RFR: 8213415: BitMap::word_index_round_up overflow problems

[Kim Barrett]

> On Nov 29, 2019, at 3:03 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> 
> Hi Kim,
> 
> On Thu, Nov 28, 2019 at 11:01 PM Kim Barrett <kim.barrett at oracle.com> wrote:
> > On Nov 28, 2019, at 7:42 AM, Thomas Stüfe <thomas.stuefe at gmail.com> wrote:
> > So, if I understand this correctly, we need this since we use the same type for bit and word indices and since we have a n:1 relationship between those two the max. bit index is necessary smaller than <type>_MAX?
> 
> The point is to avoid overflow of the type used for bit indices when
> aligning a value up to a multiple of the word size. This doesn't
> really have anything to do with using the same types for bit indices
> and word indices, though using different types might affect the
> details of some of the calculations, and the range for the word type
> would need to be suitably chosen to accomodate the bit range.
> 
> 
> I still in the dark. In your current version max_size_in_words() and max_size_in_bits() there is an overflow, since both bit- and word indexes use the same type. With 64bit I come to: FFFFFFFF.FFFFFFC0 for max word index, 3FFFFFF.FFFFFFFF for max bit index. For 64bit types this does not matter much, but if we ever were to use smaller types, e.g. uint16_t, it would matter. Also, I find it surprising that max bit index is smaller than max word index. 

You have the values backward.

The max bit index is certainly not the smaller, since it is a multiple of max word size.