RawBlock: seal most fields, replace top with a capacity field

src/machine/stack.rs

@@ -183,7 +183,7 @@ impl Stack {
         let frame_size = AndFrame::size_of(num_cells);
 
         unsafe {
-            let e = (*self.buf.ptr.get_mut()).addr() - self.buf.base.addr();
+            let e = self.buf.used_bytes();
             let new_ptr = self.alloc(frame_size)?;
             let mut offset = prelude_size::<AndFramePrelude>();
 
@@ -209,14 +209,14 @@ impl Stack {
     }
 
     pub(crate) fn top(&self) -> usize {
-        unsafe { (*self.buf.ptr.get()).addr() - self.buf.base.addr() }
+        self.buf.used_bytes()
     }
 
     pub(crate) fn allocate_or_frame(&mut self, num_cells: usize) -> Result<usize, AllocError> {
         let frame_size = OrFrame::size_of(num_cells);
 
         unsafe {
-            let b = (*self.buf.ptr.get_mut()).addr() - self.buf.base.addr();
+            let b = self.buf.used_bytes();
             let new_ptr = self.alloc(frame_size)?;
             let mut offset = prelude_size::<OrFramePrelude>();
 
@@ -267,11 +267,7 @@ impl Stack {
 
     #[inline(always)]
     pub(crate) fn truncate(&mut self, b: usize) {
-        let base = unsafe { self.buf.base.add(b) };
+        self.buf.shrink(b);
-
-        if base < (*self.buf.ptr.get_mut()) {
-            *self.buf.ptr.get_mut() = base.cast_mut();
-        }
     }
 }
 

src/raw_block.rs

@@ -11,11 +11,13 @@ pub trait RawBlockTraits {
     fn align() -> usize;
 }
 
+/// A block of memory with fast, lock-free appends.
 #[derive(Debug)]
 pub struct RawBlock<T: RawBlockTraits> {
     pub base: *const u8,
-    pub top: *const u8,
+    capacity: usize,
-    pub ptr: UnsafeCell<*mut u8>,
+
+    ptr: UnsafeCell<*mut u8>,
     _marker: PhantomData<T>,
 }
 
@@ -24,7 +26,7 @@ impl<T: RawBlockTraits> RawBlock<T> {
     pub fn empty_block() -> Self {
         RawBlock {
             base: ptr::null(),
-            top: ptr::null(),
+            capacity: 0,
             ptr: UnsafeCell::new(ptr::null_mut()),
             _marker: PhantomData,
         }
@@ -48,7 +50,7 @@ impl<T: RawBlockTraits> RawBlock<T> {
             return Err(AllocError);
         }
         self.base = new_base;
-        self.top = self.base.add(cap);
+        self.capacity = cap;
         *self.ptr.get_mut() = self.base.cast_mut();
         Ok(())
     }
@@ -57,7 +59,7 @@ impl<T: RawBlockTraits> RawBlock<T> {
         if self.base.is_null() {
             self.init_at_size(T::init_size())
         } else {
-            let size = self.size();
+            let size = self.capacity();
             let layout = alloc::Layout::from_size_align_unchecked(size, T::align());
 
             let new_base = alloc::realloc(self.base.cast_mut(), layout, size * 2).cast_const();
@@ -65,46 +67,69 @@ impl<T: RawBlockTraits> RawBlock<T> {
                 Err(AllocError)
             } else {
                 self.base = new_base;
-                self.top = self.base.add(size * 2);
+                self.capacity = size * 2;
-                *self.ptr.get_mut() = self.base.add(size).cast_mut();
+                *self.ptr.get_mut() = (self.base as usize + size) as *mut _;
                 Ok(())
             }
         }
     }
 
     pub unsafe fn grow_new(&self) -> Result<Self, AllocError> {
+        self.debug_check_invariants();
         if self.base.is_null() {
             Self::new()
         } else {
             let mut new_block = Self::empty_block();
-            new_block.init_at_size(self.size() * 2)?;
+            new_block.init_at_size(self.capacity() * 2)?;
-            let allocated = (*self.ptr.get()).addr() - self.base.addr();
+            let allocated = self.used_bytes();
             self.base.copy_to(new_block.base.cast_mut(), allocated);
             *new_block.ptr.get_mut() = new_block.base.add(allocated).cast_mut();
+
+            new_block.debug_check_invariants();
+
             Ok(new_block)
         }
     }
 
