Fix a false positive. The loops in the pagemap accessor are too
complicated for the static analyser to check, so it doesn't spot that a
non-null return is impossible.
This introduces a new `address_t` type and two new casts: `pointer_cast`
and `address_cast` for casting between an `address_t` and a pointer.
These should make it easier to audit the codebase for casts between
pointers and integers. In particular, the remaining `reinterpret_cast`s
and `pointer_cast`s should be the only places where we could perform
invalid pointer arithmetic.
Also adds a `pointer_offset` helper that adds an offset (in bytes) to a
pointer, preserving its original type. This is a sufficiently common
pattern that it seemed worthwhile to centralise it.
This is a generalisation of `std::nullptr_t` that allows non-0 values of
pointer-sized-but-not-a-pointer thing. `DLList` now takes an
`InvalidPointer`, `nullptr_t`, or some other compatible class as a
sentinel value instead of a `uintptr_t`.
We don't follow the pointers passed into the pagemap directly, but
instead use them to calculate indexs into the pagemap. Use uintptr_t
means it is easier to perform address arithmetic, and not have casts
back to void* everywhere.
Rather than unconditionally storing a pointer to the pagemap and
initialising this with a global, we now provide a choice of three
compile-time options for how the pagemap should be accessed.
If you're creating a new allocator and the pagemap comes from a library
that exports the pagemap accessor function but not the pagemap symbol,
you need to be able to replace this.
The PAL can now advertise that it supports aligned allocation. If it
does not, then the memory provider will do the alignment for it.
This change still leaves the PAL responsible for systematic testing, but
it should now be much easier to lift that out.
Replace them with some very simple constexpr things. This is what the
code used to look like, but it appears that I fundamentally
misunderstood why it didn't work. This version should be a lot more
maintainable.
- Rename MemoryProviderState to PAL to reflect what is now is.
- Hide calls to the PAL's low memory functions behind something that
returns a default value if they're not implemented.
This does not deallocate memory until the OS tells us that we are short
on memory, then tries to decommit all of the cached chunks (except for
the first page, used for the linked lists).
Nowhere near enough testing to commit to master yet!
