Rewrite stacking arena allocator to be correct
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523509d300
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2f4cab3cc7
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@ -7,35 +7,47 @@ const list = @import("./list.zig");
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const math = @import("./math.zig");
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pub const Stacking = struct {
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page_allocator: io.Allocator,
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min_page_size: usize,
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allocations: list.Stack(usize),
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pages: list.Stack(Page),
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allocator: io.Allocator,
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min_region_size: usize,
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head_region: ?*Region,
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tail_region: ?*Region,
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const AllocationsList = list.Stack(usize);
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const Region = struct {
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next: ?*Region,
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count: usize,
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capacity: usize,
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const Page = struct {
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buffer: []io.Byte,
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used: usize,
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fn allocate(self: *Region, region_size: usize) []usize {
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debug.assert(self.can_allocate(region_size));
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fn available(self: Page) usize {
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return self.buffer.len - self.used;
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const offset = (@sizeOf(Region) / @alignOf(usize)) + self.count;
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const allocation = @as([*]usize, @ptrCast(self))[offset .. (offset + region_size)];
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self.count += region_size;
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return allocation;
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}
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fn as_raw(self: *Region) []usize {
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return @as([*]usize, @ptrCast(self))[0 .. ((@sizeOf(Region) / @alignOf(usize)) + self.capacity)];
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}
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fn can_allocate(self: Region, region_size: usize) bool {
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return (self.count + region_size) <= self.capacity;
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}
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};
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const PageList = list.Stack(Page);
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fn allocate_region(self: *Stacking, requested_region_size: usize) io.AllocationError!*Region {
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const region_size = @max(requested_region_size, self.min_region_size);
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const region = @as(*Region, @ptrCast(@alignCast(try self.allocator.reallocate(null, @alignOf(Region) + (@alignOf(usize) * region_size)))));
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fn allocate_page(self: *Stacking, page_size: usize) io.AllocationError!*Page {
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var buffer = try self.page_allocator.reallocate(null, page_size);
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region.* = .{
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.next = null,
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.count = 0,
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.capacity = region_size,
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};
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errdefer self.page_allocator.deallocate(buffer);
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try self.pages.push_one(.{
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.buffer = buffer,
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.used = 0,
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});
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return (self.current_page() orelse unreachable);
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return region;
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}
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pub fn as_allocator(self: *Stacking) io.Allocator {
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@ -45,67 +57,60 @@ pub const Stacking = struct {
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});
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}
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fn current_page(self: Stacking) ?*Page {
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if (self.pages.values.len == 0) {
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return null;
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}
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return &self.pages.values[self.pages.values.len - 1];
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}
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pub fn free(self: *Stacking) void {
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for (self.pages.values) |page| {
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self.page_allocator.deallocate(page.buffer);
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while (self.head_region) |region| {
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const next_region = region.next;
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self.allocator.deallocate(region.as_raw());
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self.head_region = next_region;
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}
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self.pages.free();
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self.allocations.free();
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self.head_region = null;
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self.tail_region = null;
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}
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pub fn deallocate(_: *Stacking, _: []io.Byte) void {
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fn deallocate(_: *Stacking, _: []io.Byte) void {
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// TODO: Decide how to implement.
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unreachable;
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}
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pub fn reallocate(self: *Stacking, return_address: usize, existing_allocation: ?[]io.Byte, size: usize) io.AllocationError![]io.Byte {
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// TODO: Safety-check existing allocation is from allocator or null.
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_ = return_address;
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const alignment = @as(usize, 4);
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const aligned_size = (size + alignment - 1) & ~(alignment - 1);
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if (self.pages.values.len == 0) {
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const page = try self.allocate_page(@max(self.min_page_size, aligned_size));
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page.used = size;
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return page.buffer[0 .. size];
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fn reallocate(self: *Stacking, _: usize, allocation: ?[]io.Byte, byte_size: usize) io.AllocationError![]io.Byte {
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if (allocation) |buffer| {
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if (byte_size < buffer.len) {
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return buffer[0 .. byte_size];
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}
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}
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var page = self.current_page() orelse unreachable;
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const region_size = (byte_size + @sizeOf(usize) - 1) / @sizeOf(usize);
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if (page.available() <= aligned_size) {
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page = try self.allocate_page(@max(self.min_page_size, aligned_size));
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const tail_region = self.tail_region orelse {
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const region = try self.allocate_region(region_size);
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self.tail_region = region;
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self.head_region = self.tail_region;
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return @as([*]io.Byte, @ptrCast(region.allocate(region_size)))[0 .. byte_size];
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};
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if (!tail_region.can_allocate(region_size)) {
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const region = try self.allocate_region(region_size);
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tail_region.next = region;
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self.tail_region = region;
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return @as([*]io.Byte, @ptrCast(region.allocate(region_size)))[0 .. byte_size];
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}
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debug.assert(page.available() >= size);
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defer page.used += aligned_size;
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const reallocation = page.buffer[page.used .. (page.used + size)];
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if (existing_allocation) |allocation| {
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io.copy(reallocation, allocation);
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return @as([*]io.Byte, @ptrCast(tail_region.allocate(region_size)))[0 .. byte_size];
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}
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return reallocation;
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}
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pub fn make(allocator: io.Allocator, min_page_size: usize) Stacking {
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pub fn make(allocator: io.Allocator, min_region_size: usize) Stacking {
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return Stacking{
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.allocations = AllocationsList.make(allocator),
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.pages = PageList.make(allocator),
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.page_allocator = allocator,
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.min_page_size = min_page_size,
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.allocator = allocator,
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.min_region_size = min_region_size,
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.head_region = null,
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.tail_region = null,
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};
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}
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};
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