[LLVMdev] access array problem

[Guangming Tan]

于 2011/5/18 14:29, Duncan Sands 写道:
> Hi Tan Guangming,
>
>> I want to access an array in my instrumentation code. For example:
>>
>> GlobalVariable:
>> int *counter; //counter the number of load/store operations in run-time
>> int *counterArray; //record the load/store addresses
> strictly speaking these are not arrays, they are pointers.  Also, you have
> written them in some kind of C-style idiom.  What are the declarations in
> LLVM IR?
const Type *IntTy = Type::getInt32Ty(M.getContext());
const Type *ATyC = ArrayType::get(Type::getInt64Ty(M.getContext()), 1);

GlobalVariable *CounterSize = new GlobalVariable(M, ATyC, false, 
GlobalValue::InternalLinkage,
Constant::getNullValue(ATyC), "MemTraceCounterSize");

const Type *ATy = ArrayType::get(Type::getInt32Ty(M.getContext()), 3000);
GlobalVariable *Counters = new GlobalVariable(M, ATy, false, 
GlobalValue::InternalLinkage,
Constant::getNullValue(ATy), "MemTraceCounters");
>> //increase the counter if a load/store is performed
>> std::vector<Constant *>index(2);
>> index[0] = Constant::getNullvalue(Type:getInt32Ty(Context));
>> index[1] = Constant::get(Type::getInt32Ty(Context), 0);
> The above two lines both compute the same thing (an i32 constant equal to
> zero) in two different ways.
>
>> Constant *ElementPtr = ConstantExpr::getGetElementPtr(counter,
>> &index[0], index.size());
>> Value *oldcounter = new LoadInst(ElementPtr, "oldcounter", InsertPos);
>> Value *newcounter = BinaryOperator::Create(Instruction::Add,
>> oldcounter, ConstantInt::get(Type::getInt64Ty(Context), 1),
>> "newcounter", InsertPos);
>> new StoreInst(newcounter, ElmentPtr, InserPos);
>>
>> //store the memory address to counterArray[oldcounter]
>> std::vector<Constant*>   indexC(2);
>> indexC[0] =  Constant::getNullvalue(Type:getInt32Ty(Context));
>> indexC[1] = dync_cast(llvm::ConstantInt>(oldcounter);
> Since oldcounter is not a constant (its value is not known at compile time...)
> this is never going to work.  Declare the vector to be of Value* not Constant*.
> Then you don't need the dynamic cast.
So, do you mean that we have no way to use the "oldcounter" as an index 
to access an array?
>> Constant *ElmentPtr = ConstantExpr::getGetElementPtr(counterArray,
>> &indexC[0], indexC.size());
> This line can then not be a ConstantExpr, it has to be an instruction.  Again,
> it cannot be a constant since the address computed isn't constant (it is not
> known at compile time...).
>
> Ciao, Duncan.
>
>> ......// other codes
>>
>> Unfortunately, the oldcounter of Value type can not be cast to
>> ConstantInt, dync_cast returns NULL.
>> Is there any way to retrieve the integer value from oldcounter?
>>
>> Thanks!
>>
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