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

I have looked more closely at the messages generated by the gcc 4.3 vectorizer 
and it seems that they fall into two categories:

1) complaining about aligmnent.

For example:

Unknown alignment for access: D.33485
Unknown alignment for access: m

I don't understand, as all my data is statically allocated doubles (no dynamic 
memory allocation) and I am using -malign-double. What more can I do?

2) complaining about "possible dependence" between some data and itself


not vectorized, possible dependence between data-refs 
m.m_storage.m_data[D.43225_112] and m.m_storage.m_data[D.43225_112]

I am wondering what to do about all that? Surely there must be documentation 
about the vectorizer and its messages somewhere but I can't find it?


On Monday 17 March 2008 15:59:21 Richard Guenther wrote:
> On Mon, Mar 17, 2008 at 3:45 PM, Benoît Jacob <> 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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