On Sun, Jul 24, 2016 at 9:44 AM, Cedric St-Jean <cedric.stj...@gmail.com> wrote: > Thank you for the explanations. With that in mind, why is stack-allocating > heap-object-containing immutables hard? Doesn't it boil down to inspecting > the immutable type at compile-time to find the offset of the heap-allocated > objects' pointers, and pushing those onto the GC frame?
Well, "boil down to" is not the right way to estimate how hard something can be implemented. There's a lot of details one need to consider to implement it and to make sure it doesn't cause regression. A few important points one need to know are, 1. Memory allocations are **NOT** as expensive as one might think. 2. Making copies can be very expensive. 3. Reboxing is hard to avoid without escape analysis or ABI change. > > On Sunday, July 24, 2016 at 8:39:18 AM UTC-4, Yichao Yu wrote: >> >> On Sun, Jul 24, 2016 at 8:26 AM, Uwe Fechner <uwe.fec...@gmail.com> wrote: >> > Thanks for your answer. Some more questions: >> > - is a gc frame just a pointer? >> >> Parent point (it forms a linked list) >> Length >> GC Roots >> >> > - if not, which information does it hold? In which file is the gc frame >> > type/ >> > structure defined? >> >> `jl_gcframe_t` in `julia.h` >> >> > - is there exactly one gc frame for each local variable? >> >> There's one GC frame for each ..... "scope" or whatever way you want >> to group roots. >> We emit one GC frame per function for JIT code. >> Each root has it's own slot in the frame. >> There might be duplicated roots in a frame and that's what the GC >> frame optimizations are talking about. >> See also the various `JL_GC_PUSH` macros in `julia.h` for how they are >> created/used. >> >> > - why is it called "frame"? The term implies, that it is around >> > something. >> > Is this the case? >> >> Since it's an array of multiple pointers. The use of "frame" is >> probably similar to that in stack frame. >> >> P.S. Although you probably know already but just to be clear, one >> don't need to worry about creating/accessing/maintaining GC frames >> except for really low level stuff (GC and codegen). You are welcome to >> learn more about julia internals of course. >> >> > >> > On Sunday, July 24, 2016 at 1:56:05 PM UTC+2, Yichao Yu wrote: >> >> >> >> On Sun, Jul 24, 2016 at 6:03 AM, Uwe Fechner <uwe.fec...@gmail.com> >> >> wrote: >> >> > Hello, >> >> > in the issues on github I see a lot that refer to gc frames. >> >> > >> >> > What is a gc frame? I know how garbage collection works in general, >> >> > but I don't understand the meaning of gc frames in the context of >> >> > Julia. >> >> > >> >> > Could someone explain: >> >> > - What they are used for? >> >> >> >> Keep reference to local variables. >> >> >> >> > - When they need to be created? >> >> >> >> When there's local variables that are heap allocated. >> >> >> >> > - If they have a lifetime, that needs to be managed. >> >> >> >> They are stack allocated. >> >> >> >> > >> >> > Thank you very much! >> >> > >> >> > Uwe
