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Subject: Re: Seeking a better constexpr
From: Michael Spencer (bigcheesegs_at_[hidden])
Date: 2019-10-14 13:10:57


On Sun, Oct 13, 2019 at 9:58 PM Jeremy Ong via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> I wanted to write to seek broader feedback and relay my experience trying
> to leverage constexpr in a real-world situation where compile-time
> computation is a requirement for performance. By way of a brief background,
> I am working on an library that computes expressions over
> higher-dimensional data types. The math itself isn't important for the
> purposes of this discussion, but compile-time techniques allow me to
> "collapse" higher dimensional data into sparse representations without
> needing runtime annotation. When working with expressions, authors of ETLs
> know that expressions can explode dramatically.
>
> For example, if I encode a polynomial using templates (variables,
> monomials, and degrees) via parameter packs as some type P and later try to
> do a 4th order taylor expansion (just as an example), the number of terms
> will increase as O(N^4). I quickly found that templates were untenable due
> to memory and processing constraints (instantiating too many types, etc).
> This is *despite* aggressive usage of folds and ensuring that parameter
> packs had an imposed total order to minimize instantiations. Under this
> approach, is was astonishingly easy to completely cripple the compiler even
> for cases I would consider easy (clang profiling was tried, as well as
> general profiling tools). Again, this is code that runs perfectly fine at
> runtime, but the cost could easily be moved to compile time.
>
> Thus, the next option is to leverage constexpr, encoding expressions as
> actual member data, and using templates solely to encode the size of the
> data. Folds become loops, and in principle, compilation time and memory
> usage should go down. Here are the things I encountered along the way:
>
> 1. The compiler is "dumb" in the sense that data that is entirely compile
> time deterministic is not usable as such. Going back to the example of
> polynomials. Suppose the number of addends in each is N1 and N2. Upon
> addition, the sum is *at most *N1 + N2, however, it is possible that some
> of the monomials combine. Trying to express this using any form of
> constexpr is extremely difficult. Common strategies include traversing the
> data twice (once to compute the size, a second time to actually compute
> it). In this case, the data itself is not encoded in types, and so this is
> not possible (despite, again, all the data being compile-time
> deterministic).
>
> 2. I have found that constexpr can ironically decrease performance in a
> number of situations. Once code was restructured as constexpr, the compiler
> often fails to inline code that should be entirely inlineable. Suppose we
> want to actually attach data to what is now a compiled expression template.
> All early attempts to do so failed because the compiler was unable to infer
> that the instantiation of the function could only possibly be invoked in
> one place (and thus should be inlined no questions asked). Part of the
> issue here is that constexpr as a keyword conveys *very little
> information* regarding what will ultimately happen when invoked. By the
> time I restructured the code to peel away the non-constexpr argument that
> had snuck in, the compile times became untenable again (once again, more
> data needed to be pushed to types) and it was back to the drawing board.
>
> 3. The proper usage of `if constexpr` feels a bit lackluster. The operand
> must be fully determined to be a constant expression, but this immediately
> qualifies "data" that is, again, static at compile time. Constant
> expression arguments proposed for C++2b is an option, but it should seem
> that an if block gated by the `is_constant_evaluated` should preclude that
> (it doesn't). Another issue is that in more complicated functions, it can
> be very difficult to reason about the branches when constexpr and
> non-constexpr branches must intermingle.
>
> 4. Allowing allocations which do not leave the constexpr context via
> constexpr new is not actually helpful in several situations I can think of.
> At the end of the data, often, the data needs to be variably sized, and in
> this scenario, you cannot use auto return type deduction to read off the
> size as a constant expression and create the appropriately sized type that
> I need.
>
> Are there situations and workarounds to the above? Yes, but only after
> extensive rewrite and fighting the compiler every step of the way. Even at
> the end, I'm not sure if I will be getting the performance I could have
> gotten with just straight C-style procedural code for each operation I need
> to support. Granted, I think my application was perhaps a bit more extreme
> in its requirements, but I think it was a decent "sniff"-test for what I
> see as problems and headaches ahead. As it is, I'm doing things like
> rounding up capacities to the nearest size of two (and other such nonsense)
> to reduce the combinatorial explosion of type instantiations.
>
> *The idea I would like to float around is the following:*
>
> - Proposal: introduce a new function signature decorator called
> `consteval` (in the spirit of the variable consteval decorator) which is an
> input into overload resolution.
> - All other overload resolution decorators are evaluated first (const, ref
> qualifier, ADL), and then `consteval` is considered. If the invocation
> occurs at compile time, the `consteval` overload is given precedence.
> - All arguments passed to the consteval function are *implicitly
> constexpr *by definition
>

The only way to do this is to make that function a template and treat all
the arguments as non-type template parameters. See
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0992r0.pdf for why
this is a problem.

> - All if statements and control statements are similarly implicitly
> constexpr (even if the notion for such a construct doesn't exist now, i.e.
> there is no `for constexpr` as of yet).
> - Functions that are `consteval` can call non-consteval functions without
> any issue (it is a weaker qualification).
> - All allocations are also permitted applying the rules with the currently
> proposed `constexpr new`, except *they may leave *the constexpr context
> and reside in the data segment (analogous to static variable
> initialization). Initially, I would imagine that only trivial destructors
> are permitted (but still this would unlock a lot of capability).
>

We almost got this for C++20, but there are some significant issues to
solve here.

- Michael Spencer

> - All data mutated in the consteval context are also considered "constant
> expressions" when referenced later.
> - Ergo, consteval produces an easy-to-reason about "frontier" of constant
> expressions. In conjunction with consteval as a variable decorator, there
> is no guesswork as to what will happen at compile time and what won't
> - We dramatically simplify code and no longer need specific constexpr
> variants of various control structures and littered code with conditionals
> to activate "compile-time-only" code.
> - If we want to share functionality, we can introduce a common entry point
> that does not have the `consteval` qualifier.
>
> The theme of the idea is simple. Code that is C++ should just be C++, and
> how it behaves at compile time and runtime should differ by at most a
> keyword. I think such a feature is far easier to reason about (and to
> teach, just piggy-backing off general overload resolution principles),
> without overcrowding the API (at our current rate, we'll have a
> constexpr-paper variant for every language feature in C++). I would not
> propose deprecating any existing functionality immediately, but tbh, if I
> had something like `consteval` today, I would outright ban constexpr in my
> codebase.
>
> Important: I don't (even remotely) claim that the above has worked out all
> the details, or that there aren't issues I haven't thought of. These are
> just thoughts that I kept coming back to now that I've had much more
> extensive experience working with constexpr (compared to just dotted
> isolated cases in the wild before). My standardese is weak, having never
> written a paper, so consider this primarily as a perspective from the lens
> of a practitioner.
>
> Thoughts/feedback appreciated,
> Jeremy
>
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