llvm dev - Jan 2022 - How to calculate the offset obtained via a GEP instruction

Hi all,
so I'm trying to understand how to manually calculate the offset calculated
by a GEP instruction. I found that this question was asked over 6 years ago
on stackoverflow[1] as well but never got a real answer.

Since I need exactly the same, is there anyone willing to help me to
understand how to calculate the offset?

Thanks


[1]
https://stackoverflow.com/questions/32444497/determine-byte-offset-of-getelementptr
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The easiest way to do so in my experience is to change the base operand to
a null pointer of the type and then convert the result of the GEP to an
integer using ptrtoint. That will give you the offset in bytes and a pass
using the target info will constant fold it as well.

On Tue, Jan 25, 2022, 08:00 Alberto Barbaro via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi all,
> so I'm trying to understand how to manually calculate the offset
> calculated by a GEP instruction. I found that this question was asked over
> 6 years ago on stackoverflow[1] as well but never got a real answer.
>
> Since I need exactly the same, is there anyone willing to help me to
> understand how to calculate the offset?
>
> Thanks
>
>
> [1]
>
https://stackoverflow.com/questions/32444497/determine-byte-offset-of-getelementptr
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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On Tue, Jan 25, 2022 at 8:00 AM Alberto Barbaro via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi all,
> so I'm trying to understand how to manually calculate the offset
> calculated by a GEP instruction. I found that this question was asked over
> 6 years ago on stackoverflow[1] as well but never got a real answer.
>
> Since I need exactly the same, is there anyone willing to help me to
> understand how to calculate the offset?
>
You can use GEPOperator::accumulateConstantOffset(). Or more generically,
there is Value::stripAndAccumulateConstantOffsets(), which can look through
multiple GEPs, bitcasts, etc.

Regards,
Nikita
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