RFR (S) 8211926: Catastrophic size_t underflow in BitMap::*_large methods

[Kim Barrett]

> On Nov 5, 2018, at 7:07 PM, Kim Barrett <kim.barrett at oracle.com> wrote:

Some more comments

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src/hotspot/share/utilities/bitMap.cpp
 269   idx_t beg_full_word = word_index_round_up(beg);

Pre-existing: Note that word_index_round_up can overflow, not that
it's a practical concern.  See JDK-8213415.

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src/hotspot/share/utilities/bitMap.cpp
 272   if ((end_full_word < beg_full_word) || (end_full_word - beg_full_word < BitsPerWord)) {

Rather than adding an additional check to prevent reaching the
underflow, consider something like the following, which doesn’t
have any under/overflow to worry about.

  // Treat range as small if not more than this many words.
  const idx_t small_word_count = 32;
  idx_t beg_full_word = word_index(beg);
  idx_t end_full_word = word_index(end);
  if (beg_full_word + small_word_count >= end_full_word) {
    return set_range(beg, end);
  }
  if (!is_word_aligned(beg)) {
    ++beg_full_word;
    set_range_within_word(beg, bit_index(beg_full_word));
  }
  set_large_range_of_words(beg_full_word, end_full_word);
  set_range_within_word(bit_index(end_full_word, end);

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