[PATCH] RFC: fail-fast iterators for DenseMap

[Sanjoy Das]

I'd say we should write the erase loop as:

    for (auto I = M.begin(), E = M.end(); I != E;)
      if (foo(*I))
        (I,E) = M.erase(I);
      else
        ++I;

On Mon, Mar 2, 2015 at 9:27 PM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
> +sanjoy (somehow I dropped you).
>
>> On 2015 Mar 2, at 20:38, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>
>>
>>> On 2015 Mar 2, at 20:37, Chandler Carruth <chandlerc at google.com> wrote:
>>>
>>>
>>> On Mon, Mar 2, 2015 at 8:35 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>>>> For `==` and `!=, you shouldn't worry about whether the handle is in sync
>>>> with the debug base -- if anything, you should just check that the two
>>>> handles are pointing at the same debug base (and have the same epoch as
>>>> each other).  (But I still don't see how you've modified `==` or `!=`.)
>>>>
>>>> The comparison operators call operator->
>>>
>>> (Sorry, should have just looked at the code myself.)
>>>
>>> That's silly though.  They should just be:
>>>
>>>    bool operator==(const ConstIterator &RHS) const { return Ptr == RHS.Ptr; }
>>>    bool operator!=(const ConstIterator &RHS) const { return Ptr != RHS.Ptr; }
>>>
>>> Calling `->` is needlessly complicated.
>>>
>>> Sure.
>>>
>>> But I think we *should* check the epoch here as comparing an invalid iterator with a valid iterator should also be caught. I would check that the address of the epoch are the same, and if they are non-null, that all three epoch's (the pointed too and both iterator's copies) are in sync.
>>
>> SGTM.
>
> Actually, I wonder if this check is too strict.  Consider an
> `erase()` loop:
>
>     for (auto I = M.begin(), E = M.end(); I != E;)
>       if (foo(*I))
>         I = M.erase(I);
>       else
>         ++I;
>
> Although it's not part of this patch, I think we should change
> `erase()` to bump the epoch in a follow-up patch (at the very
> least, it should bump the epoch on `SmallPtrSet::erase()`).
> With your strict semantics, `I` and `E` wouldn't be comparable
> after an `erase()` call since `E`'s epoch wouldn't be bumped.
>
> I'd rather just check that the addresses of the epochs are the
> same, and skip the check on the epochs.