path: root/lib/Analysis/BasicAliasAnalysis.cpp

//===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the default implementation of the Alias Analysis interface
// that simply implements a few identities (two different globals cannot alias,
// etc), but otherwise does no analysis.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
#include "llvm/Analysis/MallocHelper.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include <algorithm>
using namespace llvm;

//===----------------------------------------------------------------------===//
// Useful predicates
//===----------------------------------------------------------------------===//

static const Value *GetGEPOperands(const Value *V, 
                                   SmallVector<Value*, 16> &GEPOps) {
  assert(GEPOps.empty() && "Expect empty list to populate!");
  GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
                cast<User>(V)->op_end());

  // Accumulate all of the chained indexes into the operand array
  V = cast<User>(V)->getOperand(0);

  while (const GEPOperator *G = dyn_cast<GEPOperator>(V)) {
    if (!isa<Constant>(GEPOps[0]) || isa<GlobalValue>(GEPOps[0]) ||
        !cast<Constant>(GEPOps[0])->isNullValue())
      break;  // Don't handle folding arbitrary pointer offsets yet...
    GEPOps.erase(GEPOps.begin());   // Drop the zero index
    GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
    V = G->getOperand(0);
  }
  return V;
}

/// isKnownNonNull - Return true if we know that the specified value is never
/// null.
static bool isKnownNonNull(const Value *V) {
  // Alloca never returns null, malloc might.
  if (isa<AllocaInst>(V)) return true;
  
  // A byval argument is never null.
  if (const Argument *A = dyn_cast<Argument>(V))
    return A->hasByValAttr();

  // Global values are not null unless extern weak.
  if (const GlobalValue *GV = dyn_cast<GlobalValue>(V))
    return !GV->hasExternalWeakLinkage();
  return false;
}

/// isNonEscapingLocalObject - Return true if the pointer is to a function-local
/// object that never escapes from the function.
static bool isNonEscapingLocalObject(const Value *V) {
  // If this is a local allocation, check to see if it escapes.
  if (isa<AllocaInst>(V) || isNoAliasCall(V))
    return !PointerMayBeCaptured(V, false);

  // If this is an argument that corresponds to a byval or noalias argument,
  // then it has not escaped before entering the function.  Check if it escapes
  // inside the function.
  if (const Argument *A = dyn_cast<Argument>(V))
    if (A->hasByValAttr() || A->hasNoAliasAttr()) {
      // Don't bother analyzing arguments already known not to escape.
      if (A->hasNoCaptureAttr())
        return true;
      return !PointerMayBeCaptured(V, false);
    }
  return false;
}


/// isObjectSmallerThan - Return true if we can prove that the object specified
/// by V is smaller than Size.
static bool isObjectSmallerThan(const Value *V, unsigned Size,
                                LLVMContext &Context, const TargetData &TD) {
  const Type *AccessTy;
  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
    AccessTy = GV->getType()->getElementType();
  } else if (const AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
    if (!AI->isArrayAllocation())
      AccessTy = AI->getType()->getElementType();
    else
      return false;
  } else if (const CallInst* CI = extractMallocCall(V)) {
    if (!isArrayMalloc(V, Context, &TD))
      // The size is the argument to the malloc call.
      if (const ConstantInt* C = dyn_cast<ConstantInt>(CI->getOperand(1)))
        return (C->getZExtValue() < Size);
    return false;