+    #[inline(always)]
+    fn debug_check_invariants(&self) {
+        if cfg!(debug_assertions) {
+            unsafe {
+                assert!(
+                    *self.ptr.get() as *const _ >= self.base,
+                    "self.ptr = {:?} < {:?} = self.base",
+                    *self.ptr.get(),
+                    self.base
+                );
+            }
+
+            assert!(self.used_bytes() <= self.capacity());
+        }
+    }
+
     #[inline]
-    pub fn size(&self) -> usize {
+    pub fn capacity(&self) -> usize {
-        self.top.addr() - self.base.addr()
+        self.capacity
+    }
+
+    #[inline]
+    pub fn used_bytes(&self) -> usize {
+        // TODO: safety: UnsafeCell.get()
+        // TODO: safety: prove that ∀Γ: reachable, Γ |- (ptr, base): same alloc
+        unsafe { (*self.ptr.get()).offset_from(self.base) as usize }
     }
 
     #[inline(always)]
-    unsafe fn free_space(&self) -> usize {
+    unsafe fn free_bytes(&self) -> usize {
-        debug_assert!(
+        self.capacity() - self.used_bytes()
-            *self.ptr.get() as *const _ >= self.base,
-            "self.ptr = {:?} < {:?} = self.base",
-            *self.ptr.get(),
-            self.base
-        );
-
-        self.top.addr() - (*self.ptr.get()).addr()
     }
 
     pub unsafe fn alloc(&self, size: usize) -> *mut u8 {
-        let aligned_size = size.next_multiple_of(size);
+        self.debug_check_invariants();
-        if self.free_space() >= aligned_size {
+
+        let aligned_size = size.next_multiple_of(T::align());
+        if self.free_bytes() >= aligned_size {
+            // TODO: make this an atomic add
             let ptr = *self.ptr.get();
             *self.ptr.get() = ptr.add(aligned_size) as *mut _;
             ptr
@@ -112,17 +137,41 @@ impl<T: RawBlockTraits> RawBlock<T> {
             ptr::null_mut()
         }
     }
+
+    /// Moves `ptr` back to `new_size`.
+    ///
+    /// Note that this method does *not* deallocate what was placed in the [`RawBlock`].
+    pub fn shrink(&mut self, new_size: usize) {
+        self.debug_check_invariants();
+
+        assert!(
+            new_size <= self.used_bytes(),
+            "Shrink cannot grow: new_size = {:?} > allocated = {:?}",
+            new_size,
+            self.used_bytes()
+        );
+
+        // SAFETY:
+        // - Asserted: new_size <= self.capacity
+        // - Definition: self.base := alloc(self.capacity)
+        let new_ptr = unsafe { self.base.add(new_size) };
+
+        debug_assert!(new_ptr as usize <= (*self.ptr.get_mut()) as usize,);
+
+        *self.ptr.get_mut() = new_ptr as *mut u8;
+
+        self.debug_check_invariants();
+    }
 }
 
 impl<T: RawBlockTraits> Drop for RawBlock<T> {
     fn drop(&mut self) {
         if !self.base.is_null() {
             unsafe {
-                let layout = alloc::Layout::from_size_align_unchecked(self.size(), T::align());
+                let layout = alloc::Layout::from_size_align_unchecked(self.capacity(), T::align());
                 alloc::dealloc(self.base as *mut _, layout);
             }
 
-            self.top = ptr::null();
             self.base = ptr::null();
             *self.ptr.get_mut() = ptr::null_mut();
         }