sbrk_allocator.zig

const std = @import("../std.zig");
const builtin = @import("builtin");
const math = std.math;
const Allocator = std.mem.Allocator;
const mem = std.mem;
const assert = std.debug.assert;

SbrkAllocator()

pub fn SbrkAllocator(comptime sbrk: *const fn (n: usize) usize) type {
    return struct {
        pub const vtable = Allocator.VTable{
            .alloc = alloc,
            .resize = resize,
            .free = free,
        };

        pub const Error = Allocator.Error;

        lock: std.Thread.Mutex = .{},

        const max_usize = math.maxInt(usize);
        const ushift = math.Log2Int(usize);
        const bigpage_size = 64 * 1024;
        const pages_per_bigpage = bigpage_size / mem.page_size;
        const bigpage_count = max_usize / bigpage_size;

        const min_class = math.log2(math.ceilPowerOfTwoAssert(usize, 1 + @sizeOf(usize)));
        const size_class_count = math.log2(bigpage_size) - min_class;
        const big_size_class_count = math.log2(bigpage_count);

        var next_addrs = [1]usize{0} ** size_class_count;
        var frees = [1]usize{0} ** size_class_count;
        var big_frees = [1]usize{0} ** big_size_class_count;

        // TODO don't do the naive locking strategy
        var lock: std.Thread.Mutex = .{};
        fn alloc(ctx: *anyopaque, len: usize, log2_align: u8, return_address: usize) ?[*]u8 {
            _ = ctx;
            _ = return_address;
            lock.lock();
            defer lock.unlock();
            // Make room for the freelist next pointer.
            const alignment = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_align));
            const actual_len = @max(len +| @sizeOf(usize), alignment);
            const slot_size = math.ceilPowerOfTwo(usize, actual_len) catch return null;
            const class = math.log2(slot_size) - min_class;
            if (class < size_class_count) {
                const addr = a: {
                    const top_free_ptr = frees[class];
                    if (top_free_ptr != 0) {
                        const node = @as(*usize, @ptrFromInt(top_free_ptr + (slot_size - @sizeOf(usize))));
                        frees[class] = node.*;
                        break :a top_free_ptr;
                    }

                    const next_addr = next_addrs[class];
                    if (next_addr % mem.page_size == 0) {
                        const addr = allocBigPages(1);
                        if (addr == 0) return null;
                        //std.debug.print("allocated fresh slot_size={d} class={d} addr=0x{x}\n", .{
                        //    slot_size, class, addr,
                        //});
                        next_addrs[class] = addr + slot_size;
                        break :a addr;
                    } else {
                        next_addrs[class] = next_addr + slot_size;
                        break :a next_addr;
                    }
                };
                return @as([*]u8, @ptrFromInt(addr));
            }
            const bigpages_needed = bigPagesNeeded(actual_len);
            const addr = allocBigPages(bigpages_needed);
            return @as([*]u8, @ptrFromInt(addr));
        }

        fn resize(
            ctx: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            new_len: usize,
            return_address: usize,
        ) bool {
            _ = ctx;
            _ = return_address;
            lock.lock();
            defer lock.unlock();
            // We don't want to move anything from one size class to another, but we
            // can recover bytes in between powers of two.
            const buf_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_buf_align));
            const old_actual_len = @max(buf.len + @sizeOf(usize), buf_align);
            const new_actual_len = @max(new_len +| @sizeOf(usize), buf_align);
            const old_small_slot_size = math.ceilPowerOfTwoAssert(usize, old_actual_len);
            const old_small_class = math.log2(old_small_slot_size) - min_class;
            if (old_small_class < size_class_count) {
                const new_small_slot_size = math.ceilPowerOfTwo(usize, new_actual_len) catch return false;
                return old_small_slot_size == new_small_slot_size;
            } else {
                const old_bigpages_needed = bigPagesNeeded(old_actual_len);
                const old_big_slot_pages = math.ceilPowerOfTwoAssert(usize, old_bigpages_needed);
                const new_bigpages_needed = bigPagesNeeded(new_actual_len);
                const new_big_slot_pages = math.ceilPowerOfTwo(usize, new_bigpages_needed) catch return false;
                return old_big_slot_pages == new_big_slot_pages;
            }
        }

        fn free(
            ctx: *anyopaque,
            buf: []u8,
            log2_buf_align: u8,
            return_address: usize,
        ) void {
            _ = ctx;
            _ = return_address;
            lock.lock();
            defer lock.unlock();
            const buf_align = @as(usize, 1) << @as(Allocator.Log2Align, @intCast(log2_buf_align));
            const actual_len = @max(buf.len + @sizeOf(usize), buf_align);
            const slot_size = math.ceilPowerOfTwoAssert(usize, actual_len);
            const class = math.log2(slot_size) - min_class;
            const addr = @intFromPtr(buf.ptr);
            if (class < size_class_count) {
                const node = @as(*usize, @ptrFromInt(addr + (slot_size - @sizeOf(usize))));
                node.* = frees[class];
                frees[class] = addr;
            } else {
                const bigpages_needed = bigPagesNeeded(actual_len);
                const pow2_pages = math.ceilPowerOfTwoAssert(usize, bigpages_needed);
                const big_slot_size_bytes = pow2_pages * bigpage_size;
                const node = @as(*usize, @ptrFromInt(addr + (big_slot_size_bytes - @sizeOf(usize))));
                const big_class = math.log2(pow2_pages);
                node.* = big_frees[big_class];
                big_frees[big_class] = addr;
            }
        }

        inline fn bigPagesNeeded(byte_count: usize) usize {
            return (byte_count + (bigpage_size + (@sizeOf(usize) - 1))) / bigpage_size;
        }

        fn allocBigPages(n: usize) usize {
            const pow2_pages = math.ceilPowerOfTwoAssert(usize, n);
            const slot_size_bytes = pow2_pages * bigpage_size;
            const class = math.log2(pow2_pages);

            const top_free_ptr = big_frees[class];
            if (top_free_ptr != 0) {
                const node = @as(*usize, @ptrFromInt(top_free_ptr + (slot_size_bytes - @sizeOf(usize))));
                big_frees[class] = node.*;
                return top_free_ptr;
            }
            return sbrk(pow2_pages * pages_per_bigpage * mem.page_size);
        }
    };
}