realloc

pub fn realloc(uncasted_memory: []u8, alignment: Alignment, new_len: usize, may_move: bool) ?[*]u8 {
    const memory: []align(page_size_min) u8 = @alignCast(uncasted_memory);
    const page_size = std.heap.pageSize();
    if (alignment.toByteUnits() > page_size) return null;
    const new_size_aligned = mem.alignForward(usize, new_len, page_size);

    if (native_os == .windows) {
        if (new_len <= memory.len) {
            const base_addr = @intFromPtr(memory.ptr);
            const old_addr_end = base_addr + memory.len;
            const new_addr_end = mem.alignForward(usize, base_addr + new_len, page_size);
            if (old_addr_end > new_addr_end) {
                var decommit_addr: ?*anyopaque = @ptrFromInt(new_addr_end);
                var decommit_size: windows.SIZE_T = old_addr_end - new_addr_end;

                _ = ntdll.NtAllocateVirtualMemory(windows.GetCurrentProcess(), @ptrCast(&decommit_addr), 0, &decommit_size, .{ .RESET = true }, .{ .NOACCESS = true });
            }
            return memory.ptr;
        }
        const old_size_aligned = mem.alignForward(usize, memory.len, page_size);
        if (new_size_aligned <= old_size_aligned) {
            return memory.ptr;
        }
        return null;
    }

    const page_aligned_len = mem.alignForward(usize, memory.len, page_size);
    if (new_size_aligned == page_aligned_len)
        return memory.ptr;

    // When the stack grows down, only use `mremap` if the allocation may move.
    // Otherwise, we might grow the allocation and intrude on virtual address
    // space which we want to keep available to the stack.
    if (posix.MREMAP != void and (stack_direction == .up or may_move)) {
        // TODO: if the next_mmap_addr_hint is within the remapped range, update it
        const new_memory = posix.mremap(memory.ptr, page_aligned_len, new_size_aligned, .{ .MAYMOVE = may_move }, null) catch return null;
        return new_memory.ptr;
    }

    if (new_size_aligned < page_aligned_len) {
        const ptr = memory.ptr + new_size_aligned;
        // TODO: if the next_mmap_addr_hint is within the unmapped range, update it
        posix.munmap(@alignCast(ptr[0 .. page_aligned_len - new_size_aligned]));
        return memory.ptr;
    }

    return null;
}