What are "safe point functions" as alluded to in the Go source code?

In the file runtime/proc.go of the Go source code, there are many comments referring to safe point functions, which appear to be related to points in time where it is safe to garbage collect. However, I could not find any definitions of these functions.

What are safe point functions, what are they used for, and what are some examples of such functions?

Here is everything I could dig up on this topic.

I found some discussion about Go's GC safepoints here.

It looks like safe points (as used in the Go implementation) are effectively the same as the traditional definition of a safe point:

> key points where the GC can track what all variables and registers
> hold

Another user on the same thread mentions that the GC

> folds the preemption points into the stack checks during function
> prologs

and that the source for this claim is the following: https://github.com/golang/go/issues/10958

According to this post on the golang-dev mailing list, safepoints are also called "call sites".

To gain some insight into the nature of Go's safepoints, I think it's important to look at the evolution of its GC. This post mentions the following:

> Prior to Go 1.5, Go has used a parallel stop-the-world (STW)
> collector.
>
> Go 1.5 introduces a concurrent collector.

Another answer to that question mentions:

> As of Go 1.7, the one remaining source of unbounded and potentially
> non-trivial stop-the-world (STW) time is stack re-scanning.

As of 1.8, it looks like worst-case stop-the-world times have been improved.

Also, here is the current implementation of Go's garbage collector. If you read through the comments, you'll see that it is a non-generational mark-sweep collector. You can read more about this here: https://blog.plan99.net/modern-garbage-collection-911ef4f8bd8e#.udz1kjk3b

Finally, here is an old post by Gil Tene on the golang-dev mailing list which motivates the use of a moving garbage collector. He claimed (at the time in 2012) that Go was using a "conservative, non-relocating collector" and he discussed the characteristics of safepoints which would permit a long-running garbage collector.

While Go's garbage collector has been moving away from long pauses and is now a "concurrent, tri-color, mark-sweep collector", it's still a non-generational GC. It also looks like Go's GC is building on GC ideas from the 70s rather than modern, enterprise approaches.

It looks like Go's notion of a safepoint is more in line with the traditional notion of a safepoint rather than a safepoint with modern qualities which permit generational garbage collection.