split out vec-zsts correctly

pull/10/head
Alexis Beingessner 9 years ago committed by Manish Goregaokar
parent 9f20f453b2
commit 1809c4defb

@ -45,5 +45,6 @@
* [Insert and Remove](vec-insert-remove.md)
* [IntoIter](vec-into-iter.md)
* [Drain](vec-drain.md)
* [Handling Zero-Sized Types](vec-zsts.md)
* [Final Code](vec-final.md)
* [Implementing Arc and Mutex](arc-and-mutex.md)

@ -0,0 +1,176 @@
% Handling Zero-Sized Types
It's time. We're going to fight the spectre that is zero-sized types. Safe Rust
*never* needs to care about this, but Vec is very intensive on raw pointers and
raw allocations, which are exactly the *only* two things that care about
zero-sized types. We need to be careful of two things:
* The raw allocator API has undefined behaviour if you pass in 0 for an
allocation size.
* raw pointer offsets are no-ops for zero-sized types, which will break our
C-style pointer iterator.
Thankfully we abstracted out pointer-iterators and allocating handling into
RawValIter and RawVec respectively. How mysteriously convenient.
## Allocating Zero-Sized Types
So if the allocator API doesn't support zero-sized allocations, what on earth
do we store as our allocation? Why, `heap::EMPTY` of course! Almost every operation
with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
to be considered to store or load them. This actually extends to `ptr::read` and
`ptr::write`: they won't actually look at the pointer at all. As such we *never* need
to change the pointer.
Note however that our previous reliance on running out of memory before overflow is
no longer valid with zero-sized types. We must explicitly guard against capacity
overflow for zero-sized types.
Due to our current architecture, all this means is writing 3 guards, one in each
method of RawVec.
```rust,ignore
impl<T> RawVec<T> {
fn new() -> Self {
unsafe {
// !0 is usize::MAX. This branch should be stripped at compile time.
let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
// heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
}
}
fn grow(&mut self) {
unsafe {
let elem_size = mem::size_of::<T>();
// since we set the capacity to usize::MAX when elem_size is
// 0, getting to here necessarily means the Vec is overfull.
assert!(elem_size != 0, "capacity overflow");
let align = mem::align_of::<T>();
let (new_cap, ptr) = if self.cap == 0 {
let ptr = heap::allocate(elem_size, align);
(1, ptr)
} else {
let new_cap = 2 * self.cap;
let ptr = heap::reallocate(*self.ptr as *mut _,
self.cap * elem_size,
new_cap * elem_size,
align);
(new_cap, ptr)
};
// If allocate or reallocate fail, we'll get `null` back
if ptr.is_null() { oom() }
self.ptr = Unique::new(ptr as *mut _);
self.cap = new_cap;
}
}
}
impl<T> Drop for RawVec<T> {
fn drop(&mut self) {
let elem_size = mem::size_of::<T>();
// don't free zero-sized allocations, as they were never allocated.
if self.cap != 0 && elem_size != 0 {
let align = mem::align_of::<T>();
let num_bytes = elem_size * self.cap;
unsafe {
heap::deallocate(*self.ptr as *mut _, num_bytes, align);
}
}
}
}
```
That's it. We support pushing and popping zero-sized types now. Our iterators
(that aren't provided by slice Deref) are still busted, though.
## Iterating Zero-Sized Types
Zero-sized offsets are no-ops. This means that our current design will always
initialize `start` and `end` as the same value, and our iterators will yield
nothing. The current solution to this is to cast the pointers to integers,
increment, and then cast them back:
```rust,ignore
impl<T> RawValIter<T> {
unsafe fn new(slice: &[T]) -> Self {
RawValIter {
start: slice.as_ptr(),
end: if mem::size_of::<T>() == 0 {
((slice.as_ptr() as usize) + slice.len()) as *const _
} else if slice.len() == 0 {
slice.as_ptr()
} else {
slice.as_ptr().offset(slice.len() as isize)
}
}
}
}
```
Now we have a different bug. Instead of our iterators not running at all, our
iterators now run *forever*. We need to do the same trick in our iterator impls.
Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
basically be treating the two pointers as if they point to bytes, we'll just
map size 0 to divide by 1.
```rust,ignore
impl<T> Iterator for RawValIter<T> {
type Item = T;
fn next(&mut self) -> Option<T> {
if self.start == self.end {
None
} else {
unsafe {
let result = ptr::read(self.start);
self.start = if mem::size_of::<T>() == 0 {
(self.start as usize + 1) as *const _
} else {
self.start.offset(1);
}
Some(result)
}
}
}
fn size_hint(&self) -> (usize, Option<usize>) {
let elem_size = mem::size_of::<T>();
let len = (self.end as usize - self.start as usize)
/ if elem_size == 0 { 1 } else { elem_size };
(len, Some(len))
}
}
impl<T> DoubleEndedIterator for RawValIter<T> {
fn next_back(&mut self) -> Option<T> {
if self.start == self.end {
None
} else {
unsafe {
self.end = if mem::size_of::<T>() == 0 {
(self.end as usize - 1) as *const _
} else {
self.end.offset(-1);
}
Some(ptr::read(self.end))
}
}
}
}
```
And that's it. Iteration works!
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