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const std = @import("../std.zig");
const builtin = @import("builtin");
const math = std.math;
const Allocator = std.mem.Allocator;
const mem = std.mem;
const heap = std.heap;
const assert = std.debug.assert;
pub fn SbrkAllocator(comptime sbrk: *const fn (n: usize) usize) type {
return struct {
pub const vtable: Allocator.VTable = .{
.alloc = alloc,
.resize = resize,
.remap = remap,
.free = free,
};
pub const Error = Allocator.Error;
const max_usize = math.maxInt(usize);
const ushift = math.Log2Int(usize);
const bigpage_size = 64 * 1024;
const pages_per_bigpage = bigpage_size / heap.pageSize();
const bigpage_count = max_usize / bigpage_size;
/// Because of storing free list pointers, the minimum size class is 3.
const min_class = math.log2(math.ceilPowerOfTwoAssert(usize, 1 + @sizeOf(usize)));
const size_class_count = math.log2(bigpage_size) - min_class;
/// 0 - 1 bigpage
/// 1 - 2 bigpages
/// 2 - 4 bigpages
/// etc.
const big_size_class_count = math.log2(bigpage_count);
var next_addrs = [1]usize{0} ** size_class_count;
/// For each size class, points to the freed pointer.
var frees = [1]usize{0} ** size_class_count;
/// For each big size class, points to the freed pointer.
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, alignment: mem.Alignment, return_address: usize) ?[*]u8 {
_ = ctx;
_ = return_address;
lock.lock();
defer lock.unlock();
// Make room for the freelist next pointer.
const actual_len = @max(len +| @sizeOf(usize), alignment.toByteUnits());
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 % heap.pageSize() == 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,
alignment: mem.Alignment,
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 = alignment.toByteUnits();
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 remap(
context: *anyopaque,
memory: []u8,
alignment: mem.Alignment,
new_len: usize,
return_address: usize,
) ?[*]u8 {
return if (resize(context, memory, alignment, new_len, return_address)) memory.ptr else null;
}
fn free(
ctx: *anyopaque,
buf: []u8,
alignment: mem.Alignment,
return_address: usize,
) void {
_ = ctx;
_ = return_address;
lock.lock();
defer lock.unlock();
const buf_align = alignment.toByteUnits();
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 * heap.pageSize());
}
};
}
test SbrkAllocator {
_ = SbrkAllocator(struct {
fn sbrk(_: usize) usize {
return 0;
}
}.sbrk);
}
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