On Fri, Oct 11, 2019 at 1:40 AM Jonathan Tan <jonathantanmy@xxxxxxxxxx> wrote:
>
> > From: Jeff King <peff@xxxxxxxx>
> >
> > In a following patch we will allocate a variable number
> > of words in some bitmaps. When iterating over the words we
> > will need a mark to tell us when to stop iterating. Let's
> > always allocate 2 more words, that will always contain 0,
> > as that mark.
>
> [snip]
>
> > if (block >= self->word_alloc) {
> > size_t old_size = self->word_alloc;
> > - self->word_alloc = block * 2;
> > + self->word_alloc = (block + 1) * 2;
> > REALLOC_ARRAY(self->words, self->word_alloc);
> > memset(self->words + old_size, 0x0,
> > (self->word_alloc - old_size) * sizeof(eword_t));
>
> This patch set was mentioned as needing more thorough review in "What's
> Cooking" [1], so I thought I'd give it a try.
Thanks!
> As Peff said [2], the
> justification in the commit message looks incorrect. He suggests that it
> is most likely because "block" might be 0 (which is possible because a
> previous patch eliminated the minimum of 32), which makes sense to me.
Ok I will try to come up with a better justification, though Peff said
that he would took another look at this series and I'd rather wait
until he has done that.
> In any case, the next patch does not use 0 as a sentinel mark. Iteration
> stops when word_alloc is reached anyway, and since this is a regular
> bitmap, 0 is a valid word and cannot be used as a sentinel. (Maybe 0 is
> a valid word in a compressed EWAH bitmap too...not sure about that.)
Yeah I misread this. Hopefully Peff can shed some light on this.
> I think this should be squashed with patch 3, adding to that commit
> message "since word_alloc might be 0, we need to change the growth
> function". (Or just make the minimum word_alloc be 1 or 32 or something
> positive, if that's possible.)
Yeah, thank you for the suggestion. I still wonder why 2 is added
instead of just 1 though.
