Thanks to the `Value` locals refactoring, now primitive locals (ints,
floats, chars, bools, and the like) will not require `Allocation`s at
all, and locals that are never initialized at all because of conditional
control flow won't be wasting memory.
Previously ReturnPointer was just the first slot in the locals array,
which had type `Vec<Pointer>`. But after my recent refactoring, locals
is `Vec<Value>` and it became increasingly hacky to pull a pointer out
of the first slot to be the value. Besides, that hack wouldn't allow
ReturnPointer to ever be an `Lvalue::Local`, referring directly to a
local on a higher stack frame.
Now ReturnPointer has no presence in the locals array, instead being
upgraded to its own field on `Frame`.
This introduces a couple of new hacks, detailed by some of my FIXME
comments, so that I could get the tests passing again and commit. More
commits coming soon should clean up these hacks without much trouble,
and overall I feel that the code is converging on a cleaner, more
efficient design.
The new `Lvalue` has an additional form, `Lvalue::Local`, with the old
form being `Lvalue::Ptr`. In an upcoming commit, we will start producing
the new form for locals, and enable reading and writing of primitive
locals without ever touching `Memory`.
Statics should be able to get a similar treatment, where primitive
statics can be stored and accessed without touching `Memory`.
Turning locals into `Vec<Value>` will allow writing `PrimVal` results
directly into the locals array without creating `memory::Allocation`s
for every local.
This will entail passing around a generalized kind of `Lvalue` instead
of `Pointer`s for the destinations of operations. Replacing `Pointer`
with `Lvalue` is mostly done with this commit, but expanding `Lvalue`
will come later.
This commit turns every local from `Pointer` into `Value::ByRef(ptr)`.
Locals which are `Value::ByVal(prim_val)` will come in a later commit.
