[llvm-dev] Non-native endianness support in Clang

Guy Benyei via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 28 07:15:49 PDT 2019

AFAIK, the padding of non-packed structs shouldn't be affected, but bitfield offsets must be adjusted.
I'll test the solution thoroughly to make sure different structs with this attribute are interpreted the same on BE and LE targets.

        Guy Benyei

-----Original Message-----
From: Finkel, Hal J. <hfinkel at anl.gov> 
Sent: Monday, August 26, 2019 9:59 PM
To: Guy Benyei <guybe at mellanox.com>; cfe-dev at lists.llvm.org; llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] Non-native endianness support in Clang

On 8/26/19 9:18 AM, Guy Benyei via llvm-dev wrote:
> Hello,
> I'd like to implement the gnu attribute "scalar_storage_order" to mark structs with non-native endianness. My rational is working on network protocols over little endian architecture (for instance: RiscV).
> An old bug is open on this issue here:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs
> .llvm.org%2Fshow_bug.cgi%3Fid%3D35293&data=02%7C01%7Cguybe%40mella
> nox.com%7C5eb16d1a2f174a2b002808d72a57811a%7Ca652971c7d2e4d9ba6a4d1492
> 56f461b%7C0%7C0%7C637024427646321756&sdata=Je2g85mVic2aq5H9gySsbsQ
> 8B6Kt0I03LWThuqDmjWQ%3D&reserved=0
> I'd like to discuss the possible ways to implement this.
> The GCC implementation is pretty complicated, touching almost everything, modifying optimizations, etc...:
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgith
> ub.com%2Fgcc-mirror%2Fgcc%2Fcommit%2F292237f3c109f2e0d2752ad1e73181689
> f7d0d53&data=02%7C01%7Cguybe%40mellanox.com%7C5eb16d1a2f174a2b0028
> 08d72a57811a%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C637024427646
> 321756&sdata=e2U1YX0Qnzl4AM1SNYIWIrJuQPi6JQmLBp%2FfbeJOxD8%3D&
> reserved=0
> I think, it would be better, to implement this attribute in the frontend only, adding the required byte-swaps early.

I agree. Adding the swaps in the frontend seems relatively straightforward. As I understand it, for non-packed structs, you also need to adjust the relative position of the padding?


> Any thoughts, ideas, objections?
> Thanks
>            Guy Benyei	
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Hal Finkel
Lead, Compiler Technology and Programming Languages Leadership Computing Facility Argonne National Laboratory

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