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Hello,</div>
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I would like to revive this thread.</div>
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We started thinking about supporting complex number support, for the same reason Florian stated in his first mail. The idea before I googled and stumbled on this thread, was the same as Florian proposed. So yes, I would like to support this approach.
<span id="🙂">🙂</span> I would like to add that GCC's approach is the same/similar: complex number patterns are recognised in the SLP vectoriser, and complex number builtins are emitted (and then matched later).
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Any objections if we go for this approach? Florian, any plans to pick this up?<br>
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Cheers,</div>
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Sjoerd.<br>
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<div id="divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Steve (Numerics) Canon via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> 04 December 2020 19:23<br>
<b>To:</b> David Greene <greened@obbligato.org><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] Complex proposal v3 + roundtable agenda</font>
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<div class="PlainText">(Late to the party)<br>
<br>
I think there’s a lot of good questions in this thread, but I also want Florian to get started landing some patches and then everyone can iterate on that.<br>
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Slight pushback on this one question:<br>
<br>
<br>
> On Nov 19, 2020, at 18:11, Cameron McInally via llvm-dev <llvm-dev@lists.llvm.org> wrote:<br>
<br>
>> On Wed, Nov 18, 2020 at 4:47 PM Krzysztof Parzyszek via llvm-dev<br>
>> <llvm-dev@lists.llvm.org> wrote:<br>
>> 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).<br>
> <br>
> Is that universally true? I think it depends on the target. Let's take<br>
> Florian's FCMLA example. The inputs and output are interleaved. And if<br>
> you need just the reals/imags from an interleaved vector for something<br>
> else, LD2/ST2 should be pretty fast on recent chips.<br>
<br>
FCMLA makes it so _if your data is already interleaved and you can’t change it_, you can still operate efficiently in that format. But if you have control, and you’re doing anything beyond basic arithmetic, it’s still advantageous to use a planar [SoA] layout
instead of interleaved [AoS]. For a simple example, if you want to vectorize a complex exponential function, you’ll want to compute cos and sin of the imaginary parts, and exp of the real parts—FCMLA doesn’t help here.<br>
<br>
So Florian’s proposal:<br>
<br>
> In the short-term I think to get things rolling it would be good to focus on the layout as defined by frontends (e.g. Clang/C++).<br>
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Sounds great to me.<br>
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– Steve<br>
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