X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FAnalysis%2FBasicAliasAnalysis.cpp;h=fe71f04098b13061a0b04c4dcf2a99bc3fd7f9dd;hb=002ec1482c83003c6aef841395271aa5993d916a;hp=be8fe07f2c97c245c5c8a79ab6b6e7f671d62477;hpb=d087480166d8f0ec2a44732c7ec087f4eb1ba527;p=oota-llvm.git

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index be8fe07f2c9..fe71f04098b 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -14,14 +14,14 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/Analysis/Passes.h"
 #include "llvm/Constants.h"
 #include "llvm/DerivedTypes.h"
 #include "llvm/Function.h"
-#include "llvm/ParameterAttributes.h"
 #include "llvm/GlobalVariable.h"
 #include "llvm/Instructions.h"
-#include "llvm/Intrinsics.h"
+#include "llvm/IntrinsicInst.h"
 #include "llvm/Pass.h"
 #include "llvm/Target/TargetData.h"
 #include "llvm/ADT/SmallVector.h"
@@ -32,6 +32,105 @@
 #include <algorithm>
 using namespace llvm;
 
+//===----------------------------------------------------------------------===//
+// Useful predicates
+//===----------------------------------------------------------------------===//
+
+static const User *isGEP(const Value *V) {
+  if (isa<GetElementPtrInst>(V) ||
+      (isa<ConstantExpr>(V) &&
+       cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
+    return cast<User>(V);
+  return 0;
+}
+
+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 User *G = isGEP(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<AllocationInst>(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,
+                                const TargetData &TD) {
+  const Type *AccessTy;
+  if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) {
+    AccessTy = GV->getType()->getElementType();
+  } else if (const AllocationInst *AI = dyn_cast<AllocationInst>(V)) {
+    if (!AI->isArrayAllocation())
+      AccessTy = AI->getType()->getElementType();
+    else
+      return false;
+  } else if (const Argument *A = dyn_cast<Argument>(V)) {
+    if (A->hasByValAttr())
+      AccessTy = cast<PointerType>(A->getType())->getElementType();
+    else
+      return false;
+  } else {
+    return false;
+  }
+  
+  if (AccessTy->isSized())
+    return TD.getTypePaddedSize(AccessTy) < Size;
+  return false;
+}
+
+//===----------------------------------------------------------------------===//
+// NoAA Pass
+//===----------------------------------------------------------------------===//
+
 namespace {
   /// NoAA - This class implements the -no-aa pass, which always returns "I
   /// don't know" for alias queries.  NoAA is unlike other alias analysis
@@ -40,8 +139,8 @@ namespace {
   ///
   struct VISIBILITY_HIDDEN NoAA : public ImmutablePass, public AliasAnalysis {
     static char ID; // Class identification, replacement for typeinfo
-    NoAA() : ImmutablePass((intptr_t)&ID) {}
-    explicit NoAA(intptr_t PID) : ImmutablePass(PID) { }
+    NoAA() : ImmutablePass(&ID) {}
+    explicit NoAA(void *PID) : ImmutablePass(PID) { }
 
     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       AU.addRequired<TargetData>();
@@ -56,11 +155,6 @@ namespace {
       return MayAlias;
     }
 
-    virtual ModRefBehavior getModRefBehavior(Function *F, CallSite CS,
-                                         std::vector<PointerAccessInfo> *Info) {
-      return UnknownModRefBehavior;
-    }
-
     virtual void getArgumentAccesses(Function *F, CallSite CS,
                                      std::vector<PointerAccessInfo> &Info) {
       assert(0 && "This method may not be called on this function!");
@@ -79,32 +173,34 @@ namespace {
     virtual void deleteValue(Value *V) {}
     virtual void copyValue(Value *From, Value *To) {}
   };
+}  // End of anonymous namespace
 
