Re: Auto-vectorization: need to know what to expect

[Benoît Jacob <jacob at math dot jussieu dot fr>]

Thanks Richard for the answer.

It sounds like it's worth betting on gcc's autovectorizer and submitting bug 
reports -- so expect to hear again from us :)

Cheers,
Benoît

On Monday 17 March 2008 15:59:21 Richard Guenther wrote:
> On Mon, Mar 17, 2008 at 3:45 PM, Benoît Jacob <jacob@math.jussieu.fr> wrote:
> > Dear All,
> >
> >  I am currently (co-)developing a Free (GPL/LGPL) C++ library for
> > vector/matrix math.
> >
> >  A major decision that we need to take is, what to do regarding
> > vectorization instructions (SSE). Either we rely on GCC to
> > auto-vectorize, or we control explicitly the vectorization using GCC's
> > special primitives. The latter solution is of course more difficult, and
> > would to some degree obfuscate our source code, so we wish to know
> > whether or not it's really necessary.
> >
> >  GCC 4.3.0 does auto-vectorize our loops, but the resulting code has
> > worse performance than a version with unrolled loops and no
> > vectorization. By contrast, ICC auto-vectorizes the same loops in a way
> > that makes them significantly faster than the unrolled-loops
> > non-vectorized version.
> >
> >  If you want to know, the loops in question typically look like:
> >  for(int i = 0; i < COMPILE_TIME_CONSTANT; i++)
> >  {
> >         // some abstract c++ code with deep recursive templates and
> >         // deep recursive inline functions, but resulting in only a
> >         // few assembly instructions
> >         a().b().c().d(i) = x().y().z(i);
> >  }
> >
> >  As said above, it's crucial for us to be able to get an idea of what to
> >  expect, because design decisions depend on that. Should we expect large
> >  improvements regarding autovectorization in 4.3.x, in 4.4 or 4.5 ?
>
> In general GCCs autovectorization capabilities are quite good, cases
> where we miss opportunities do of course exist.  There were improvements
> regarding autovectorization capabilities in every GCC release and I expect
> that to continue for future releases (though I cannot promise anything
> as GCC is a volunteer driven project - but certainly testcases where we
> miss optimizations are welcome - often we don't know of all corner cases).
>
> If you require to get the absolute most out of your CPU I recommend to
> provide special routines tuned for the different CPU families and I
> recommend the use of the standard intrinsics headers (*mmintr.h) for
> this.  Of course this comes at a high cost of maintainance (and initial
> work), so autovectorization might prove good enough.  Often tuning the
> source for a given compiler has a similar effect than producing vectorized
> code manually.  Looking at GCC tree dumps and knowing a bit about
> GCC internals helps you here ;)
>
> >  A roadmap or a GCC developer sharing his thoughts would be very helpful.
>
> Thanks,
> Richard.

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