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

Zachary Turner via llvm-commits llvm-commits at lists.llvm.org
Wed May 17 09:00:07 PDT 2017


Doesn't it already do this for *all* types?  It's only using placement new,
but the storage is stack-allocated.

On Wed, May 17, 2017 at 8:57 AM Daniel Berlin <dberlin at dberlin.org> wrote:

> Yeah, I was about to suggest this.
> We could specialize Optional for POD types so it doesn't spend time trying
> to new a buffer to store them :)
>
>
> On Wed, May 17, 2017 at 8:50 AM, Zachary Turner <zturner at google.com>
> wrote:
>
>> 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
>>>
>>>
>>>
>>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170517/8afa8bab/attachment-0001.html>


More information about the llvm-commits mailing list