-  // Register this pass...
-  char NoAA::ID = 0;
-  RegisterPass<NoAA>
-  U("no-aa", "No Alias Analysis (always returns 'may' alias)");
+// Register this pass...
+char NoAA::ID = 0;
+static RegisterPass<NoAA>
+U("no-aa", "No Alias Analysis (always returns 'may' alias)", true, true);
 
-  // Declare that we implement the AliasAnalysis interface
-  RegisterAnalysisGroup<AliasAnalysis> V(U);
-}  // End of anonymous namespace
+// Declare that we implement the AliasAnalysis interface
+static RegisterAnalysisGroup<AliasAnalysis> V(U);
 
 ImmutablePass *llvm::createNoAAPass() { return new NoAA(); }
 
+//===----------------------------------------------------------------------===//
+// BasicAA Pass
+//===----------------------------------------------------------------------===//
+
 namespace {
   /// BasicAliasAnalysis - This is the default alias analysis implementation.
   /// Because it doesn't chain to a previous alias analysis (like -no-aa), it
   /// derives from the NoAA class.
   struct VISIBILITY_HIDDEN BasicAliasAnalysis : public NoAA {
     static char ID; // Class identification, replacement for typeinfo
-    BasicAliasAnalysis() : NoAA((intptr_t)&ID) { }
+    BasicAliasAnalysis() : NoAA(&ID) {}
     AliasResult alias(const Value *V1, unsigned V1Size,
                       const Value *V2, unsigned V2Size);
 
     ModRefResult getModRefInfo(CallSite CS, Value *P, unsigned Size);
-    ModRefResult getModRefInfo(CallSite CS1, CallSite CS2) {
-      return NoAA::getModRefInfo(CS1,CS2);
-    }
+    ModRefResult getModRefInfo(CallSite CS1, CallSite CS2);
 
     /// hasNoModRefInfoForCalls - We can provide mod/ref information against
     /// non-escaping allocations.
@@ -124,116 +220,30 @@ namespace {
                          const Type *BasePtr2Ty,
                          Value **GEP2Ops, unsigned NumGEP2Ops, unsigned G2Size);
   };
+}  // End of anonymous namespace
 
-  // Register this pass...
-  char BasicAliasAnalysis::ID = 0;
-  RegisterPass<BasicAliasAnalysis>
-  X("basicaa", "Basic Alias Analysis (default AA impl)");
+// Register this pass...
+char BasicAliasAnalysis::ID = 0;
+static RegisterPass<BasicAliasAnalysis>
+X("basicaa", "Basic Alias Analysis (default AA impl)", false, true);
 
-  // Declare that we implement the AliasAnalysis interface
-  RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
-}  // End of anonymous namespace
+// Declare that we implement the AliasAnalysis interface
+static RegisterAnalysisGroup<AliasAnalysis, true> Y(X);
 
 ImmutablePass *llvm::createBasicAliasAnalysisPass() {
   return new BasicAliasAnalysis();
 }
 
-/// getUnderlyingObject - This traverses the use chain to figure out what object
-/// the specified value points to.  If the value points to, or is derived from,
-/// a unique object or an argument, return it.  This returns:
-///    Arguments, GlobalVariables, Functions, Allocas, Mallocs.
-static const Value *getUnderlyingObject(const Value *V) {
-  if (!isa<PointerType>(V->getType())) return 0;
-
-  // If we are at some type of object, return it. GlobalValues and Allocations
-  // have unique addresses. 
-  if (isa<GlobalValue>(V) || isa<AllocationInst>(V) || isa<Argument>(V))
-    return V;
-
-  // Traverse through different addressing mechanisms...
-  if (const Instruction *I = dyn_cast<Instruction>(V)) {
-    if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I))
-      return getUnderlyingObject(I->getOperand(0));
-  } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
-    if (CE->getOpcode() == Instruction::BitCast || 
-        CE->getOpcode() == Instruction::GetElementPtr)
-      return getUnderlyingObject(CE->getOperand(0));
-  }
-  return 0;
-}
-
-static const User *isGEP(const Value *V) {
-  if (isa<GetElementPtrInst>(V) ||
-      (isa<ConstantExpr>(V) &&
-       cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
-    return cast<User>(V);
-  return 0;
-}
-
-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 User *G = isGEP(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;
-}
 
