[PATCH] D33104: [BitVector] Implement find_[first/last]_[set/unset]_in

[Zachary Turner via llvm-commits]

At the risk of asking a stupid question, how about returning
Optional<unsigned> (in a subsequent patch of course)?
On Wed, May 17, 2017 at 8:36 AM Chandler Carruth via Phabricator <
reviews at reviews.llvm.org> wrote:

> chandlerc accepted this revision.
> chandlerc added a comment.
> This revision is now accepted and ready to land.
>
> LGTM with the assert changes.
>
>
>
> ================
> Comment at: llvm/include/llvm/ADT/BitVector.h:151
> +  /// [Begin, End).  Returns -1 if all bits in the range are unset.
> +  int find_first_in(unsigned Begin, unsigned End) const {
> +    if (Begin >= End)
> ----------------
> zturner wrote:
> > chandlerc wrote:
> > > Given that we return an int, I would prefer that the arguments also be
> int (and the intermediate variables within).
> > The only reason we return an int is because we have to use a sentinel
> value to indicate "no bits found".  The methods that these originated from
> `find_next`, `find_prev`, etc have always taken `unsigned` arguments even
> before I started tinkering on `BitVector`.  Do you still think it's a good
> idea to change all of them?
> I mean, my view is that we're doing a lot of arithmetic on these things
> including potentially subtraction. So generally, unless there is a problem
> with using a signed integer, I would prefer that.
>
> If we return `int`, then we've given up on having more than `2^31 - 1`
> bits in one of these containers.
>
> That said, a bitvector is one of the few datastructures where it seems
> entirely conceivable that we'll actually end up with 500mb vector of bits
> and an `int` index won't work. But an `unsigned` won't either, so I suspect
> that isn't the deciding factor and if we care we need a 64-bit type.
>
>
> So I guess my feeling is that we should do one of two things here:
> a) Switch the return types to an unsigned type and use something like
> `npos`, or
> b) Switch the argument types to a signed type and continue to use `-1`.
>
> (Whether we use 32-bits or 64-bits should be orthogonal as I see no reason
> why for a *bit* count 2gb vs 4gb would be "enough".)
>
> However, looking at the code, I agree that the patch as-is remains
> consistent, and you shouldn't do either (a) or (b) in *this* patch. But I'd
> somewhat like to consider doing one of them in a subsequent patch.
>
> My personal, strong preference is to use (b) because I find asserts easier
> to write for signed types in the face of arithmetic and we have strong
> usage of UBSan to ensure we don't develop bugs here. I also have a somewhat
> powerful dislike of `npos`. But I think either (a) or (b) would be an
> improvement. Anyways, sorry for the digression.
>
>
> ================
> Comment at: llvm/include/llvm/ADT/BitVector.h:152-153
> +  int find_first_in(unsigned Begin, unsigned End) const {
> +    if (Begin >= End)
>        return -1;
>
> ----------------
> zturner wrote:
> > chandlerc wrote:
> > > Why isn't this an assert?
> > I suppose I should assert if `Begin > End`, but `Begin == End` should
> probably still just return -1 I think.
> Ah, yes, that makes sense.
>
>
> https://reviews.llvm.org/D33104
>
>
>
>
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