diff options
| -rw-r--r-- | compiler/rustc_arena/src/lib.rs | 37 |
1 files changed, 16 insertions, 21 deletions
diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs index 6f9cccf58dd..32783951a31 100644 --- a/compiler/rustc_arena/src/lib.rs +++ b/compiler/rustc_arena/src/lib.rs @@ -299,11 +299,13 @@ unsafe impl<#[may_dangle] T> Drop for TypedArena<T> { unsafe impl<T: Send> Send for TypedArena<T> {} pub struct DroplessArena { - /// A pointer to the next object to be allocated. - ptr: Cell<*mut u8>, + /// A pointer to the start of the free space. + start: Cell<*mut u8>, - /// A pointer to the end of the allocated area. When this pointer is - /// reached, a new chunk is allocated. + /// A pointer to the end of free space. + /// + /// The allocation proceeds from the end of the chunk towards the start. + /// When this pointer crosses the start pointer, a new chunk is allocated. end: Cell<*mut u8>, /// A vector of arena chunks. @@ -316,7 +318,7 @@ impl Default for DroplessArena { #[inline] fn default() -> DroplessArena { DroplessArena { - ptr: Cell::new(ptr::null_mut()), + start: Cell::new(ptr::null_mut()), end: Cell::new(ptr::null_mut()), chunks: Default::default(), } @@ -348,7 +350,7 @@ impl DroplessArena { new_cap = cmp::max(additional, new_cap); let mut chunk = TypedArenaChunk::<u8>::new(new_cap); - self.ptr.set(chunk.start()); + self.start.set(chunk.start()); self.end.set(chunk.end()); chunks.push(chunk); } @@ -359,24 +361,17 @@ impl DroplessArena { /// request. #[inline] fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> { - let ptr = self.ptr.get() as usize; + let start = self.start.get() as usize; let end = self.end.get() as usize; + let align = layout.align(); let bytes = layout.size(); - // The allocation request fits into the current chunk iff: - // - // let aligned = align_to(ptr, align); - // ptr <= aligned && aligned + bytes <= end - // - // Except that we work with fixed width integers and need to be careful - // about potential overflow in the calcuation. If the overflow does - // happen, then we definitely don't have enough free and need to grow - // the arena. - let aligned = ptr.checked_add(align - 1)? & !(align - 1); - let new_ptr = aligned.checked_add(bytes)?; - if new_ptr <= end { - self.ptr.set(new_ptr as *mut u8); - Some(aligned as *mut u8) + + let new_end = end.checked_sub(bytes)? & !(align - 1); + if start <= new_end { + let new_end = new_end as *mut u8; + self.end.set(new_end); + Some(new_end) } else { None } |