 /// pointsToConstantMemory - Chase pointers until we find a (constant
 /// global) or not.
 bool BasicAliasAnalysis::pointsToConstantMemory(const Value *P) {
-  if (const Value *V = getUnderlyingObject(P))
-    if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(V))
-      return GV->isConstant();
+  if (const GlobalVariable *GV = 
+        dyn_cast<GlobalVariable>(P->getUnderlyingObject()))
+    return GV->isConstant();
   return false;
 }
 
-// Determine if an AllocationInst instruction escapes from the function it is
-// contained in. If it does not escape, there is no way for another function to
-// mod/ref it.  We do this by looking at its uses and determining if the uses
-// can escape (recursively).
-static bool AddressMightEscape(const Value *V) {
-  for (Value::use_const_iterator UI = V->use_begin(), E = V->use_end();
-       UI != E; ++UI) {
-    const Instruction *I = cast<Instruction>(*UI);
-    switch (I->getOpcode()) {
-    case Instruction::Load: 
-      break; //next use.
-    case Instruction::Store:
-      if (I->getOperand(0) == V)
-        return true; // Escapes if the pointer is stored.
-      break; // next use.
-    case Instruction::GetElementPtr:
-      if (AddressMightEscape(I))
-        return true;
-      break; // next use.
-    case Instruction::BitCast:
-      if (!isa<PointerType>(I->getType()))
-        return true;
-      if (AddressMightEscape(I))
-        return true;
-      break; // next use
-    case Instruction::Ret:
-      // If returned, the address will escape to calling functions, but no
-      // callees could modify it.
-      break; // next use
-    default:
-      return true;
-    }
-  }
-  return false;
-}
 
 // getModRefInfo - Check to see if the specified callsite can clobber the
 // specified memory object.  Since we only look at local properties of this
@@ -243,22 +253,32 @@ static bool AddressMightEscape(const Value *V) {
 AliasAnalysis::ModRefResult
 BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
   if (!isa<Constant>(P)) {
-    const Value *Object = getUnderlyingObject(P);
-    // Allocations and byval arguments are "new" objects.
-    if (Object &&
-        (isa<AllocationInst>(Object) ||
-         (isa<Argument>(Object) && cast<Argument>(Object)->hasByValAttr()))) {
-      // Okay, the pointer is to a stack allocated object.  If we can prove that
-      // the pointer never "escapes", then we know the call cannot clobber it,
-      // because it simply can't get its address.
-      if (!AddressMightEscape(Object))
-        return NoModRef;
-
-      // If this is a tail call and P points to a stack location, we know that
-      // the tail call cannot access or modify the local stack.
+    const Value *Object = P->getUnderlyingObject();
+    
+    // If this is a tail call and P points to a stack location, we know that
+    // the tail call cannot access or modify the local stack.
+    // We cannot exclude byval arguments here; these belong to the caller of
+    // the current function not to the current function, and a tail callee
+    // may reference them.
+    if (isa<AllocaInst>(Object))
       if (CallInst *CI = dyn_cast<CallInst>(CS.getInstruction()))
-        if (CI->isTailCall() && !isa<MallocInst>(Object))
+        if (CI->isTailCall())
           return NoModRef;
+    
+    // If the pointer is to a locally allocated object that does not escape,
+    // then the call can not mod/ref the pointer unless the call takes the
+    // argument without capturing it.
+    if (isNonEscapingLocalObject(Object) && CS.getInstruction() != Object) {
+      bool passedAsArg = false;
+      // TODO: Eventually only check 'nocapture' arguments.
+      for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end();
+           CI != CE; ++CI)
+        if (isa<PointerType>((*CI)->getType()) &&
+            alias(cast<Value>(CI), ~0U, P, ~0U) != NoAlias)
+          passedAsArg = true;
+      
+      if (!passedAsArg)
+        return NoModRef;
     }
   }
 
