This patch adds two macros to declare per-type allocators:
- JS_DECLARE_ALLOCATOR(TypeName)
- JS_DEFINE_ALLOCATOR(TypeName)
When used, they add a type-specific CellAllocator that the Heap will
delegate allocation requests to.
The result of this is that GC objects of the same type always end up
within the same HeapBlock, drastically reducing the ability to perform
type confusion attacks.
It also improves HeapBlock utilization, since each block now has cells
sized exactly to the type used within that block. (Previously we only
had a handful of block sizes available, and most GC allocations ended
up with a large amount of slack in their tails.)
There is a small performance hit from this, but I'm sure we can make
up for it elsewhere.
Note that the old size-based allocators still exist, and we fall back
to them for any type that doesn't have its own CellAllocator.
We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Instead of passing a GlobalObject everywhere, we will simply pass a VM,
from which we can get everything we need: common names, the current
realm, symbols, arguments, the heap, and a few other things.
In some places we already don't actually need a global object and just
do it for consistency - no more `auto& vm = global_object.vm();`!
This will eventually automatically fix the "wrong realm" issue we have
in some places where we (incorrectly) use the global object from the
allocating object, e.g. in call() / construct() implementations. When
only ever a VM is passed around, this issue can't happen :^)
I've decided to split this change into a series of patches that should
keep each commit down do a somewhat manageable size.
