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Subject: Re: sg7 varid proposal
From: Ville Voutilainen (ville.voutilainen_at_[hidden])
Date: 2020-12-21 17:43:59


On Tue, 22 Dec 2020 at 01:03, Dominic Jones <dominic.jones_at_[hidden]> wrote:
>
> Thank you for your comments.
>
> Regarding the first: my use case is compile-time automatic differentiation (AD). I gave a talk on the approach at a London C++ meetup, and at a Euro AD workshop [slides].
>
> The gist of it is that I want to be able to evaluate an expression from its root to its terminals. This is the opposite of what one normally wishes to do, but it just is the nature of the so-called 'adjoint' method of automatic differentiation. Doing this naively, one ends up performing duplicate evaluations when a term is used repeatedly.
>
> Consider the code corresponding to Figure 1 (I now realise I should have included the code snippet in the paper!): the term 'ab' occurs twice in the expression for 'result'. In typical expression evaluation, 'ab' is evaluated once then its result used in many places. However, in back-propagation evaluations (like those found in adjoint automatic differentiation), evaluation begins at 'result' and propagates to the terminals 'a' and 'b'. The problem is that the back-propagation will visit the sub-expression tree for 'ab' twice since it occurs twice upstream.
>
> float c0;
> float c1;
> auto ab = a * b;
> auto result = (c0 + ab) / (c1 + ab);
>
> Regarding the second point: I came across Andrew Sutton's paper, P2237R0, after submitting my own. Whilst skimming it I saw something that may be doing what I am proposing, namely 'meta::location_of(e)', p15 ff. Are the reflection mechanisms you are referring to those mentioned by Andrew in his paper or are you referring to another language proposal? If it is another, I'd be grateful if I could get a link to the paper.
>
> I agree that if there are not sufficiently broad use case examples for my proposal then it does not warrant further consideration; amassing a broad range of use cases is something I still need to work on. (Klaus Iglberger, I believe, has a use case for his math library 'Blaze' which I need to ask him for.) However, presently, I don't know how the same end could be achieved without a top level API function. Early on, I thought it could be by proposing a language extension whereby `address of' in a constexpr context would return a constexpr variable ID, much like what varid would do. However, even with this, a second language change would need to be made, one whereby a constexpr function could return a constexpr result despite taking non-constexpr arguments (i.e. resolving the compilation error presented in Listing 3).
>
> In response to your final comment: unfortunately, only having equality wouldn't do; it would be minimally useful, facilitating expression transformations for only trivial expressions, such as 'c0*c1' or 'c0*c0', but nothing more complicated than that.

Interesting, very interesting. I think this is certainly worth further
study, considering that we are in a Study Group.
It's also a very interesting use in general, something that pushes the
boundaries of what C++ can do, which is
a sizeable part of what we're trying to do here anyway. :)

So yeah, the reflection mechanisms I referred to are indeed what's
catalogued by P2237. I have two further questions:

1) What do you use to solve such problems today?
2) Would the introduction of this language facility perhaps, vaguely
guessing at a possible answer to (1), to be able to
avoid using a compiler-specific solution or a custom parser?

A further remark: we are yet fairly far from being able to introspect
and analyze function bodies, sequences of statements
and expressions, that is. Based on a brief look at the talk this might
fall into that bucket that we hope to get at later
on, but perhaps there's something about the answer to the question (1)
above that makes this more straightforward
than full expression/statement-reflection?


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