@@ -266,9 +286,27 @@ BasicAliasAnalysis::getModRefInfo(CallSite CS, Value *P, unsigned Size) {
   return AliasAnalysis::getModRefInfo(CS, P, Size);
 }
 
+
+AliasAnalysis::ModRefResult 
+BasicAliasAnalysis::getModRefInfo(CallSite CS1, CallSite CS2) {
+  // If CS1 or CS2 are readnone, they don't interact.
+  ModRefBehavior CS1B = AliasAnalysis::getModRefBehavior(CS1);
+  if (CS1B == DoesNotAccessMemory) return NoModRef;
+  
+  ModRefBehavior CS2B = AliasAnalysis::getModRefBehavior(CS2);
+  if (CS2B == DoesNotAccessMemory) return NoModRef;
+  
+  // If they both only read from memory, just return ref.
+  if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
+    return Ref;
+  
+  // Otherwise, fall back to NoAA (mod+ref).
+  return NoAA::getModRefInfo(CS1, CS2);
+}
+
+
 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
-// as array references.  Note that this function is heavily tail recursive.
-// Hopefully we have a smart C++ compiler.  :)
+// as array references.
 //
 AliasAnalysis::AliasResult
 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
@@ -284,127 +322,88 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
   // Are we checking for alias of the same value?
   if (V1 == V2) return MustAlias;
 
-  if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
-      V1->getType() != Type::Int64Ty && V2->getType() != Type::Int64Ty)
+  if (!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType()))
     return NoAlias;  // Scalars cannot alias each other
 
-  // Strip off cast instructions...
+  // Strip off cast instructions.   Since V1 and V2 are pointers, they must be
+  // pointer<->pointer bitcasts.
   if (const BitCastInst *I = dyn_cast<BitCastInst>(V1))
     return alias(I->getOperand(0), V1Size, V2, V2Size);
   if (const BitCastInst *I = dyn_cast<BitCastInst>(V2))
     return alias(V1, V1Size, I->getOperand(0), V2Size);
 
-  // Figure out what objects these things are pointing to if we can...
-  const Value *O1 = getUnderlyingObject(V1);
-  const Value *O2 = getUnderlyingObject(V2);
-
-  // Pointing at a discernible object?
-  if (O1) {
-    if (O2) {
-      if (const Argument *O1Arg = dyn_cast<Argument>(O1)) {
-        // Incoming argument cannot alias locally allocated object!
-        if (isa<AllocationInst>(O2)) return NoAlias;
-        
-        // If they are two different objects, and one is a noalias argument
-        // then they do not alias.
-        if (O1 != O2 && O1Arg->hasNoAliasAttr())
-          return NoAlias;
-
-        // Byval arguments can't alias globals or other arguments.
-        if (O1 != O2 && O1Arg->hasByValAttr()) return NoAlias;
-        
-        // Otherwise, nothing is known...
-      } 
-      
-      if (const Argument *O2Arg = dyn_cast<Argument>(O2)) {
-        // Incoming argument cannot alias locally allocated object!
-        if (isa<AllocationInst>(O1)) return NoAlias;
-        
-        // If they are two different objects, and one is a noalias argument
-        // then they do not alias.
-        if (O1 != O2 && O2Arg->hasNoAliasAttr())
-          return NoAlias;
-          
-        // Byval arguments can't alias globals or other arguments.
-        if (O1 != O2 && O2Arg->hasByValAttr()) return NoAlias;
-        
-        // Otherwise, nothing is known...
-      
-      } else if (O1 != O2 && !isa<Argument>(O1)) {
-        // If they are two different objects, and neither is an argument,
-        // we know that we have no alias.
-        return NoAlias;
-      }
-
-      // If they are the same object, they we can look at the indexes.  If they
-      // index off of the object is the same for both pointers, they must alias.
-      // If they are provably different, they must not alias.  Otherwise, we
-      // can't tell anything.
-    }
-
-    // Unique values don't alias null, except non-byval arguments.
-    if (isa<ConstantPointerNull>(V2)) {
-      if (const Argument *O1Arg = dyn_cast<Argument>(O1)) {
-        if (O1Arg->hasByValAttr()) 
-          return NoAlias;
-      } else {
-        return NoAlias;                    
-      }
-    }
+  // Figure out what objects these things are pointing to if we can.
+  const Value *O1 = V1->getUnderlyingObject();
+  const Value *O2 = V2->getUnderlyingObject();
 
