Re: Complex numbers support: discussions summary

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Sep 25, 2023 at 5:17 PM Sylvain Noiry via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi,
>
> We had very interesting discussions during our presentation with Paul on
> the
> support of complex numbers in gcc at the Cauldron.
>
> Thank you all for your participation !
>
> Here is a small summary from our viewpoint:
>
> - Replace CONCAT with a backend defined internal representation in RTL
> --> No particular problems
>
> - Allow backend to write patterns for operation on complex modes
> --> No particular problems
>
> - Conditional lowering depending on whether a pattern exists or not
> --> Concerns when the vectorization of split complex operations performs
> better
>     than not vectorized unified complex operations
>
> - Centralize complex lowering in cplxlower
> --> No particular problems if it doesn't prevent IEEE compliance and
>     optimizations (like const folding)
>
> - Vectorization of complex operations
> --> 2 representations (interleaved and separated real/imag): cannot
> impose one
>     if some machines prefer the other
> --> Complex are composite modes, the vectorizer assumes that the inner
> mode is
>     scalar to do some optimizations (which ones ?)
> --> Mixed split/unified complex operations cannot be vectorized easely
> --> Assuming that the inner representation of complex vectors is let to
> target
>     backends, the vectorizer doesn't know it, which prevent some
> optimizations
>     (which ones ?)
>
> - Explicit vectors of complex
> --> Cplxlower cannot lower it, and moving veclower before cplxlower is a
> bad
>     idea as it prevents some optimizations
> --> Teaching cplxlower how to deal with vectors of complex seems to be a
>     reasonable alternative
> --> Concerns about ABI or indexing if the internal representation is let
> to the
>     backend and differs from the representation in memory
>
> - Impact of the current SLP pattern matching of complex operations
> --> Only with -ffast-math
> --> It can match user defined operations (not C99) that can be
> simplified with a
>     complex instruction
> --> Dedicated opcode and real vector type choosen VS standard opcode and
> complex
>     mode in our implementation
> --> Need to preserve SLP pattern matching as too many applications
> redefines
>     complex and bypass C99 standard.
> --> So need to harmonize with our implementation
>
> - Support of the pure imaginary type (_Imaginary)
> --> Still not supported by gcc (and llvm), neither in our implementation
> --> Issues comes from the fact that an imaginary is not a complex with
> real part
>     set to 0
> --> The same issue with complex multiplication by a real (which is split
> in the
>     frontend, and our implementation hasn't changed it yet)
> --> Idea: Add an attribute to the Tree complex type which specify pure
> real / pure
>     imaginary / full complex ?
>
> - Fast pattern for IEEE compliant emulated operations
> --> Not enough time to discuss about it
>
> Don't hesitate to add something or bring more precision if you want.
>
> As I said at the end of the presentation, we have written a paper which
> explains
> our implementation in details. You can find it on the wiki page of the
> Cauldron
> (https://gcc.gnu.org/wiki/cauldron2023talks?action=AttachFile&do=view&target=Exposing+Complex+Numbers+to+Target+Back-ends+%28paper%29.pdf).

Thanks for the detailed presentation at the Cauldron.

My personal summary is that I'm less convinced delaying lowering is
the way to go.
I do think that if targets implement complex optabs we should use them but
eventually re-discovering complex operations from lowered form is going to be
more useful.  That's because as you said, use of _Complex is limited and people
inventing their own representation.  SLP vectorization can discover some ops
already with the limiting factor being that we don't specifically search for
only complex operations (plus we expose the result as vector operations,
requiring target support for the vector ops rather than [SD]Cmode operations).

There's the gimple-isel.cc or the widen-mul pass that perform
instruction selection
which could be enhanced to discover scalar [SD]Cmode operations.

Richard.

> Sylvain
>
>
>
>
>