alloc

        fn alloc(context: *anyopaque, len: usize, alignment: mem.Alignment, ret_addr: usize) ?[*]u8 {
            const self: *Self = @ptrCast(@alignCast(context));
            if (have_mutex) std.Io.Threaded.mutexLock(&self.mutex);
            defer if (have_mutex) std.Io.Threaded.mutexUnlock(&self.mutex);

            if (config.enable_memory_limit) {
                const new_req_bytes = self.total_requested_bytes + len;
                if (new_req_bytes > self.requested_memory_limit) return null;
                self.total_requested_bytes = new_req_bytes;
            }

            const size_class_index: usize = @max(@bitSizeOf(usize) - @clz(len - 1), @backingInt(alignment));
            if (size_class_index >= self.buckets.len) {
                @branchHint(.unlikely);
                self.large_allocations.ensureUnusedCapacity(self.backing_allocator, 1) catch return null;
                const ptr = self.backing_allocator.rawAlloc(len, alignment, ret_addr) orelse return null;
                const slice = ptr[0..len];

                const gop = self.large_allocations.getOrPutAssumeCapacity(@intFromPtr(slice.ptr));
                if (config.retain_metadata and !config.never_unmap) {
                    // Backing allocator may be reusing memory that we're retaining metadata for
                    assert(!gop.found_existing or gop.value_ptr.freed);
                } else {
                    assert(!gop.found_existing); // This would mean the kernel double-mapped pages.
                }
                gop.value_ptr.bytes = slice;
                if (config.enable_memory_limit)
                    gop.value_ptr.requested_size = len;
                gop.value_ptr.captureStackTrace(ret_addr, .alloc);
                if (config.retain_metadata) {
                    gop.value_ptr.freed = false;
                    if (config.never_unmap) {
                        gop.value_ptr.alignment = alignment;
                    }
                }

                if (config.verbose_log) {
                    log.info("large alloc {d} bytes at {*}", .{ slice.len, slice.ptr });
                }
                return slice.ptr;
            }

            const slot_count = slot_counts[size_class_index];

            if (self.buckets[size_class_index]) |bucket| {
                @branchHint(.likely);
                const slot_index = bucket.allocated_count;
                if (slot_index < slot_count) {
                    @branchHint(.likely);
                    bucket.allocated_count = slot_index + 1;
                    const used_bits_byte = bucket.usedBits(slot_index / @bitSizeOf(usize));
                    const used_bit_index: Log2USize = @intCast(slot_index % @bitSizeOf(usize));
                    used_bits_byte.* |= (@as(usize, 1) << used_bit_index);
                    const size_class = @as(usize, 1) << @as(Log2USize, @intCast(size_class_index));
                    if (config.stack_trace_frames > 0) {
                        bucket.captureStackTrace(ret_addr, slot_count, slot_index, .alloc);
                    }
                    if (config.safety) {
                        bucket.requestedSizes(slot_count)[slot_index] = @intCast(len);
                        bucket.log2PtrAligns(slot_count)[slot_index] = alignment;
                    }
                    const page_addr = @intFromPtr(bucket) & ~(page_size - 1);
                    const addr = page_addr + slot_index * size_class;
                    if (config.verbose_log) {
                        log.info("small alloc {d} bytes at 0x{x}", .{ len, addr });
                    }
                    return @ptrFromInt(addr);
                }
            }

            const page = self.backing_allocator.rawAlloc(page_size, page_align, @returnAddress()) orelse
                return null;
            const bucket: *BucketHeader = .fromPage(@intFromPtr(page), slot_count);
            bucket.* = .{
                .allocated_count = 1,
                .freed_count = 0,
                .prev = self.buckets[size_class_index],
                .next = null,
            };
            if (self.buckets[size_class_index]) |old_head| {
                old_head.next = bucket;
            }
            self.buckets[size_class_index] = bucket;

            if (!config.backing_allocator_zeroes) {
                @memset(@as([*]usize, @as(*[1]usize, bucket.usedBits(0)))[0..usedBitsCount(slot_count)], 0);
                if (config.safety) @memset(bucket.requestedSizes(slot_count), 0);
            }

            bucket.usedBits(0).* = 0b1;

            if (config.stack_trace_frames > 0) {
                bucket.captureStackTrace(ret_addr, slot_count, 0, .alloc);
            }

            if (config.safety) {
                bucket.requestedSizes(slot_count)[0] = @intCast(len);
                bucket.log2PtrAligns(slot_count)[0] = alignment;
            }

            if (config.verbose_log) {
                log.info("small alloc {d} bytes at 0x{x}", .{ len, @intFromPtr(page) });
            }

            return page;
        }