-    if (isa<GlobalVariable>(O1) ||
-        (isa<AllocationInst>(O1) &&
-         !cast<AllocationInst>(O1)->isArrayAllocation()))
-      if (cast<PointerType>(O1->getType())->getElementType()->isSized()) {
-        // If the size of the other access is larger than the total size of the
-        // global/alloca/malloc, it cannot be accessing the global (it's
-        // undefined to load or store bytes before or after an object).
-        const Type *ElTy = cast<PointerType>(O1->getType())->getElementType();
-        unsigned GlobalSize = getTargetData().getABITypeSize(ElTy);
-        if (GlobalSize < V2Size && V2Size != ~0U)
-          return NoAlias;
-      }
-  }
+  if (O1 != O2) {
+    // If V1/V2 point to two different objects we know that we have no alias.
+    if (isIdentifiedObject(O1) && isIdentifiedObject(O2))
+      return NoAlias;
+  
+    // Arguments can't alias with local allocations or noalias calls.
+    if ((isa<Argument>(O1) && (isa<AllocationInst>(O2) || isNoAliasCall(O2))) ||
+        (isa<Argument>(O2) && (isa<AllocationInst>(O1) || isNoAliasCall(O1))))
+      return NoAlias;
 
-  if (O2) {
-    if (!isa<Argument>(O2) && isa<ConstantPointerNull>(V1))
-      return NoAlias;                    // Unique values don't alias null
-
-    if (isa<GlobalVariable>(O2) ||
-        (isa<AllocationInst>(O2) &&
-         !cast<AllocationInst>(O2)->isArrayAllocation()))
-      if (cast<PointerType>(O2->getType())->getElementType()->isSized()) {
-        // If the size of the other access is larger than the total size of the
-        // global/alloca/malloc, it cannot be accessing the object (it's
-        // undefined to load or store bytes before or after an object).
-        const Type *ElTy = cast<PointerType>(O2->getType())->getElementType();
-        unsigned GlobalSize = getTargetData().getABITypeSize(ElTy);
-        if (GlobalSize < V1Size && V1Size != ~0U)
-          return NoAlias;
-      }
+    // Most objects can't alias null.
+    if ((isa<ConstantPointerNull>(V2) && isKnownNonNull(O1)) ||
+        (isa<ConstantPointerNull>(V1) && isKnownNonNull(O2)))
+      return NoAlias;
   }
-
+  
+  // If the size of one access is larger than the entire object on the other
+  // side, then we know such behavior is undefined and can assume no alias.
+  const TargetData &TD = getTargetData();
+  if ((V1Size != ~0U && isObjectSmallerThan(O2, V1Size, TD)) ||
+      (V2Size != ~0U && isObjectSmallerThan(O1, V2Size, TD)))
+    return NoAlias;
+  
+  // If one pointer is the result of a call/invoke and the other is a
+  // non-escaping local object, then we know the object couldn't escape to a
+  // point where the call could return it.
+  if ((isa<CallInst>(O1) || isa<InvokeInst>(O1)) &&
+      isNonEscapingLocalObject(O2) && O1 != O2)
+    return NoAlias;
+  if ((isa<CallInst>(O2) || isa<InvokeInst>(O2)) &&
+      isNonEscapingLocalObject(O1) && O1 != O2)
+    return NoAlias;
+  
   // If we have two gep instructions with must-alias'ing base pointers, figure
   // out if the indexes to the GEP tell us anything about the derived pointer.
   // Note that we also handle chains of getelementptr instructions as well as
   // constant expression getelementptrs here.
   //
   if (isGEP(V1) && isGEP(V2)) {
+    const User *GEP1 = cast<User>(V1);
+    const User *GEP2 = cast<User>(V2);
+    
