llvm dev - Aug 2019 - Non-native endianness support in Clang

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://bugs.llvm.org/show_bug.cgi?id=35293

I'd like to discuss the possible ways to implement this.

The GCC implementation is pretty complicated, touching almost everything,
modifying optimizations, etc...:
https://github.com/gcc-mirror/gcc/commit/292237f3c109f2e0d2752ad1e73181689f7d0d53

I think, it would be better, to implement this attribute in the frontend only,
adding the required byte-swaps early.

Any thoughts, ideas, objections?

Thanks
          Guy Benyei

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://bugs.llvm.org/show_bug.cgi?id=35293
>
> I'd like to discuss the possible ways to implement this.
>
> The GCC implementation is pretty complicated, touching almost everything,
modifying optimizations, etc...:
>
https://github.com/gcc-mirror/gcc/commit/292237f3c109f2e0d2752ad1e73181689f7d0d53
>
> 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?

  -Hal

>
> Any thoughts, ideas, objections?
>
> Thanks
>            Guy Benyei	
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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.

Thanks
        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&amp;data=02%7C01%7Cguybe%40mella
> nox.com%7C5eb16d1a2f174a2b002808d72a57811a%7Ca652971c7d2e4d9ba6a4d1492
> 56f461b%7C0%7C0%7C637024427646321756&amp;sdata=Je2g85mVic2aq5H9gySsbsQ
> 8B6Kt0I03LWThuqDmjWQ%3D&amp;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&amp;data=02%7C01%7Cguybe%40mellanox.com%7C5eb16d1a2f174a2b0028
> 08d72a57811a%7Ca652971c7d2e4d9ba6a4d149256f461b%7C0%7C0%7C637024427646
>
321756&amp;sdata=e2U1YX0Qnzl4AM1SNYIWIrJuQPi6JQmLBp%2FfbeJOxD8%3D&amp;
> 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?

  -Hal

>
> Any thoughts, ideas, objections?
>
> Thanks
>            Guy Benyei	
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Flist
> s.llvm.org%2Fcgi-bin%2Fmailman%2Flistinfo%2Fllvm-dev&amp;data=02%7C01%
> 7Cguybe%40mellanox.com%7C5eb16d1a2f174a2b002808d72a57811a%7Ca652971c7d
> 2e4d9ba6a4d149256f461b%7C0%7C0%7C637024427646321756&amp;sdata=gk%2Fnh7
> laaSGjUxAXMixQ76jtTiRxJehLlG3b%2F7fLDOY%3D&amp;reserved=0
--
Hal Finkel
Lead, Compiler Technology and Programming Languages Leadership Computing
Facility Argonne National Laboratory