Re: [eigen] How do you link multiple versions (e.g. AVX vs SSE) of the same Eigen code?

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi,

I thought that we once (informally) agreed on statically aligning to 16 
bytes only (also because AVX is a bit more relaxed regarding alignment), 
but do 32 byte alignment for dynamic objects. Admittedly, that was long 
time ago before we had a working AVX implementation. Also, I did not yet 
look much into the details of the current AVX code.

But I agree with Benoit that losing ABI compatibility is severe enough 
to make 32 byte alignment not the default when compiling for AVX. IMO 
the best solution would be to define the desired alignment by using an 
option flag. Then it is still debatable what shall be the default 
behavior. Requiring 32 byte alignment if the object size is a multiple 
of 32 can still destroy ABI compatibility, e.g., when having 
to-be-aligned objects in a struct which requires padding bytes.

Fortunately, so far AVX is only in the devel-branch, so it's not too 
late to re-discuss any design decisions.


Christoph


Am 28.01.2015 um 00:30 schrieb Benoit Jacob:
> Benoit, Gael, could you confirm that this change is intentional?
>
> Pro:
>    improves perf of certain 4d ops
> Con:
>   - regresses perf of 4f ops, as now Vector4f isn't aligned anymore!!!!
>   - breaks long-standing invariant that Eigen ABI is independent of compile
> flags
>
> Benoit
>
> 2015-01-27 18:22 GMT-05:00 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
>
> > Ouch... you're right. From Macros.h:
> >
> >    #if defined(__AVX__)
> >      #undef EIGEN_ALIGN_BYTES
> >      #define EIGEN_ALIGN_BYTES 32
> >    #endif
> >
> >
> >
> > 2015-01-27 18:19 GMT-05:00 Benoit Jacob <jacob.benoit.1@xxxxxxxxx>:
> >
> > Oh --- I was still living in a world where the Eigen ABI is unaffected by
> > > vectorization, which was the case at least until AVX.
> > >
> > > Is alignof(Vector4d) actually 32 with AVX by default? So ABI is now
> > > dependent on compile flags?
> > >
> > > Benoit
> > >
> > > 2015-01-27 18:17 GMT-05:00 Marc Glisse <marc.glisse@xxxxxxxx>:
> > >
> > > On Tue, 27 Jan 2015, Benoit Jacob wrote:
> > > >   An approach that works is to #define Eigen EigenAVX, but this is very
> > > > > annoying because you can't anymore pass around Eigen objects, you have
> > > > > to
> > > > > somehow reinterpret_cast them to EigenAVX objects.
> > > >
> > > > I would expect EigenAVX::Vector4d to have an alignment of 32, and only
> > > > 16 for EigenSSE::Vector4d (I did not check), which would mean they really
> > > > are different types and you cannot pass the second one to a function
> > > > expecting the first. So I am not sure how you intend to "pass around Eigen
> > > > objects".
> > > >
> > > > --
> > > > Marc Glisse


--
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Dipl.-Inf., Dipl.-Math. Christoph Hertzberg
Cartesium 0.049
Universität Bremen
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