[llvm-dev] Complex proposal v3 + roundtable agenda

[Cameron McInally via llvm-dev]

On Wed, Nov 18, 2020 at 4:47 PM Krzysztof Parzyszek via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Complex type would pose another issue for vectorization: in general it's better to have a vector of the real parts and a vector of the imaginary parts for the purpose of arithmetic, than having vectors of complex elements (i.e. real and imaginary parts interleaved).

Is that universally true? I think it depends on the target. Let's take
Florian's FCMLA example. The inputs and output are interleaved. And if
you need just the reals/imags from an interleaved vector for something
else, LD2/ST2 should be pretty fast on recent chips.

On the other hand, if we had a non-interleaved complex representation
and wanted to use FCMLA, we'd need some number of zips and unzips to
interleave and deinterleave between the load and store. Those probably
aren't cheap in aggregate.

I haven't studied this across all targets, but my intuition says we
should leave the representation decision up to the targets. Maybe we
should have a larger discussion about it.

Although, it's worth noting that predication would likely be *much*
easier with a non-interleaved representation. I think. Again, I
haven't thought this completely through, but it's probably worth
talking about.