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

[Duncan P. N. Exon Smith via llvm-commits]

> 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?

>> 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.

>> 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.
>>>> 
>>>> 
>> 
>>