arena.rs

// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// Dynamic arenas.

// Arenas are used to quickly allocate objects that share a
// lifetime. The arena uses ~[u8] vectors as a backing store to
// allocate objects from. For each allocated object, the arena stores
// a pointer to the type descriptor followed by the
// object. (Potentially with alignment padding after each of them.)
// When the arena is destroyed, it iterates through all of its chunks,
// and uses the tydesc information to trace through the objects,
// calling the destructors on them.
// One subtle point that needs to be addressed is how to handle
// failures while running the user provided initializer function. It
// is important to not run the destructor on uninitialized objects, but
// how to detect them is somewhat subtle. Since alloc() can be invoked
// recursively, it is not sufficient to simply exclude the most recent
// object. To solve this without requiring extra space, we use the low
// order bit of the tydesc pointer to encode whether the object it
// describes has been fully initialized.

// As an optimization, objects with destructors are stored in
// different chunks than objects without destructors. This reduces
// overhead when initializing plain-old-data and means we don't need
// to waste time running the destructors of POD.

#[allow(missing_doc)];


use list::{MutList, MutCons, MutNil};

use std::at_vec;
use std::cast::{transmute, transmute_mut, transmute_mut_region};
use std::cast;
use std::num;
use std::ptr;
use std::sys;
use std::uint;
use std::vec;
use std::unstable::intrinsics;
use std::unstable::intrinsics::{TyDesc, get_tydesc};

// The way arena uses arrays is really deeply awful. The arrays are
// allocated, and have capacities reserved, but the fill for the array
// will always stay at 0.
struct Chunk {
    data: @[u8],
    fill: uint,
    is_pod: bool,
}

#[no_freeze]
pub struct Arena {
    // The head is separated out from the list as a unbenchmarked
    // microoptimization, to avoid needing to case on the list to
    // access the head.
    priv head: Chunk,
    priv pod_head: Chunk,
    priv chunks: @mut MutList<Chunk>,
}

#[unsafe_destructor]
impl Drop for Arena {
    fn drop(&self) {
        unsafe {
            destroy_chunk(&self.head);
            do self.chunks.each |chunk| {
                if !chunk.is_pod {
                    destroy_chunk(chunk);
                }
                true
            };
        }
    }
}

fn chunk(size: uint, is_pod: bool) -> Chunk {
    let mut v: @[u8] = @[];
    unsafe { at_vec::raw::reserve(&mut v, size); }
    Chunk {
        data: unsafe { cast::transmute(v) },
        fill: 0u,
        is_pod: is_pod,
    }
}

pub fn arena_with_size(initial_size: uint) -> Arena {
    Arena {
        head: chunk(initial_size, false),
        pod_head: chunk(initial_size, true),
        chunks: @mut MutNil,
    }
}

pub fn Arena() -> Arena {
    arena_with_size(32u)
}

#[inline]
fn round_up_to(base: uint, align: uint) -> uint {
    (base + (align - 1)) & !(align - 1)
}

// Walk down a chunk, running the destructors for any objects stored
// in it.
unsafe fn destroy_chunk(chunk: &Chunk) {
    let mut idx = 0;
    let buf = vec::raw::to_ptr(chunk.data);
    let fill = chunk.fill;

    while idx < fill {
        let tydesc_data: *uint = transmute(ptr::offset(buf, idx as int));
        let (tydesc, is_done) = un_bitpack_tydesc_ptr(*tydesc_data);
        let (size, align) = ((*tydesc).size, (*tydesc).align);

        let after_tydesc = idx + sys::size_of::<*TyDesc>();

        let start = round_up_to(after_tydesc, align);

        //debug!("freeing object: idx = %u, size = %u, align = %u, done = %b",
        //       start, size, align, is_done);
        if is_done {
            ((*tydesc).drop_glue)(ptr::offset(buf, start as int) as *i8);
        }

        // Find where the next tydesc lives
        idx = round_up_to(start + size, sys::pref_align_of::<*TyDesc>());
    }
}

// We encode whether the object a tydesc describes has been
// initialized in the arena in the low bit of the tydesc pointer. This
// is necessary in order to properly do cleanup if a failure occurs
// during an initializer.
#[inline]
unsafe fn bitpack_tydesc_ptr(p: *TyDesc, is_done: bool) -> uint {
    let p_bits: uint = transmute(p);
    p_bits | (is_done as uint)
}
#[inline]
unsafe fn un_bitpack_tydesc_ptr(p: uint) -> (*TyDesc, bool) {
    (transmute(p & !1), p & 1 == 1)
}

impl Arena {
    // Functions for the POD part of the arena
    fn alloc_pod_grow(&mut self, n_bytes: uint, align: uint) -> *u8 {
        // Allocate a new chunk.
        let chunk_size = at_vec::capacity(self.pod_head.data);
        let new_min_chunk_size = num::max(n_bytes, chunk_size);
        self.chunks = @mut MutCons(self.pod_head, self.chunks);
        self.pod_head =
            chunk(uint::next_power_of_two(new_min_chunk_size + 1u), true);

        return self.alloc_pod_inner(n_bytes, align);
    }

    #[inline]
    fn alloc_pod_inner(&mut self, n_bytes: uint, align: uint) -> *u8 {
        unsafe {
            let this = transmute_mut_region(self);
            let start = round_up_to(this.pod_head.fill, align);
            let end = start + n_bytes;
            if end > at_vec::capacity(this.pod_head.data) {
                return this.alloc_pod_grow(n_bytes, align);
            }
            this.pod_head.fill = end;

