[llvm-dev] Force casting a Value*

[don hinton via llvm-dev]

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

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()->getOrInsertFunction("
> 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
>
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>
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