[llvm-dev] getelementptr

Anastasiya Ruzhanskaya via llvm-dev llvm-dev at lists.llvm.org
Sat Sep 2 23:06:09 PDT 2017

I also looked at some examples and I noticed that these too function
declaration result in totally different code:
#define N 100
void pivot (int x[], int a[][N], int n, int k) {

void pivot (int *x, int **a, int n, int k) {

To access element if array a, in the second case I have to do a load
at first , after getelementptr of the first dimension and then make
getelementptr again :


In the first case there is only one getelementptr on the two
dimensions simultaneously(getelementptr with three arguments) :

What is the difference?

2017-09-03 7:18 GMT+02:00 Anastasiya Ruzhanskaya <
anastasiya.ruzhanskaya at frtk.ru>:

> So, I am trying to do smth like this. Basically, we are developing are own
> hls tool for fpga based on llvm. Getelementptr is not representable in
> hardware (as far as I understand, I am here a person , who deals with llvm
> mostly). The easiest case will be to derive the index and pass it to memory
> controller, which is connected to load and store instructions. However,
> when there is a getelementptr for structures - the things become too
> complicated to derive the offset (index). So, by now I am trying to
> understand what are rules for getelementptr in code, what can be it's
> successors , what can be it's predecessors in order to come to another
> solution.
> 2017-09-02 23:31 GMT+02:00 Daniel Berlin <dberlin at dberlin.org>:
>> No.
>> It would be helpful to understand what you are trying to accomplish
>> overall, which may help people give you details about the best way to
>> accomplish it.
>> For example, if you are trying to understand or recover array indexes
>> from GEP's,  that is non-trivial.
>> On Sat, Sep 2, 2017 at 3:53 AM, Anastasiya Ruzhanskaya via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>> Is there a way to set always a defined behavior here - for example that
>>> we will always one getelemntptr with complex index?
>>> 2017-09-02 12:53 GMT+02:00 Anastasiya Ruzhanskaya <
>>> anastasiya.ruzhanskaya at frtk.ru>:
>>>> Ok, thank you. I have also one question about getelementptr. In
>>>> different versions of clang I see that sometimes array[i][i] is preceded by
>>>> two getelementptr instructions and sometimes only by one - with an already
>>>> complex index.
>>>> 2017-09-01 12:50 GMT+02:00 David Chisnall <David.Chisnall at cl.cam.ac.uk>
>>>> :
>>>>> On 1 Sep 2017, at 11:44, Anastasiya Ruzhanskaya via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>> >
>>>>> > Hello,
>>>>> > I wonder if the getelementptr can have other successors than load,
>>>>> store in some other cases when I directly print or directly return the
>>>>> result. every time I would like to assign the result - it will have a
>>>>> load/store successor?
>>>>> > So, basically the overall question is to clarify the possible
>>>>> successors of getelementptr.
>>>>> Any instruction that may take a pointer operand might be a user of a
>>>>> GEP.  For example, consider this C function:
>>>>> int x(struct S *s)
>>>>> {
>>>>>         y(&s->field);
>>>>> }
>>>>> Here, there will be a GEP to get the address of the field and then the
>>>>> user will be a call (or possibly invoke) instruction.
>>>>> David
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
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