+    // If V1 and V2 are identical GEPs, just recurse down on both of them.
+    // This allows us to analyze things like:
+    //   P = gep A, 0, i, 1
+    //   Q = gep B, 0, i, 1
+    // by just analyzing A and B.  This is even safe for variable indices.
+    if (GEP1->getType() == GEP2->getType() &&
+        GEP1->getNumOperands() == GEP2->getNumOperands() &&
+        GEP1->getOperand(0)->getType() == GEP2->getOperand(0)->getType() &&
+        // All operands are the same, ignoring the base.
+        std::equal(GEP1->op_begin()+1, GEP1->op_end(), GEP2->op_begin()+1))
+      return alias(GEP1->getOperand(0), V1Size, GEP2->getOperand(0), V2Size);
+    
+    
     // Drill down into the first non-gep value, to test for must-aliasing of
     // the base pointers.
-    const User *G = cast<User>(V1);
-    while (isGEP(G->getOperand(0)) &&
-           G->getOperand(1) ==
-           Constant::getNullValue(G->getOperand(1)->getType()))
-      G = cast<User>(G->getOperand(0));
-    const Value *BasePtr1 = G->getOperand(0);
-
-    G = cast<User>(V2);
-    while (isGEP(G->getOperand(0)) &&
-           G->getOperand(1) ==
-           Constant::getNullValue(G->getOperand(1)->getType()))
-      G = cast<User>(G->getOperand(0));
-    const Value *BasePtr2 = G->getOperand(0);
+    while (isGEP(GEP1->getOperand(0)) &&
+           GEP1->getOperand(1) ==
+           Constant::getNullValue(GEP1->getOperand(1)->getType()))
+      GEP1 = cast<User>(GEP1->getOperand(0));
+    const Value *BasePtr1 = GEP1->getOperand(0);
+
+    while (isGEP(GEP2->getOperand(0)) &&
+           GEP2->getOperand(1) ==
+           Constant::getNullValue(GEP2->getOperand(1)->getType()))
+      GEP2 = cast<User>(GEP2->getOperand(0));
+    const Value *BasePtr2 = GEP2->getOperand(0);
 
     // Do the base pointers alias?
     AliasResult BaseAlias = alias(BasePtr1, ~0U, BasePtr2, ~0U);
@@ -498,7 +497,7 @@ BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
   return MayAlias;
 }
 
-// This function is used to determin if the indices of two GEP instructions are
+// This function is used to determine if the indices of two GEP instructions are
 // equal. V1 and V2 are the indices.
 static bool IndexOperandsEqual(Value *V1, Value *V2) {
   if (V1->getType() == V2->getType())
@@ -666,7 +665,7 @@ BasicAliasAnalysis::CheckGEPInstructions(
         if (isa<ConstantInt>(GEP1Ops[i]) && 
             !cast<ConstantInt>(GEP1Ops[i])->isZero()) {
           // Yup, there's a constant in the tail.  Set all variables to
-          // constants in the GEP instruction to make it suiteable for
+          // constants in the GEP instruction to make it suitable for
           // TargetData::getIndexedOffset.
           for (i = 0; i != MaxOperands; ++i)
             if (!isa<ConstantInt>(GEP1Ops[i]))
@@ -681,9 +680,15 @@ BasicAliasAnalysis::CheckGEPInstructions(
           int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops,
                                                 MinOperands);
 
+          // Make sure we compare the absolute difference.
+          if (Offset1 > Offset2)
+            std::swap(Offset1, Offset2);
+
           // If the tail provided a bit enough offset, return noalias!
           if ((uint64_t)(Offset2-Offset1) >= SizeMax)
             return NoAlias;
+          // Otherwise break - we don't look for another constant in the tail.
+          break;
         }
     }