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Re: guile 3 update: instruction explosion for pairs, vectors

From: Mikael Djurfeldt
Subject: Re: guile 3 update: instruction explosion for pairs, vectors
Date: Fri, 19 Jan 2018 15:02:59 +0100

On Tue, Jan 16, 2018 at 4:55 PM, Andy Wingo <address@hidden> wrote:
Thinking more globally, there are some more issues -- one is that
ideally we need call-site specialization.  A GF could be highly
polymorphic globally but monomorphic for any given call site.  We need
away to specialize.

Yes, but I imagine that the gain of having polymorphic inline caches compared to just GFs will decrease the more that type dispatch can be eliminated from compiled method bodies (the monomorphic case will be replaced by a direct call of the IM (compiled method)). Then, of course, a polymorphic inline cache can perhaps be regarded as an anonymous GF such that there isn't really much difference.

Dynamically typed data structures would be the remaining main source of type dispatch.

Secondly, it would be nice of course to have speculative optimization,
including speculative inlining and type specialization not only on GF
arguments but also arguments to regular calls, types of return values,
and so on.

Yes! I think that dispatch on the return values is interesting.

What I'm now going to write is based on almost zero knowledge of compiler construction, and I still will have to learn the Guile compiler infrastructure (where is a good start?), so please bear with me. For the same reason, what I write might be completely obvious and well-known already.

Imagine that we are compiling the body of a method and we arrive at an integer addition. At some step of compilation, there has been a conversion to CPS such that we (at some level) can regard the operation as:

 (+ a b k)

where k is the continuation. This means that k will be called with the result of the addition:

  (k <sum>)

In a framework where essentially all procedures are GFs, also k, here, is a GF. This allows it to dispatch on its argument such that it can treat inums, bignums, floats and doubles differently.

Note now that in such a framework, a GF might have only one method (M) but the instantiated/compiled methods (IMs) can still be many. If k above is called with an inum, there will be an IM of k which is specialized to inums. This means that the compiler later can choose operations relevant for inums inside k.

The "exploded" (in a slightly different sense) code for + above might, in turn, contain a branch which handles the transition into bignums at "inum overflow". If now k above come to occur in a branch of the +-code, inlined in an outer function, where the argument of k is guaranteed to be an inum, then our GF dispatch elimination will replace k with with the k-IM for inums. So, the *only* branch remaining in the code will be the overflow check in +. (BTW, I wonder if this inlining/specialization to an outer function could in some sense rely on type dispatch on the continuation k?)

Finally I wonder that if we had the latter, if it matters so much about
optimizing generic functions in any kind of specific way -- instead you
could just have a generic optimizer.

Yes. I guess I'm mostly using my GFs as a "hook" for my thoughts on this. The reason I do this is that I can imagine a reasonably simple implementation where (almost) everything is a GF. :) There, the GFs would, in some sense, work as data structures for the compiler/optimizer.

Of course the speculative optimizer could work on traces instead of
methods, and in that case we'd get a lot of this stuff for free... but
that's a question for farther down the road.  See

Yes, this is very nice and exciting work. :)

Best regards,

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