> GC/runtime

1. Runtime: A runtime is any code that is not a direct result of compiling the program's code (i.e. it is used across different programs) that is linked, either statically or dynamically, into the executable. I remember that when I learnt C in the eighties, the book said that C isn't just a language but a rich runtime. Rust also has a rich runtime. It's true that you can write Rust in a mode without a runtime, but then you can barely even use strings, and most Rust programs use the runtime. What's different about Java (in the way it's most commonly used) isn't that it has a runtime, but that it relies on a JIT compiler included in the runtime. A JIT has pros and cons, but they're not a general feature of "a runtime".

2. GC: A garbage collector is any mechanism that automatically reuses a heap object's memory after it becomes unreachable. The two classic GC designs, reference counting and tracing, date back to the sixties, and have evolved in different ways. E.g. in the eighties and nineties there were GC designs where the compiler could either infer a non-escaping object's lifetime and statically insert a `free` or have the language track lifetimes ("regions", 1994) and have the compiler statically insert a `free` based on information annotated in the language. On the other hand, in the eighties Andrew Appel famously showed that moving tracing collectors "can be faster than stack allocation". So different GCs employ different combination of static inference and dynamic information on object reachability to optimise for different things, such as footprint or throughput. There are tradeoffs between having a GC or not, and they also exist between Rust (GC) and Zig (no GC), e.g. around arenas, but most tradeoffs are among the different GC algorithms. Java, Go, and Rust use very different GCs with different tradeoffs.

So the problem with using the terms "runtime" and "GC" colloquially as they're used today is not so much that it differs from the literature, but that it misses what the actual tradeoffs are. We can talk about the pros and cons of linking a runtime statically or dynamically, we can talk about the pros and cons of AOT vs. JIT compilation, and we can talk about the pros and cost of a refcounting/"static" GC algorithm vs a moving tracing algorithm, but talking in general about having a GC/runtime or not, even if these things mean something specific in the colloquial usage, is not very useful because it doesn't express the most relevant properties.

▲

ceteia 3 hours ago | parent [-]

Does Rust really require reference counting? I thought Rust programs only used reference counting when types like Rc and Arc are used.

Swift seems to require reference counting significantly more than Rust.

▲

brabel 3 hours ago | parent [-]

Op saying Rust has a kind of GC is absurd. Rust keeps track of the lifetime of variables and drops them at the end of their lifecycle. If you really want to call that a GC you should at least make a huge distinction that it works at compile time: the generated code will have drop calls inserted without any overhead at runtime. But no one calls that a GC.

You see OP is trying to murk the waters when they claim C has a runtime. While there is a tiny amount of truth to that, in the sense that there’s some code you don’t write present at runtime, if that’s how you define runtime the term loses all meaning since even Assemblers insert code you don’t have to write yourself, like keeping track of offsets and so on. Languages like Java and D have a runtime that include lots of things you don’t call yourself, like GC obviously, but also many stdlib functions that are needed and you can’t remove because it may be used internally. That’s a huge difference from inserting some code like Rust and C do. To be fair, D does let you remove the runtime or even replace it. But it’s not easy by any means.

▲

pron 2 hours ago | parent [-]

> If you really want to call that a GC you should at least make a huge distinction that it works at compile time: the generated code will have drop calls inserted without any overhead at runtime. But no one calls that a GC.

Except for the memory management literature, because it's interested in the actual tradeoffs of memory management. A compiler inferring lifetimes, either automatically for some objects or for most objects based on language annotations, has been part of GC research for decades now.

The distinction of working at compile time or runtime is far from huge. Working at compile time reduces the work associated with modifying the counters in a refcounting GC in many situations, but the bigger differences are between optimising for footprint or for throughput. When you mathematically model the amount of CPU spent on memory management and the heap size as functions of the allocation rate and live set size (residency), the big differences are not whether calling `free` is determined statically or not.

So you can call that GC (as is done in academic memory management research) or not (as is done in colloquial use), but that's not where the main distinction is. A refcounting algorithm, like that found in Rust's (and C++'s) runtime is such a classic GC that not calling it a GC is just confusing.

	▲	ceteia 24 minutes ago \| parent [-]
		> A refcounting algorithm, like that found in Rust's (and C++'s) runtime is such a classic GC that not calling it a GC is just confusing. But is it not easy to opt out of in C, C++, Zig and Rust, by simply not using the types that use reference counting? And how does your performance analysis consider techniques like arenas and allocating at startup only?