[LLVMdev] First class aggregates of small size: split when used in function call

[Reid Kleckner]

The current recommendation for matching external C ABIs is actually "use
Clang", as in literally link it into your program and try to leverage the
methods of clang::CodeGen::CodeGenFunction. This is the only way to get the
lowering 100% correct, but it's a lot of work, so depending on your needs,
you may want to roll your own lowering of high-level function prototype to
LLVM function prototype.

If you roll your own, then LLVM generally passes FCAs as though they were
split into constituent elements. I think ARM does something different here,
though. :(

Clang sometimes uses integers of appropriate size on non-x86 architectures
to try to model the usage of an integer register to pass the whole struct.
For a small struct, this is good because element accesses can be
transformed into shifts, masks, and truncs.

If you just want to match MSVC's C ABI, byval is probably the way to go.
LLVM is not very good at optimizing it, but it will do the right thing.
Splitting into i32-sized chunks would also work.

On Tue, Dec 30, 2014 at 11:41 PM, Virgile Bello <virgile.bello at gmail.com>
wrote:

> Hello,
>
> In my LLVM frontend (CLR/MSIL), I am currently using first-class
> aggregates to represent loaded value types on the "CLR stack".
>
> However, I noticed that when calling external method taking those
> aggregate by value, they were not passed as I expected:
>
> %COLORREF = type { i8, i8, i8, i8 }
>
> declare i32 @SetLayeredWindowAttributes(i8*, %COLORREF, i8, i32)
> I call this function with call x86_stdcallcc (it's a Win32 function,
> loaded with GetProcAddress)
>
> However, checking the assembly code, it seems that the %COLORREF gets
> split due to the calling convention: first i8 field go through %edx, but
> the 3 next fields go through the stacks.
> I would like all of it to go through either a single 32bit register or a
> 32bit stack value (since all of the structure fits in a i32 and it is
> already packed in memory that way before the call).
>
> I was thinking using alloca with sret/byval might help, but I am not even
> sure since it is enough, since clang also seems to actually use i16 or i32
> (and even i32+i16 or i32+i32) to represent such struct <= 8 bytes  when
> passing them to a method (even if they contain many smaller i8 fields).
>
> Does somebody know if only alloca with sret/byval is enough or if I also
> need to concat myself smaller struct into i32 types like clang does to be
> sure it won't be split across registers?
> Any other hint or idea on how I can achieve this?
>
> Also, I was wondering which is the current recommendation (sret/byval with
> alloca for every copy vs first-class aggregate) considering the current
> state of LLVM and supported optimizations. Since clang uses sret/byval, I
> expect it to be more optimized/mature, but I might be wrong.
>
> I suppose LLVM will easily understand/optimize all those additional
> aggregate alloca/memcpy I will end up doing if I were to switch to a
> sret/byval approach?
>
> Thanks,
>
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