[LLVMdev] size of const_ptr for array index

Alexander Poddey
Tue, 17 Mar 2015 18:09:41 -0000

Hi,

I tried different things, but none did give me the correct size.
Any ideas?



Mehdi Amini wrote:

> Hi,
>
>
>> On Mar 16, 2015, at 12:44 PM, Alexander Poddey <alexander.poddey at
gmx.net>
>> wrote:
>>
>> Hi all,
>>
>> when e.g. accessing a global variable residing in a structure, the
c++API
>> code looks like:
>>
>> std::vector<Constant*> const_ptr_183_indices;
>> const_ptr_183_indices.push_back(const_int32_172);
>> const_ptr_183_indices.push_back(const_int32_184);
>> const_ptr_183_indices.push_back(const_int64_175);
>> Constant* const_ptr_183 =
ConstantExpr::getGetElementPtr(gvar_struct_foo,
>> const_ptr_183_indices);
>>
>>
>> the struct could be
>>
>> struct stest {
>>    double age;
>>   double darray[3];
>> }
>>
>> The indices for accessing 'age' would be
>> 0 (32bit)
>> 0 (32bit).
>>
>> For an element of the darray, one however needs
>> 0 (32bit)
>> 1 (32bit)
>> 0-2 (64bit)
>>
>> The same for indices of vectors.
>>
>> The question:
>> Can I at runtime determine (how?) which size I have to use per case?
>> I made some tests but could not figure it out.
>
> I believe you have to check the pointer size in the DataLayout.
>
> Mehdi
>
>
>
>>
>> So for now (at my architecture) it seems hardcoding 32 bit as default,
>> and 64bit for indices (of arrays and vectors) works. But this is not
>> portable.
>>
>> Thanks
>> Alexander
>>
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