[llvm] r243996 - Avoid passing nullptr to std::equal.

[Aaron Ballman via llvm-commits]

On Thu, Aug 20, 2015 at 2:28 PM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
>
>> On 2015-Aug-20, at 10:53, Aaron Ballman <aaron at aaronballman.com> wrote:
>>
>> On Thu, Aug 20, 2015 at 1:49 PM, Yaron Keren <yaron.keren at gmail.com> wrote:
>>> So, there are three options
>>>
>>> 1) Go _DEBUG_POINTER_IMPL route for VC2013 only. This will also disable the
>>> null checks in the other headers where they are beneficial and may miss real
>>> bugs.
>>
>> If we limit it to just MSVC 2013, I think this is a reasonable route
>> to go because we can still get reasonable debug testing from MSVC 2015
>> users who build in debug mode. Then, when we drop support for 2013,
>> the check can go away.
>>
>> ~Aaron
>>
>
> If it works, I like this best, but didn't you say that MSVC 2013 calls
> std::memcmp(nullptr) from std::equal()?  IIUC, then the interesting uses
> of std::equal() would invoke UB when compiled with MSVC 2013.  Whether or
> not it's a bug in MSVC, we shouldn't introduce UB :(.
>
> Or did I miss a beat somewhere?

Only some of the overloads call std::memcmp(); if we are not using
std::equal() over container elements that are of type char, signed
char, or unsigned char, we should be fine. It seems those are the only
overloads for _Equal() (which is used to implement std::equal()) that
invoke std::memcmp().

>
>>> 2) Replace std::equal with llvm::isEqual or stl::equal or ...
>>>
>>> 3) Solve the llvm::equal ambiguity using namespace trick while making peace
>>> with the llvm::equal functor.
>
> If #1 doesn't work, this is the option I like for the short-term, and
> we can remove the workaround (go back to std::equal()) when we drop
> support for MSVC 2013.

I would be fine with this as well if #1 cannot work out for some reason.

~Aaron

>
>>> 2015-08-18 16:13 GMT+03:00 Aaron Ballman <aaron at aaronballman.com>:
>>>>
>>>> On Mon, Aug 17, 2015 at 8:28 PM, Duncan P. N. Exon Smith
>>>> <dexonsmith at apple.com> wrote:
>>>>>
>>>>>> On 2015-Aug-17, at 16:38, Aaron Ballman <aaron at aaronballman.com> wrote:
>>>>>>
>>>>>> On Mon, Aug 17, 2015 at 7:23 PM, Yaron Keren <yaron.keren at gmail.com>
>>>>>> wrote:
>>>>>>> An  issue that was patched several times in LLVM is
>>>>>>> std::equal(nullptr,
>>>>>>> nullptr, nullptr) where VC asserts on the third nullptr. Earlier in
>>>>>>> the
>>>>>>> thread Marshal wrote this should return true and not assert.
>>>>>>
>>>>>> Yes, I've fixed some myself, IIRC. Marshall's logic makes sense to me,
>>>>>> and from my reading of the standard, it also seems to fit.
>>>>>
>>>>> 25.2.11 is pretty clear:
>>>>>
>>>>>> Returns: true if for every iterator i in the range [first1,last1) the
>>>>>> following corresponding condi- tions hold: *i == *(first2 + (i - first1)),
>>>>>> pred(*i, *(first2 + (i - first1))) != false. Otherwise, returns false.
>>>>>
>>>>>
>>>>> Unless you're suggesting that `[nullptr, nullptr)` is an invalid
>>>>> iterator range, or that `nullptr` is an invalid iterator?  But
>>>>> `[nullptr, nullptr)` supports all the necessary operations for an
>>>>> empty iterator range, so how could that be?  (Maybe I'm guessing
>>>>> wrong and this is a straw man... in which case, how could this be
>>>>> a conforming extension?)
>>>>
>>>> I took longer to read more of the standard (specifically, the parts
>>>> about library requirements in [res.on.arguments] and [iterators]), and
>>>> I don't think this is a conforming extension. Then I fired up MSVC
>>>> 2015 and did some experiments, and realized the behavior is different
>>>> in 2015, so I'm not even certain the behavior was intentional in the
>>>> first place.
>>>>
>>>> In MSVC 2013, equal would check the range and then check the last
>>>> pointer for null, and this check would trigger the assertion.
>>>> In MSVC 2015, equal would check the range, and then check the last
>>>> pointer for null only if the range is not empty, and so no assertion
>>>> is triggered.
>>>>
>>>> (There is still a degenerate case bug where std::equal(nullptr,
>>>> nullptr, nullptr) refuses to compile that I've reported.)
>>>>
>>>>>> However,
>>>>>> I'm not convinced this isn't a valid extension that prevents UB from
>>>>>> calling std::memcmp() with a null pointer value.
>>>>>
>>>>> `std::equal` isn't defined in terms of `std::memcmp()`, it's defined
>>>>> as above.  How could it be valid to return anything but `true`?  The
>>>>> wording is clear.
>>>>>
>>>>> But if MSVC is calling `std::memcmp()` directly then we probably
>>>>> don't have a choice but to reimplement `std::equal()` ourselves,
>>>>> correctly :(.
>>>>
>>>> I think that's true in MSVC 2013, unfortunately. However, I also think
>>>> that speaks to a hackish solution limited to just MSVC 2013 being
>>>> somewhat more acceptable because we will be getting rid of 2013
>>>> sometime in the future (next few years).
>>>>
>>>> ~Aaron
>>>>
>>>>>
>>>>>> I think LWG would
>>>>>> need to weigh in on the matter, but I don't know of any issues that
>>>>>> are open on this topic that we could point to for a definitive answer.
>>>>>> Perhaps Marshall is aware of something, however.
>>>>>
>>>>>
>>>
>>>
>