[LLVMdev] size of const_ptr for array index

Alexander Poddey
Tue, 24 Mar 2015 18:39:07 -0000

Hi Christoph,

thanks a lot for the hint!
Alex

Christoph Grenz wrote:

> Hi,
>
> AFAIK GEP indices into structs always (independent of the target
> architecture) use i32 because of historical reasons [1]. I don't know
for
> sure for other indices, but i64 always seems to work for anything except
> struct indices.
>
> http://llvm.org/docs/GetElementPtr.html#why-do-struct-member-indices-
always-use-i32
>
> Regards
> Christoph Grenz
>
> Am Montag, 16. März 2015, 20:44:46 schrieb Alexander Poddey:
>> 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.
>>
>> 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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