[llvm-dev] Force casting a Value*

[Dipanjan Das via llvm-dev]

On 11 June 2017 at 14:04, don hinton <hintonda at gmail.com> wrote:

> Assuming you know it's a 64 bit value, and the function you are calling
> takes a uint64_t, try this:
>

The values from the test program are of type: i32/i32*/i32**. Can't I
interpret these as uint64_t some way?


>
> Value* args[] = {store_inst->getOperand(0)};
>
>
> On Sun, Jun 11, 2017 at 1:16 PM, Dipanjan Das via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>>
>> On 11 June 2017 at 12:05, Tim Northover <t.p.northover at gmail.com> wrote:
>>
>>> On 11 June 2017 at 11:56, Dipanjan Das via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> > I can't pass var_value to a function accepting uint64_t. LLVM complains
>>> > about broken function call.
>>>
>>> Well, yes. var_value has type "ConstantInt *", not uint64_t. Assuming
>>> the value being stored actually is a constant known at compile-time
>>> you should be able to use cast<ConstantInt *>(vo)->getZExtValue() to
>>> retrieve a uint64_t value (careful of i128s!).
>>>
>>> If that cast fails then you're not dealing with a constant store. This
>>> is either a nuisance (if you were expecting the possibility and can
>>> deal with it) or a huge misunderstanding over compile-time vs runtime
>>> values.
>>>
>>
>> I think it's a misunderstanding in may part. Let me explain my intention
>> first. I want to retrieve the value (if isPointerTy() returns true) to be
>> stored by a store instruction at run-time and pass on that value as either
>> an unsigned int (64) or a pointer (char*) to an external function. The call
>> to that function is inserted using IRBuilder.CreateCall().
>>
>> Consider the code snippet below.
>>
>> ==================================================
>>
>> if (StoreInst *store_inst = dyn_cast<StoreInst>(&I)) {
>>                     Value* vo = store_inst->getValueOperand();
>>                      uint64 var_value = /* cast vo to unsigned int 64 bit
>> */
>>
>>                    // Pass on this value to external function
>>                     IRBuilder<> builder(&I);
>>                     builder.SetInsertPoint(&B,
>> ++builder.GetInsertPoint());
>>                     Constant *func = F.getParent()->getOrInsertFunc
>> tion("instrument_store",
>>
>> Type::getVoidTy(Ctx), Type::getInt8PtrTy(Ctx), NULL);
>>                     Value* args[] = {var_value};
>>                     builder.CreateCall(func, args);
>>  }
>>
>> ==================================================
>>
>> To reiterate, I want to wrap up the run-time value (var_value) as a
>> Value* and call the method (instrument_store) with that value as an
>> argument.
>>
>>
>>
>>> If it's a nuisance, you can use "dyn_cast" instead of "cast". That'll
>>> return nullptr if the store wasn't a constant instead of crashing; you
>>> can decide what to do at that point.
>>>
>>> Cheers.
>>>
>>> Tim.
>>>
>>
>>
>>
>> --
>>
>> Thanks & Regards,
>> Dipanjan
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>


-- 

Thanks & Regards,
Dipanjan
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