[PATCH] x86-64 ABI: unwrap single element structs / arrays of 256-bit vectors to pass and return in registers

[H.J. Lu]

On Mon, Feb 16, 2015 at 8:40 AM, hfinkel at anl.gov <hfinkel at anl.gov> wrote:
> In http://reviews.llvm.org/D7614#124323, @spatel wrote:
>
>> In http://reviews.llvm.org/D7614#124120, @hfinkel wrote:
>>
>> > What happens when you try to pass a struct of { v4f32 x[2]; }?
>>
>>
>> Great question. There's a check in classify() around here:
>>  https://github.com/llvm-mirror/clang/blob/master/lib/CodeGen/TargetInfo.cpp#L2044
>>
>>   // The only case a 256-bit wide vector could be used is when the struct
>>   // contains a single 256-bit element. Since Lo and Hi logic isn't extended
>>   // to work for sizes wider than 128, early check and fallback to memory.
>>   //
>>   if (Size > 128 && getContext().getTypeSize(i->getType()) != 256) {
>>
>>
>> And so we don't try to pass/return anything in registers with an array of two 128-bit vectors.
>>
>> And this also triggers on the even simpler case of:
>>  struct v4f32_wrapper {
>>
>>   v4f32 v1;
>>   v4f32 v2;
>>
>> };
>>
>> Based on my reading of the ABI, I would've thought that could be passed as 2 xmm registers, but we pass by memory for this too. FWIW, gcc 4.9 and icc 15 do the same in these cases, so I think this is just the way things are supposed to be.
>
>
> Okay, great. Please commit.
>
> Also, if the ABI spec differs from implementation practice, we should also send a note to whomever maintains that and try to get it clarified.
>

All x86-64 psABI questions should go to

https://groups.google.com/forum/#!forum/x86-64-abi


-- 
H.J.