precise GC

[Adam D. Ruppe]

On Monday, 4 March 2019 at 10:38:29 UTC, KnightMare wrote:
> For example, we have some rooted memory block as
> auto rooted = new long[1_000_000];
> 1) conservative-GC will scan it for false pointers every 
> GC-cycle. is it true?

Well, conservative GC in general might, but D's GC would NOT.

D's old GC is arguably semi-precise. It would scan void[] 
conservatively, anything that looks like a pointer is considered 
maybe a pointer. But it would NOT scan blocks allocated as long[] 
or ubyte[]. It would allocate those as NO_SCAN blocks.

The difference is in combination things, like the stack or 
structs with static blocks.

struct {
    int a;
    void* b;
}

The old GC would treat that whole struct as potentially pointers, 
both a and b. The new precise GC would know only b needs to be 
scanned inside that struct.

The even bigger deal with precise is it also knows only b would 
need to be changed if the pointer were to move - that's the big 
gain precise is setting the groundwork for, to enable moving GC 
optimizations.