[LLVMdev] How to Load a Value?

[Zheng Wang]

Thanks guys!!

I got it!

I was confuse with the global variables, which require a explicit load!

Something like load i32* @variable.

Cheers,
Zheng

On 8 April 2010 18:58, Óscar Fuentes <ofv at wanadoo.es> wrote:
> The following message is a courtesy copy of an article
> that has been posted to gmane.comp.compilers.llvm.devel as well.
>
> Zheng Wang <jason.wangz at gmail.com> writes:
>
>> I have a problem of generating a load instruction. The LLVM bytecode is:
>>
>> ------------------------
>> entry:
>>  ...
>>  %2 = call i32 (...)* @atoi(i8*%1) nounwind
>>  /*<- Insertpos*/
>> ...
>>
>> --
>> bb1:
>>  ..
>>  %5 = icmp sgt i32 %2, %i.0
>>  ...
>> -----------------
>>
>>
>> Now I have
>>
>> pb: pointer to the Value object of *%2* of bb1.
>>
>> Here, I want to generate a load instruction and I did it as:
>>
>>  new LoadInst(pb, "load_2", InsertPos);
>>
>> where InsertPos points to the position immediately after "%2 = call
>> i32 (...)* @atoi(i8*%1) nounwind".
>>
>>
>> BUT I got a runtime error as:
>>
>> "
>> include/llvm/Support/Casting.h:199: typename llvm::cast_retty<To,
>> From>::ret_type llvm::cast(const Y&) [with X = llvm::PointerType, Y =
>> const llvm::Type*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of
>> incompatible type!"' failed."
>>
>> This is because LoadInst is implemented as:
>>
>> LoadInst::LoadInst(Value *Ptr, const Twine &Name, BasicBlock *InsertAE)
>>   : UnaryInstruction(cast<PointerType>(Ptr->getType())->getElementType(),
>>                      Load, Ptr, InsertAE)
>> {
>>   ...
>> }
>>
>> and pb->getType() is not a "PointerType"!!! WHY? Is it because it is a
>> function call? In this situation, how can I load %2 assuming it has
>> been load before.
>>
>> Can anybody gives me some hints?
>
> You don't need to load the value. `pb' IS the value. The `load'
> instruction is used for getting a value through a pointer, but your call
> to `atoi' does not return a pointer, it returns an integer value, which
> is what you want, so `load' makes no sense. Do not confuse LLVM
> Assembler pointer types with C++ pointers to LLVM instructions.
>
> When confused, use the online demo on
>
> http://www.llvm.org/demo/
>
> feed it with some C/C++ and look at both the generated LLVM assembler
> and the equivalent C++ code. Remember that there is a one-to-one
> relation among them.
>



-- 
Best regards,

WANG Zheng