[llvm-dev] getelementptr

Anastasiya Ruzhanskaya via llvm-dev llvm-dev at lists.llvm.org
Sat Sep 2 22:18:12 PDT 2017

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

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
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>> llvm-dev at lists.llvm.org
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