determinable.  If this is not determinable,
/// return null.
Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C,
                                             const TargetData *TD) {
  // First, try the easy cases:
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
    if (GV->isConstant() && GV->hasDefinitiveInitializer())
      return GV->getInitializer();

  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
    if (CE->getOpcode() == Instruction::GetElementPtr) {
      if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
        if (GV->isConstant() && GV->hasDefinitiveInitializer())
          if (Constant *V = 
               ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE))
            return V;
    }
    
    // Instead of loading constant c string, use corresponding integer value
    // directly if string length is small enough.
    std::string Str;
    if (TD