[llvm-dev] Structure Padding and GetElementPtr

[Hong Hu via llvm-dev]

Add the example:

class A {
    bool b1, b2;
    double d1;
    int i1;
};

%class.A = type <{ i8, i8, [6 x i8], double, i32, [4 x i8] }>

Regards,
Hu Hong

On 29 December 2016 at 10:57, Hong Hu <huhong789 at gmail.com> wrote:

> Yes, Reid. I have used these methods to figure out the layout.
>
> Now my question is to build a map between the original layout and the new
> layout. I show one example below. When LLVM IR access the 4th (starting
> from 0th) member (i32) of the class A, the map will tell that in fact it is
> accessing the originally 3rd member (i1). Any suggestion?
>
> Regards,
> Hu Hong
>
> On 29 December 2016 at 10:50, Reid Kleckner <rnk at google.com> wrote:
>
>> Only Clang really knows the original structure layout. You can pass
>> '-Xclang -fdump-record-layouts', though, to see the layout during
>> compilation. The DICompositeType metadata produced when compiling with
>> debug info might contain enough information to describe the original layout.
>>
>> On Wed, Dec 28, 2016 at 6:44 PM, Hong Hu via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Thanks, Eli.
>>>
>>> Next question is how to get the layout of the original C++ class from
>>> LLVM IR?
>>>
>>> Regards,
>>> Hu Hong
>>>
>>> On 29 December 2016 at 01:57, Friedman, Eli <efriedma at codeaurora.org>
>>> wrote:
>>>
>>>> On 12/28/2016 5:41 AM, Hong Hu via llvm-dev wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I'm writing a pass to understand the memory access to C++ class
>>>> members. For each GetElementPtr instruction, I check the second index to
>>>> the class pointer, to figure out which member it is intended to access.
>>>>
>>>> However, due to the structure padding, there are some fake members
>>>> inserted into the structure. For example, when GEP works on the 5th element
>>>> of the padded structure, it may in fact works on the originally 3rd one, if
>>>> there are two paddings before the original 3rd member.
>>>>
>>>> Is there any way to map this "5th" access to the original "3rd" one?
>>>> For example, some APIs to tell whether one member is a real member, or a
>>>> padded one?
>>>>
>>>>
>>>> I would suggest converting the index of the struct GEP into an offset
>>>> in bytes; see StructLayout::getElementOffset.  You can then compare
>>>> that to the layout of the original C++ class.
>>>>
>>>> -Eli
>>>>
>>>> --
>>>> Employee of Qualcomm Innovation Center, Inc.
>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
>>>>
>>>>
>>>
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>>>
>>>
>>
>
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