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

Sanjay Patel spatel at rotateright.com
Mon Feb 16 08:28:10 PST 2015

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:

  // 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.



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