This wrapper will allow us to pass `AuthPtr<T,B> p` to zero() without needing to
write `p.unsafe_auth_ptr` to get to a `T*` inside. Moreover, it will give us a
convenient point to assert that `B` is such that the pointer can be used to
manipulate the memory map (i.e. is not exported).
Going forward, this gives us explicit pointers with which to carry bounds
annotations. Otherwise, assuming AuthPtr overloads operator->, a OOP-style call
like
AuthPtr<Slab, Bounds> slab;
slab->foo()
will create a `Slab* this` within the body of `Slab::foo`, leaving it unable to
see or propagate the Bounds annotation. If it invokes callees that expect
`AuthPtr` arguments, it will therefore have to fabricate new `Bounds` unsafely.
This had not been observed as an issue prior to
923705e514 because CMakeLists.txt had, until
then, been using EQUAL, not STREQUAL, to test for oe (and to then enable
USE_SMALL_CHUNKS). This test would fail, and so the default SLAB_SIZE was
used. Absent this min operation, the use of a whole page on a 64KiB page
causes a crash when using the largest medium size class, as, ultimately, size
classes are not based on page sizes, and so committing a whole page to the
header leaves too little room for that class.
See also 3d3b048776.
With large pages (e.g. the 64K that Debian defaults to for ppc64), this
is a bit much to ask. It's only not true for the bottom few medium size
classes, tho', as all sizes above 256K are multiples of 64K with the
current two mantissa bits size schedule.
This is useful as codegen is nicer if we use size_t, but the semantics
is uint8_t, and is stored as that in many places in the metadata.
Ultimately should introduce a wrapper to check this invariant.
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.
