[LLVMdev] dyn_cast<Instruction *> returns NULL where it should return a valid instruction

[Chuck Zhao]

John,

Thank you for the quick response, that is precisely the problem.
It is a global variable indeed, and the embedded form is not an 
instruction, but an cast expression :-)

Based on your suggestion, I worked out a quick fix for the problem.

Thank you very much

Chuck

On 5/16/2011 10:50 AM, John Criswell wrote:
> On 5/16/11 9:35 AM, Chuck Zhao wrote:
>> I have the following prototype for a function:
>> void bkp_memory(char *, int);
>>
>> Inside my LLVM IR, I have a callsite looks like the following:
>> tail call void @bkp_memory(i8* bitcast (i32** @P to i8*), i32 4) nounwind
>>
>>
>> When I try to obtain its 1st argument and check whether it is a valid 
>> instruction, using:
>> Instruction *Inst = dyn_cast<Instruction *>(I->getOperand(0));
>>
>> it gives me a NULL Ptr, which is quite a surprise to me.
>>
>> Am I doing anything wrong here?
>
> The bitcast you see above is not an instruction.  Rather, it is a 
> constant expression and is represented by a ConstExpr object in the 
> C++ API.  That is most likely because @P is a constant (a function 
> pointer, global value, or something similar).
>
> So,
>
> 1) Use the stripPointerCasts() method of llvm::Value to strip away all 
> pointer casts (whether they are cast instructions or ConstExpr casts):
>
> I->getOperand(0)->stripPointerCasts()
>
> 2) Realize that not all operands are instructions.  In this case, the 
> operand is probably a global variable.  Chances are good that:
>
> dyn_cast<Instruction>(I->getOperand(0)->stripPointerCasts())
>
> ... will return NULL because the operand is a constant and not an 
> instruction.
>
>
>>
>>
>> Note that in many other cases, the bkp_memory() callsite won't have 
>> the embedded form. They look like:
>>    store i32* %21, i32** @P, align 4
>>    %22 = bitcast i32* %21 to i8*
>>    tail call void @bkp_memory(i8* %22, i32 4) nounwind
>>
>> And the dyn_cast<>  conversion is fine in these cases.
>> Would that be the problem?
>
> The "embedded" form is a constant expression.  The "unembedded" form 
> is an instruction.  LLVM has several constant expressions that are 
> modeled after instructions; the difference is that a constant 
> expression's operands are all constants, and therefore the constant 
> expression can also be used like a constant.
>
> For more details, please see the LLVM Language Reference manual.
>
> -- John T.
>
>>
>>
>> Thank you
>>
>> Chuck
>>
>>
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>
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