RFR: 8213415: BitMap::word_index_round_up overflow problems

[Thomas Stüfe]

Hi Stefan,

On Mon, Dec 2, 2019 at 11:11 AM Stefan Karlsson <stefan.karlsson at oracle.com>
wrote:

> On 2019-11-29 21:37, Kim Barrett wrote:
> >> On Nov 29, 2019, at 3:03 AM, Thomas Stüfe <thomas.stuefe at gmail.com>
> wrote:
> >>
> >> Side note, I was interested in using smaller types because long term I
> would like to have a BitMap class in cases where I today use little hand
> written bitmaps. As it is now, BitMap has a pointer and a size, which makes
> it a 16byte structure on 64 bit, which is rather fat. The indirection is
> also often unwanted. I would like to have a BitMap class which contains
> directly the data as member(s), e.g. one where it just has a 16bit word or,
> maybe, an array of multiple words. That would make this structure a lot
> smaller and better suited to be included in space sensitive structures.
> > There was some discussion here in Oracle about this sort of thing a
> > while ago. Looking back over that discussion, I don't think we got
> > very far with a statically sized bitmap, just some handwaving. I think
> > the interest is there, but ideas (and time to persue them!) are
> > needed.
>
> FWIW, see the usage of BitMapView in zLiveMap.hpp and zLiveMap.inline.hpp.
>
> The bitmap backing is a single word inside ZLiveMap:
>    BitMap::bm_word_t _segment_live_bits;
>
> and the size is constant:
>    static const size_t nsegments = 64;
>
> The views are created with:
> inline BitMapView ZLiveMap::segment_live_bits() {
>    return BitMapView(&_segment_live_bits, nsegments);
> }
>
> and then used with:
> inline bool ZLiveMap::is_segment_live(BitMap::idx_t segment) const {
>    return segment_live_bits().par_at(segment);
> }
>
> All this gets inlined as expected.
>
> It doesn't support sizes less than a word, but it does support arrays of
> multiple words.
>
> StefanK
>

I was aware that we could put the Bitmap over existing memory, but it did
not occur to me to do this just temporarily for one operation... So, all
that business above (Bitmap construction, copying etc) gets optimized away
to a simple load and an or?

Thanks, Thomas