[llvm-dev] Structure Padding and GetElementPtr

[Friedman, Eli via llvm-dev]

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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