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.
http://reviews.llvm.org/D7614
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