From fb80c73fb3fd7fd18cedf716ff3c3cdc8d90f93d Mon Sep 17 00:00:00 2001 From: Nicholas Nethercote Date: Mon, 15 Nov 2021 17:15:42 +1100 Subject: Remove `DropArena`. Most arena-allocate types that impl `Drop` get their own `TypedArena`, but a few infrequently used ones share a `DropArena`. This sharing adds complexity but doesn't help performance or memory usage. Perhaps it was more effective in the past prior to some other improvements to arenas. This commit removes `DropArena` and the sharing of arenas via the `few` attribute of the `arena_types` macro. This change removes over 100 lines of code and nine uses of `unsafe` (one of which affects the parallel compiler) and makes the remaining code easier to read. --- compiler/rustc_arena/src/lib.rs | 141 ++-------------------------------------- 1 file changed, 7 insertions(+), 134 deletions(-) (limited to 'compiler/rustc_arena/src/lib.rs') diff --git a/compiler/rustc_arena/src/lib.rs b/compiler/rustc_arena/src/lib.rs index 6d5f47aceeb..e54fcaf6fc1 100644 --- a/compiler/rustc_arena/src/lib.rs +++ b/compiler/rustc_arena/src/lib.rs @@ -19,7 +19,6 @@ #![feature(rustc_attrs)] #![cfg_attr(test, feature(test))] -use rustc_data_structures::sync; use smallvec::SmallVec; use std::alloc::Layout; @@ -517,130 +516,12 @@ impl DroplessArena { } } -/// Calls the destructor for an object when dropped. -struct DropType { - drop_fn: unsafe fn(*mut u8), - obj: *mut u8, -} - -// SAFETY: we require `T: Send` before type-erasing into `DropType`. -#[cfg(parallel_compiler)] -unsafe impl sync::Send for DropType {} - -impl DropType { - #[inline] - unsafe fn new(obj: *mut T) -> Self { - unsafe fn drop_for_type(to_drop: *mut u8) { - std::ptr::drop_in_place(to_drop as *mut T) - } - - DropType { drop_fn: drop_for_type::, obj: obj as *mut u8 } - } -} - -impl Drop for DropType { - fn drop(&mut self) { - unsafe { (self.drop_fn)(self.obj) } - } -} - -/// An arena which can be used to allocate any type. -/// -/// # Safety -/// -/// Allocating in this arena is unsafe since the type system -/// doesn't know which types it contains. In order to -/// allocate safely, you must store a `PhantomData` -/// alongside this arena for each type `T` you allocate. -#[derive(Default)] -pub struct DropArena { - /// A list of destructors to run when the arena drops. - /// Ordered so `destructors` gets dropped before the arena - /// since its destructor can reference memory in the arena. - destructors: RefCell>, - arena: DroplessArena, -} - -impl DropArena { - #[inline] - pub unsafe fn alloc(&self, object: T) -> &mut T - where - T: sync::Send, - { - let mem = self.arena.alloc_raw(Layout::new::()) as *mut T; - // Write into uninitialized memory. - ptr::write(mem, object); - let result = &mut *mem; - // Record the destructor after doing the allocation as that may panic - // and would cause `object`'s destructor to run twice if it was recorded before. - self.destructors.borrow_mut().push(DropType::new(result)); - result - } - - #[inline] - pub unsafe fn alloc_from_iter(&self, iter: I) -> &mut [T] - where - T: sync::Send, - I: IntoIterator, - { - let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect(); - if vec.is_empty() { - return &mut []; - } - let len = vec.len(); - - let start_ptr = self.arena.alloc_raw(Layout::array::(len).unwrap()) as *mut T; - - let mut destructors = self.destructors.borrow_mut(); - // Reserve space for the destructors so we can't panic while adding them. - destructors.reserve(len); - - // Move the content to the arena by copying it and then forgetting - // the content of the SmallVec. - vec.as_ptr().copy_to_nonoverlapping(start_ptr, len); - mem::forget(vec.drain(..)); - - // Record the destructors after doing the allocation as that may panic - // and would cause `object`'s destructor to run twice if it was recorded before. - for i in 0..len { - destructors.push(DropType::new(start_ptr.add(i))); - } - - slice::from_raw_parts_mut(start_ptr, len) - } -} - -pub macro arena_for_type { - ([][$ty:ty]) => { - $crate::TypedArena<$ty> - }, - ([few $(, $attrs:ident)*][$ty:ty]) => { - ::std::marker::PhantomData<$ty> - }, - ([$ignore:ident $(, $attrs:ident)*]$args:tt) => { - $crate::arena_for_type!([$($attrs),*]$args) - }, -} - -pub macro which_arena_for_type { - ([][$arena:expr]) => { - ::std::option::Option::Some($arena) - }, - ([few$(, $attrs:ident)*][$arena:expr]) => { - ::std::option::Option::None - }, - ([$ignore:ident$(, $attrs:ident)*]$args:tt) => { - $crate::which_arena_for_type!([$($attrs),*]$args) - }, -} - #[rustc_macro_transparency = "semitransparent"] pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) { #[derive(Default)] pub struct Arena<$tcx> { pub dropless: $crate::DroplessArena, - drop: $crate::DropArena, - $($name: $crate::arena_for_type!($a[$ty]),)* + $($name: $crate::TypedArena<$ty>,)* } pub trait ArenaAllocatable<'tcx, T = Self>: Sized { @@ -670,13 +551,9 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) { #[inline] fn allocate_on<'a>(self, arena: &'a Arena<$tcx>) -> &'a mut Self { if !::std::mem::needs_drop::() { - return arena.dropless.alloc(self); - } - match $crate::which_arena_for_type!($a[&arena.$name]) { - ::std::option::Option::<&$crate::TypedArena>::Some(ty_arena) => { - ty_arena.alloc(self) - } - ::std::option::Option::None => unsafe { arena.drop.alloc(self) }, + arena.dropless.alloc(self) + } else { + arena.$name.alloc(self) } } @@ -686,13 +563,9 @@ pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) { iter: impl ::std::iter::IntoIterator, ) -> &'a mut [Self] { if !::std::mem::needs_drop::() { - return arena.dropless.alloc_from_iter(iter); - } - match $crate::which_arena_for_type!($a[&arena.$name]) { - ::std::option::Option::<&$crate::TypedArena>::Some(ty_arena) => { - ty_arena.alloc_from_iter(iter) - } - ::std::option::Option::None => unsafe { arena.drop.alloc_from_iter(iter) }, + arena.dropless.alloc_from_iter(iter) + } else { + arena.$name.alloc_from_iter(iter) } } } -- cgit 1.4.1-3-g733a5