[PUSHED] Fix off-by-one storage problem in irange_allocator.

[Andrew MacLeod]

On 10/6/20 6:40 AM, Andreas Schwab wrote:
> On Okt 06 2020, Jakub Jelinek wrote:
>
>> On Tue, Oct 06, 2020 at 10:47:34AM +0200, Andreas Schwab wrote:
>>> On Okt 06 2020, Jakub Jelinek via Gcc-patches wrote:
>>>
>>>> I mean, we could just use:
>>>>    size_t nbytes = sizeof (irange) + sizeof (tree) * 2 * num_pairs;
>>>>    irange *r = (irange *) obstack_alloc (&m_obstack, nbytes);
>>>>    return new (r) irange ((tree *) (r + 1), num_pairs);
>>>> without any new type.
>>> Modulo proper alignment.
>> Sure, but irange's last element is tree * which is pointer to pointer,
>> and we need here an array of tree, i.e. pointers.  So, it would indeed break
>> on a hypothetical host that has smaller struct X ** alignment than struct X *
>> alignment.  I'm not aware of any.
>> One could add a static_assert to verify that (that alignof (irange) >= alignof (tree)
>> and that sizeof (irange) % alignof (tree) == 0).
> I think the proper alignment will be guaranteed if irange and tree[] are
> obstack_alloc'd separately.  They don't need to be adjacent, do they?
>
>
They do not, it just seemed wasteful to do 2 allocs each time, and it'd 
be nice to have them co-located since accessing one inevitable leads to 
accessing the other.