[llvm-dev] How to change CLang struct alignment behaviour?

Joan Lluch via llvm-dev llvm-dev at lists.llvm.org
Sun May 12 23:53:13 PDT 2019

Hi Tim,

Thanks for your reply. That’s what I was afraid of. I will try on the cfe-list as you suggest though.

The reason I want structs to be aligned/padded to 2 bytes is because my architecture only has 16 bit operations. I can read (sign and zero extended) and write (truncated) 8 bit data from/to memory, but all intermediate operations in registers are performed in 16 bit registers. This causes LLVM to generate odd tricks such as shifts and byte-swaps, when trying to replace struct ‘memcpy’s by word sized load/store instructions. For 16 bit aligned structs the ‘memcpy’ replacement code is much cleaner. That’s the reason I would want structs to be always 16 bit aligned/padded. 

Joan Lluch
Puigsacalm, 7 
17458 - Fornells de la Selva

Tel: 620 28 45 13

> On 13 May 2019, at 08:36, Tim Northover <t.p.northover at gmail.com> wrote:
> Hi Joan,
> On Sun, 12 May 2019 at 21:02, Joan Lluch via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> How do I change that behaviour to get structs always (at least) 2 byte aligned ?
> I don't think there's a feature you can toggle for this (except,
> maybe, making the alignment of every basic type 2 bytes; but that
> would obviously affect arrays and even normal variables too). So you
> probably have to modify lib/AST/RecordLayoutBuilder.cpp directly.
> Might be worth asking this on the cfe-dev mailing list though. That's
> where most of the Clang experts live.
> Cheers.
> Tim.

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