const WasmPageAllocator = @This();
const std = @import("../std.zig");
const builtin = @import("builtin");
const Allocator = std.mem.Allocator;
const mem = std.mem;
const maxInt = std.math.maxInt;
const assert = std.debug.assert;
comptime {
if (!builtin.target.isWasm()) {
@compileError("WasmPageAllocator is only available for wasm32 arch");
}
}
pub const vtable = Allocator.VTable{
.alloc = alloc,
.resize = resize,
.free = free,
};
const PageStatus = enum(u1) {
used = 0,
free = 1,
pub const none_free: u8 = 0;
};
const FreeBlock = struct {
data: []u128,
const Io = std.packed_int_array.PackedIntIo(u1, .Little);
fn totalPages(self: FreeBlock) usize {
return self.data.len * 128;
}
fn isInitialized(self: FreeBlock) bool {
return self.data.len > 0;
}
fn getBit(self: FreeBlock, idx: usize) PageStatus {
const bit_offset = 0;
return @intToEnum(PageStatus, Io.get(mem.sliceAsBytes(self.data), idx, bit_offset));
}
fn setBits(self: FreeBlock, start_idx: usize, len: usize, val: PageStatus) void {
const bit_offset = 0;
var i: usize = 0;
while (i < len) : (i += 1) {
Io.set(mem.sliceAsBytes(self.data), start_idx + i, bit_offset, @enumToInt(val));
}
}
const not_found = maxInt(usize);
fn useRecycled(self: FreeBlock, num_pages: usize, log2_align: u8) usize {
@setCold(true);
for (self.data, 0..) |segment, i| {
const spills_into_next = @bitCast(i128, segment) < 0;
const has_enough_bits = @popCount(segment) >= num_pages;
if (!spills_into_next and !has_enough_bits) continue;
var j: usize = i * 128;
while (j < (i + 1) * 128) : (j += 1) {
var count: usize = 0;
while (j + count < self.totalPages() and self.getBit(j + count) == .free) {
count += 1;
const addr = j * mem.page_size;
if (count >= num_pages and mem.isAlignedLog2(addr, log2_align)) {
self.setBits(j, num_pages, .used);
return j;
}
}
j += count;
}
}
return not_found;
}
fn recycle(self: FreeBlock, start_idx: usize, len: usize) void {
self.setBits(start_idx, len, .free);
}
};
var _conventional_data = [_]u128{0} ** 16;
const conventional = FreeBlock{ .data = &_conventional_data };
var extended = FreeBlock{ .data = &[_]u128{} };
fn extendedOffset() usize {
return conventional.totalPages();
}
fn nPages(memsize: usize) usize {
return mem.alignForward(memsize, mem.page_size) / mem.page_size;
}
fn alloc(ctx: *anyopaque, len: usize, log2_align: u8, ra: usize) ?[*]u8 {
_ = ctx;
_ = ra;
if (len > maxInt(usize) - (mem.page_size - 1)) return null;
const page_count = nPages(len);
const page_idx = allocPages(page_count, log2_align) catch return null;
return @intToPtr([*]u8, page_idx * mem.page_size);
}
fn allocPages(page_count: usize, log2_align: u8) !usize {
{
const idx = conventional.useRecycled(page_count, log2_align);
if (idx != FreeBlock.not_found) {
return idx;
}
}
const idx = extended.useRecycled(page_count, log2_align);
if (idx != FreeBlock.not_found) {
return idx + extendedOffset();
}
const next_page_idx = @wasmMemorySize(0);
const next_page_addr = next_page_idx * mem.page_size;
const aligned_addr = mem.alignForwardLog2(next_page_addr, log2_align);
const drop_page_count = @divExact(aligned_addr - next_page_addr, mem.page_size);
const result = @wasmMemoryGrow(0, @intCast(u32, drop_page_count + page_count));
if (result <= 0)
return error.OutOfMemory;
assert(result == next_page_idx);
const aligned_page_idx = next_page_idx + drop_page_count;
if (drop_page_count > 0) {
freePages(next_page_idx, aligned_page_idx);
}
return @intCast(usize, aligned_page_idx);
}
fn freePages(start: usize, end: usize) void {
if (start < extendedOffset()) {
conventional.recycle(start, @min(extendedOffset(), end) - start);
}
if (end > extendedOffset()) {
var new_end = end;
if (!extended.isInitialized()) {
new_end -= 1;
extended.data = @intToPtr([*]u128, new_end * mem.page_size)[0 .. mem.page_size / @sizeOf(u128)];
mem.set(u128, extended.data, PageStatus.none_free);
}
const clamped_start = @max(extendedOffset(), start);
extended.recycle(clamped_start - extendedOffset(), new_end - clamped_start);
}
}
fn resize(
ctx: *anyopaque,
buf: []u8,
log2_buf_align: u8,
new_len: usize,
return_address: usize,
) bool {
_ = ctx;
_ = log2_buf_align;
_ = return_address;
const aligned_len = mem.alignForward(buf.len, mem.page_size);
if (new_len > aligned_len) return false;
const current_n = nPages(aligned_len);
const new_n = nPages(new_len);
if (new_n != current_n) {
const base = nPages(@ptrToInt(buf.ptr));
freePages(base + new_n, base + current_n);
}
return true;
}
fn free(
ctx: *anyopaque,
buf: []u8,
log2_buf_align: u8,
return_address: usize,
) void {
_ = ctx;
_ = log2_buf_align;
_ = return_address;
const aligned_len = mem.alignForward(buf.len, mem.page_size);
const current_n = nPages(aligned_len);
const base = nPages(@ptrToInt(buf.ptr));
freePages(base, base + current_n);
}
test "internals" {
const page_allocator = std.heap.page_allocator;
const testing = std.testing;
const conventional_memsize = WasmPageAllocator.conventional.totalPages() * mem.page_size;
const initial = try page_allocator.alloc(u8, mem.page_size);
try testing.expect(@ptrToInt(initial.ptr) < conventional_memsize);
var inplace = try page_allocator.realloc(initial, 1);
try testing.expectEqual(initial.ptr, inplace.ptr);
inplace = try page_allocator.realloc(inplace, 4);
try testing.expectEqual(initial.ptr, inplace.ptr);
page_allocator.free(inplace);
const reuse = try page_allocator.alloc(u8, 1);
try testing.expectEqual(initial.ptr, reuse.ptr);
page_allocator.free(reuse);
const padding = try page_allocator.alloc(u8, conventional_memsize);
page_allocator.free(padding);
const ext = try page_allocator.alloc(u8, conventional_memsize);
try testing.expect(@ptrToInt(ext.ptr) >= conventional_memsize);
const use_small = try page_allocator.alloc(u8, 1);
try testing.expectEqual(initial.ptr, use_small.ptr);
page_allocator.free(use_small);
inplace = try page_allocator.realloc(ext, 1);
try testing.expectEqual(ext.ptr, inplace.ptr);
page_allocator.free(inplace);
const reuse_extended = try page_allocator.alloc(u8, conventional_memsize);
try testing.expectEqual(ext.ptr, reuse_extended.ptr);
page_allocator.free(reuse_extended);
}