[llvm-dev] How to calculate the offset obtained via a GEP instruction

[Alberto Barbaro via llvm-dev]

Hello,
I did some more digging and I think that those numbers where indeed correct:

```
➜  /tmp cat struct.c
#include<stdio.h>

struct Point
{
   int x;
   char y;
   char z;
   long w;
};

int main()
{
   struct Point p1;

   // Accessing members of point p1
   p1.x = 1;
   p1.y = 2;
   p1.z = 3;
   p1.w = 4;

   printf("p1: 0x%x\n", &p1);
   printf("p1.x: 0x%x\n", &p1.x);
   printf("p1.y: 0x%x\n", &p1.y);
   printf("p1.z: 0x%x\n", &p1.z);
   printf("p1.w: 0x%x\n", &p1.w);
   return 0;
}
➜  /tmp
```
➜  /tmp ./struct
p1: 0x9791b30
p1.x: 0x9791b30
p1.y: 0x9791b34
p1.z: 0x9791b35
p1.w: 0x9791b38
➜  /tmp
```

```
  %3 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 0
ap_offset: 0
Accumulated offset: 1
ap_offset: 0
  %4 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 1
ap_offset: 0
Accumulated offset: 1
ap_offset: 4
  %5 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 2
ap_offset: 0
Accumulated offset: 1
ap_offset: 5
  %6 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 3
ap_offset: 0
Accumulated offset: 1
ap_offset: 8
```

Let me know if I'm missing anything.

Thanks
Alberto

Il giorno mar 25 gen 2022 alle ore 20:03 Alberto Barbaro <
barbaro.alberto at gmail.com> ha scritto:

> Hi Nikita,
> so I think I made some progress but I'm not quite there yet.
>
> So the GEP Instruction that I'm interested to analyze are:
>
> %3 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 0
> %4 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 1
> %5 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32 2
>
> and the Point struct is declared in the following way:
>
> struct Point
> {
>    int x;
>    char y;
>    long z;
> };
>
> Reading some doc online I tried the following:
>
> Module *M = I.getModule();
> I.dump();
> APInt ap_offset(32, 0, false);
> std::cout << "ap_offset: " << ap_offset.getSExtValue() << "\n";
> std::cout << "Accumulated offset: " <<
> I.accumulateConstantOffset(M->getDataLayout(), ap_offset) << "\n";
> std::cout << "ap_offset: " << ap_offset.getSExtValue() << "\n";
>
> The output is something like:
>   %3 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 0
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 0
>   %4 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 1
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 4
>   %5 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 2
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 8
>   %3 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 0
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 0
>   %4 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 1
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 4
>   %5 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 2
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 8
>   %3 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 0
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 0
>   %4 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 1
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 4
>   %5 = getelementptr inbounds %struct.Point, %struct.Point* %2, i32 0, i32
> 2
> ap_offset: 0
> Accumulated offset: 1
> ap_offset: 8
>
> I think the output is almost correct because the offset is increased by 4
> each time... but the struct has char and long elements so the offset does
> not always look right. I think it due to the fact that the GEP is referring
> only to i32.
>
> How should I fix this situation?  Few lines of code would be very helpful
>
> Thanks
> Alberto
>
>
> Il giorno mar 25 gen 2022 alle ore 08:25 Nikita Popov <
> nikita.ppv at gmail.com> ha scritto:
>
>> 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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