            //debug!("idx = %u, size = %u, align = %u, fill = %u",
            //       start, n_bytes, align, head.fill);

            ptr::offset(vec::raw::to_ptr(this.pod_head.data), start as int)
        }
    }

    #[inline]
    fn alloc_pod<'a, T>(&'a mut self, op: &fn() -> T) -> &'a T {
        unsafe {
            let tydesc = get_tydesc::<T>();
            let ptr = self.alloc_pod_inner((*tydesc).size, (*tydesc).align);
            let ptr: *mut T = transmute(ptr);
            intrinsics::move_val_init(&mut (*ptr), op());
            return transmute(ptr);
        }
    }

    // Functions for the non-POD part of the arena
    fn alloc_nonpod_grow(&mut self, n_bytes: uint, align: uint)
                         -> (*u8, *u8) {
        // Allocate a new chunk.
        let chunk_size = at_vec::capacity(self.head.data);
        let new_min_chunk_size = num::max(n_bytes, chunk_size);
        self.chunks = @mut MutCons(self.head, self.chunks);
        self.head =
            chunk(uint::next_power_of_two(new_min_chunk_size + 1u), false);

        return self.alloc_nonpod_inner(n_bytes, align);
    }

    #[inline]
    fn alloc_nonpod_inner(&mut self, n_bytes: uint, align: uint)
                          -> (*u8, *u8) {
        unsafe {
            let start;
            let end;
            let tydesc_start;
            let after_tydesc;

            {
                let head = transmute_mut_region(&mut self.head);

                tydesc_start = head.fill;
                after_tydesc = head.fill + sys::size_of::<*TyDesc>();
                start = round_up_to(after_tydesc, align);
                end = start + n_bytes;
            }

            if end > at_vec::capacity(self.head.data) {
                return self.alloc_nonpod_grow(n_bytes, align);
            }

            let head = transmute_mut_region(&mut self.head);
            head.fill = round_up_to(end, sys::pref_align_of::<*TyDesc>());

            //debug!("idx = %u, size = %u, align = %u, fill = %u",
            //       start, n_bytes, align, head.fill);

            let buf = vec::raw::to_ptr(self.head.data);
            return (ptr::offset(buf, tydesc_start as int), ptr::offset(buf, start as int));
        }
    }

    #[inline]
    fn alloc_nonpod<'a, T>(&'a mut self, op: &fn() -> T) -> &'a T {
        unsafe {
            let tydesc = get_tydesc::<T>();
            let (ty_ptr, ptr) =
                self.alloc_nonpod_inner((*tydesc).size, (*tydesc).align);
            let ty_ptr: *mut uint = transmute(ty_ptr);
            let ptr: *mut T = transmute(ptr);
            // Write in our tydesc along with a bit indicating that it
            // has *not* been initialized yet.
            *ty_ptr = transmute(tydesc);
            // Actually initialize it
            intrinsics::move_val_init(&mut(*ptr), op());
            // Now that we are done, update the tydesc to indicate that
            // the object is there.
            *ty_ptr = bitpack_tydesc_ptr(tydesc, true);

            return transmute(ptr);
        }
    }

    // The external interface
    #[inline]
    pub fn alloc<'a, T>(&'a self, op: &fn() -> T) -> &'a T {
        unsafe {
            // XXX: Borrow check
            let this = transmute_mut(self);
            if intrinsics::needs_drop::<T>() {
                this.alloc_nonpod(op)
            } else {
                this.alloc_pod(op)
            }
        }
    }
}

#[test]
fn test_arena_destructors() {
    let arena = Arena();
    for i in range(0u, 10) {
        // Arena allocate something with drop glue to make sure it
        // doesn't leak.
        do arena.alloc { @i };
        // Allocate something with funny size and alignment, to keep
        // things interesting.
        do arena.alloc { [0u8, 1u8, 2u8] };
    }
}

#[test]
#[should_fail]
#[ignore(cfg(windows))]
fn test_arena_destructors_fail() {
    let arena = Arena();
    // Put some stuff in the arena.
    for i in range(0u, 10) {
        // Arena allocate something with drop glue to make sure it
        // doesn't leak.
        do arena.alloc { @i };
        // Allocate something with funny size and alignment, to keep
        // things interesting.
        do arena.alloc { [0u8, 1u8, 2u8] };
    }
    // Now, fail while allocating
    do arena.alloc::<@int> {
        // Now fail.
        fail!();
    };
}