[LLVMdev] Accessing constant indexes in GetElementPtr

Casey Carter Casey at Carter.net
Mon Sep 23 08:10:00 PDT 2002

Vikram S. Adve wrote:

>>Also sprach Wojciech Stryjewski:
>>} Ok. Let's say I have a GetElementPtrInst that is used to access
>>} elements. In this case the indexes will be constants and I want
>>to get the
>>} constant values as an a native C int.
>>} Now I can iterate over all the indexes with idx_begin(). I can cast
>>} all of the indexes to a ConstantIntegral. However, I cannot cast the
>>} indexes to a ConstantSInt or ConstantUInt (cast<> throws an
>>assertion) and
>>} these two classes are the ones that have the getValue() method I need.
>>} Am I missing something here?
>>I'm not quite sure what particular problem you're having, but a few
>>things to remember...
>>    - Not all of the operands of getelementptr are constants. (In
>>      particular, Operand(0) isn't).
>Bill is right -- not all operands are necessarily constant.  Basically, any
>index into an array may not be a constant.  Any index into a structure must
>be a constant of type UByte, and so it should cast to a ConstantUInt.
>(Operand 0 is not a special case, but it is always an array index so it
>sometimes may not be a constant.  Very often, though, it is the constant 0.)
That should be _index_ 0, operand 0 is the base pointer itself.

>anyway, indexVal->getType() == Type::UByteTy should tell you if you have a
>structure index or not.
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