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

[Daniel Berlin via llvm-commits]

Does it? I didn't stare hard enough

On Wed, May 17, 2017 at 9:00 AM, Zachary Turner <zturner at google.com> wrote:

> 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
>>>>
>>>>
>>>>
>>>>
>>
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