[llvm-dev] Structure Padding and GetElementPtr

[Daniel Berlin via llvm-dev]

Yes.
LLVM types != C++ types.
There is no mapping except that produced if you add debug info.


On Wed, Dec 28, 2016 at 6:51 PM, Hong Hu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Here is an example:
>
> I can define two classes: A and Apad:
>
> class A {
>     bool b1, b2;
>     double d1;
>     int i1;
> };
>
> class Apad {
>     bool b1, b2;
>     bool pad1[6];
>     double d1;
>     int i1;
>     bool pad2[4];
> };
>
> A and Apad will have the same layout, from the LLVM IR level:
>
> %class.A = type <{ i8, i8, [6 x i8], double, i32, [4 x i8] }>
> %class.Apad = type { i8, i8, [6 x i8], double, i32, [4 x i8] }
>
> Regards,
> Hu Hong
>
> On 29 December 2016 at 10:44, Hong Hu <huhong789 at gmail.com> 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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