Why many programmers don't like GC?

[Ola Fosheim Grøstad]

On Friday, 15 January 2021 at 15:48:07 UTC, welkam wrote:
> On Friday, 15 January 2021 at 14:35:55 UTC, Ola Fosheim Grøstad 
> wrote:
>> improved with precise collection
> Precise GC is slower than default GC.

D does not have a fully precise GC. The "precise" collector still 
scans things conservatively when it cannot be certain.

If you combine fully precise collection it with static analysis, 
then you can reduce the number of paths you follow, but it is a 
lot of work to implement. So it would take a very motivated 
individual.

> -lowmem flag replaces all* allocations with GC allocations so 
> you can benchmark that

Interesting idea. There are compilers that use GC written in 
other languages. It is a nice baseline test, especially since 
there are not many large commonly known programs for D to do 
realistic benchmarks with.

> A write barrier is a peace of code that is inserted before a 
> write to an [object].

Not a write to the object, but a modified pointer. The write 
barrier is invoked when you switch a pointer from one object to 
another one. Then you mark the object, so you need 2 free bits in 
each object to use for marking.

But my uncertainty was related to how to optimize away barrier 
that has no impact on the final collection. It is easy to make 
mistakes when doing such optimizations. The goal should be to 
invoke as few barriers as possible by static analysis.

> Reference counting needs mutation. How do you define immutable 
> RC slice that needs to mutate its reference count? Thats a 
> unsolved problem in D.

D needs more fine grained immutable, for sure.