Re: Strange Bitmapset manipulation in DiscreteKnapsack()

On Thu, Jan 18, 2024 at 8:35 AM David Rowley <dgrowleyml@gmail.com> wrote:

> The functions's header comment mentions "The bitmapsets are all
> pre-initialized with an unused high bit so that memory allocation is
> done only once.".


Ah, I neglected to notice this when reviewing the v1 patch.  I guess
it's implemented this way due to performance considerations, right?


> I've attached a patch which adds bms_replace_members(). It's basically
> like bms_copy() but attempts to reuse the member from another set. I
> considered if the new function should be called bms_copy_inplace(),
> but left it as bms_replace_members() for now.


Do you think we can use 'memcpy(a, b, BITMAPSET_SIZE(b->nwords))'
directly in the new bms_replace_members() instead of copying the
bitmapwords one by one, like:

-   i = 0;
-   do
-   {
-       a->words[i] = b->words[i];
-   } while (++i < b->nwords);
-
-   a->nwords = b->nwords;
+   memcpy(a, b, BITMAPSET_SIZE(b->nwords));

But I'm not sure if this is an improvement or not.

Thanks
Richard