Improve GC performance by using typed interface to bdwgc #1239
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We now generate a GC descriptor for structs. It describes the memory structure, where there are pointers, to the GC. The descriptor itself is stored in the _class struct in the first field. The old $GCINFO was meant for this type of GC information but we never got around to using it before, instead it held a string of the name of the object class - useful for debugging. We still think that's useful so we keep the name but it is now stored in $name - slightly more intuitive. The GC descriptor is built from a bitmap. As it can vary in size, it is kept outside of the _class struct as a new global of the name ClassD_gcbm.
This adds GC descriptors for most of the C code we have checked in, like builtins and RTS. Mallocs have been turned into GC_MALLOC_EXPLICITLY_TYPED with a corresponding GC descriptor stored in the method table of the associated class. The GC descriptor is in turn based on a GC bitmap, which is of varying length and thus not suitable for placing directly in the method table. There are instead separate global variables, one per class. We initialize these structures a bit differently, so where there are static initializations the bitmap is currently initialized to 0, which is usually wrong as it indicates that the GC should not scan the object. For classes initialized with a method table, as the current code generated by actonc, we currently set it to 0xFF, as to indicate all bits should be scanned. The goal right now is just to add in the GC descriptors and start using them for allocations without breaking anything. We will start fiddling with the bits later on to speed things up.
We err on the safe side by initializing the GC bitmaps to all 1s which means they will be scanned in their entirety.
A little cleaner I think! :)
Set correct GC bitmaps for all builtin types!
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