On Thu May 1, 2025 at 3:16 AM JST, Danilo Krummrich wrote: > On Wed, Apr 30, 2025 at 10:38:11AM -0400, Joel Fernandes wrote: >> On 4/30/2025 9:25 AM, Alexandre Courbot wrote: >> > On Tue Apr 22, 2025 at 11:44 PM JST, Danilo Krummrich wrote: >> >> >>> +/// Returns a boxed falcon HAL adequate for the passed `chipset`. >> >>> +/// >> >>> +/// We use this function and a heap-allocated trait object instead of >> >>> statically defined trait >> >>> +/// objects because of the two-dimensional (Chipset, Engine) lookup >> >>> required to return the >> >>> +/// requested HAL. >> >> >> >> Do we really need the dynamic dispatch? AFAICS, there's only E::BASE that >> >> is >> >> relevant to FalconHal impls? >> >> >> >> Can't we do something like I do in the following example [1]? >> >> >> >> [1] >> >> https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=bf7035a07e79a4047fb6834eac03a9f2 >> > >> > So are you have noticed there are two dimensions from which the falcons >> > can be instantiated: >> > >> > - The engine, which determines its register BASE, >> > - The HAL, which is determined by the chipset. >> > >> > For the engine, I want to keep things static for the main reason that if >> > BASE was dynamic, we would have to do all our IO using >> > try_read()/try_write() and check for an out-of-bounds error at each >> > register access. The cost of monomorphization is limited as there are >> > only a handful of engines. >> > >> > But the HAL introduces a second dimension to this, and if we support N >> > engines then the amount of monomorphized code would then increase by N >> > for each new HAL we add. Chipsets are released at a good cadence, so >> > this is the dimension that risks growing the most. > > I agree, avoiding the dynamic dispatch is probably not worth in this case > considering the long term. However, I wanted to point out an alternative with > [2]. > >> > It is also the one that makes use of methods to abstract things (vs. >> > fixed parameters), so it is a natural candidate for using virtual >> > methods. I am not a fan of having ever-growing boilerplate match >> > statements for each method that needs to be abstracted, especially since >> > this is that virtual methods do without requiring extra code, and for a >> > runtime penalty that is completely negligible in our context and IMHO >> > completely balanced by the smaller binary size that results from their >> > use. >> >> Adding to what Alex said, note that the runtime cost is still there even >> without >> using dyn. Because at runtime, the match conditionals need to route function >> calls to the right place. > > Honestly, I don't know how dynamic dispatch scales compared to static dispatch > with conditionals. > > OOC, I briefly looked for a benchmark and found [3], which doesn't look > unreasonable at a first glance. > > I modified it real quick to have more than 2 actions. [4] > > 2 Actions > --------- > Dynamic Dispatch: time: [2.0679 ns 2.0825 ns 2.0945 ns] > Static Dispatch: time: [850.29 ps 851.05 ps 852.36 ps] > > 20 Actions > ---------- > Dynamic Dispatch: time: [21.368 ns 21.827 ns 22.284 ns] > Static Dispatch: time: [1.3623 ns 1.3703 ns 1.3793 ns] > > 100 Actions > ----------- > Dynamic Dispatch: time: [103.72 ns 104.33 ns 105.13 ns] > Static Dispatch: time: [4.5905 ns 4.6311 ns 4.6775 ns] > > Absolutely take it with a grain of salt, I neither spend a lot of brain power > nor time on this, which usually is not a great combination with benchmarking > things. :) > > However, I think it's probably not too important here. Hence, feel free to go > with dynamic dispatch for this.
Indeed, it looks like the cost of dispatch will be completely shadowed by the IO behind it anyway. And these HAL calls are like a few here and there anyway, it's not like they are on a critical path. > >> I am just not seeing the benefits of not using dyn for >> this use case and only drawbacks. IMHO, we should try to not be doing the >> compiler's job. >> >> Maybe the only benefit is you don't need an Arc or Kbox wrapper? > > That's not a huge concern for me, it's only one single allocation per Engine, > correct? Correct. Note that for other engines we will be able to store the HALs as static singletons instead of building them on the heap like I am currently doing. The reason for doing this on falcon is that the dual-dimension of the instances makes it more complex to build and look them up. ... or maybe I could just use a macro? Let me try that and see whether it works.
