(buy me a drink and we’ll talk about the size of the zero in zero cost exceptions)
Conversation
Yeah I’d take you up on that (if not for the big ocean in the way)! There’s definitely a cost to zero cost. I’m also interested to see if we can define what people mean by ‘zero cost’ more precisely. had some interesting ideas on the Pikelet Gitter actually.
1
2
It was here: gitter.im/pikelet-lang/L
The notion is “indirections that the compiler can see through”
1
I don't think it's a very meaningful term tbh. It's close to a definition but if you're going to allow translation only "as good as if by hand" you get into a weird place: by hand an author would write different code, not the numerous as-if-by-hand instances the compiler emits.
2
2
I agree it’s not per se terribly meaningful, but I need a similar concept as a touchstone for thinking about my language design—what are the criteria by which I judge perf-sensitive features?—so I need a working definition that does mean something in the context of that design
1
1
There’s a subtlety, I think, between “would write” (willingly) and “could write” (conceivably, but it’s tedious and error-prone)—it’s more the latter that I care about: automatic rearrangement of the higher-level code I want to read & write into the lower-level code I want to run
1
1
Some interesting barometers:
- would I be willing to lean on this abstraction in a tight inner loops?
- does the optimization that this abstraction lean on break easily/silently?
- how does this it impact compilation time?
- how much damage does it do to the UX?
1
1
If you're willing to talk about compilation time or other not-just-runtime-microbenchmark costs in your cost model (hello ABI stability / version-stable separate compilation! hello user cognitive load / predictability!) the picture gets _much_ murkier.
2
3
Yeah it's an interesting landscape. I bring it up because often 'zero cost' stuff sacrifices it. I'm just interested in having a healthier, more honest design discussion.
1
Even if the _runtime_ cost model is just a bit more complex -- eg. if you start wondering whether all the specialized copies or unrolling/inlining is hurting cache -- you lose sight of clear answers. Speculative devirt, polymorphic inline cache, trace-jit: ??? heuristic-ville
2
5
Hey, I just want to say, I really appreciate these responses. Lots of really good food for though. Thanks! 🥰
It's been on my mind a lot lately when thinking of where to _not_ spend optimization energy in new languages. I think the perspective of staging / specializing is helpful. Like consider your compiler as deciding not just "abstractions to look through" but values to specialize-on.
1
3
It's tempting to think that a compiler is only looking-through abstractions; but in many cases it's also baking-in (possibly to a partial replica) assumptions made when looking-through, and those carry costs both for the replicas and for handling cases when assumptions are wrong.
1
1
Show replies