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

[Joan Lluch via cfe-dev]

This has been tracked down to a bug in LLVM FindOptimalMemOpLowering function that affects all targets not supporting misaligned memory accesses (such as MSP430 and mips16)

The entire discussion starts here:

http://lists.llvm.org/pipermail/llvm-dev/2019-May/132349.html

John Lluch


> On 16 May 2019, at 10:49, llvm at dyatkovskiy.com wrote:
> 
> Depending on your target, alignment may be increased by final code generation (IR -> native). Did you check final code?
>  
> As radical measure you can hack your clang.
>  
> Namely, artificially add "Aligned" attr to your structure declaration (which is CXXRecordDecl).
>  See "handleAlignedAttr" static method. And then "Sema::AddAlignedAttr". Latter should be called with some fake params, for it's not user defined.
>  
> Or may be you could alter DataLayout for your target somehow.
>  
> P.S.: If you're under C++11 or higher you always can use "alignas" in cpp code.
> P.S.2: But the goal is weird. If I got right, you rely on target alignment and then fullfill gaps between structures with some data?
>  
> Stepan Dyatkovskiy
>  
> 13.05.2019, 20:47, "Joan Lluch via cfe-dev" <cfe-dev at lists.llvm.org>:
> My target implementation has 2 byte (16 bit ints) 
> For my target implementation I would want that Clang would always use at least 2 byte aligned padding for Structs, which would match the size of an int. 
>  
> The current situation is that for this struct:
>  
> struct AA
> {
>   char n;
>   char m;
>   char j;
> };
>  
> I get it aligned by 1:
>  
> %a = alloca %struct.AA, align 1     
>  
> I would want it to implicitly use 4 bytes instead of 3, (or it be aligned by 2 bytes instead of 1).
>  
> If I replace the above struct by this:
>  
> struct AA
> {
>   int n;
>   char m;
> };
>  
> or this:
>  
> struct AA
> {
>   char n;
>   char m;
>   char j;
>   char k
> };
> 
>  
> I correctly get it aligned by 2:
>  
> %a = alloca %struct.AA, align 2
>  
> or:
>  
> %a = alloca i32, align 2
> %tmpcast = bitcast i32* %a to %struct.AA*
>  
> In summary, I noticed that Clang will compute struct alignments as an int (2 bytes) if it has one, or if the struct size is already a multiple of 2, but not in other cases.
>  
> How do I change that behaviour to get structs always (at least) 2 byte aligned ?
>  
> John Lluch
> ,
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