> Are you saying that a C99 compiler won’t complaint if the user passes
a 64b int to a 32b int argument? That’s a pretty stupid compiler if you
ask me.
It is perfectly valid in C *and* C++ to pass a 64bit integer value as an
argument to a 32bit integer parameter. The 64bit integer value will be
converted to 32bit integer and thus potentially truncated. Depending on
the original value this may result in a negative or positive 32bit
signed integer value so depending on the size you want to send, MPI_Send
may or may not detect the overflow.
Passing a pointer to a 64bit integer (int64_t*) for a parameter that is
a pointer to a 32bit integer (int32_t*) is a bit different. In C, this
is still valid and the compiler will only issue a warning about the type
mismatch. C++ handles types stricter and pointers are not implicitly
converted.
> I’m fine with putting MPI C11 in separate header that can #error if
C11 isn’t supported.
I don't have a strong opinion on this and I don't know if this (and the
implications of introducing a new header file) has been discussed in the
forum already. Maybe Jeff S has considered this already?
> In any case, we can’t design MPI around ignorant users who don’t read
about features they’re using.
I'm not advocating to change the design, i.e., I'm not arguing against
having C11 _Generic selectors. I just think it would be beneficial to
clearly mark the potential caveat.
Joseph
On 8/2/19 7:19 AM, Jeff Hammond wrote:
Are you saying that a C99 compiler won’t complaint if the user passes a
64b int to a 32b int argument? That’s a pretty stupid compiler if you
ask me.
I’m fine with putting MPI C11 in separate header that can #error if C11
isn’t supported. That’s a pretty obvious user experience win that costs
nothing. We are basically doing that for Fortran already.
Maclaren can just skip C11 and teach MPI in Fortran 2008 or C99 with the
_x symbols if the C11 is too scary.
In any case, we can’t design MPI around ignorant users who don’t read
about features they’re using. Doing so makes it impossible to do
anything at all because there’s always somebody too ignorant to use some
feature correctly, and the union of the ignorance covers all nontrivial
changes.
Jeff
On Thu, Aug 1, 2019 at 7:28 AM Joseph Schuchart via mpi-forum
<mpi-forum@lists.mpi-forum.org <mailto:mpi-forum@lists.mpi-forum.org>>
wrote:
I think the point he wanted to make was that you won't see a
compile-time error if you /think/ you're using the MPI_Count overloads
but are in fact not, i.e., you are modernizing a legacy code base that
is stuck in the nineties and you introduce MPI_Count for size arguments
because the standard says you may do so now without a clear warning
that
this is only possible if you bump to C11. The compiler won't warn you,
the MPI standard may mention it in a subsection somewhere, which of
course you didn't read because you haven't had the time to read all ~1k
pages of 4.0, yet. Having a clear note attached to listings that these
overloads are constrained to C11 may help reduce the risk of
introducing
another caveat. MPI developers may be (more) familiar with the
differences between C99 and C11 but the average developer of HPC
software likely is not. And there are plenty of codes out there that
constrain themselves to ancient language versions, for reasons unknown
to me...
Joseph
On 8/1/19 12:56 PM, Jeff Hammond via mpi-forum wrote:
> That’s why there will be C90/C99 compatible symbols as well. If
you don’t like C11, don’t use it. Nothing will happen. BigCount will
still work.
>
> C11 has been the default in GCC and Clang for a while. What
compilers are going to limit users to C99 for years to come?
>
> Jeff
>
>> On Aug 1, 2019, at 3:23 AM, N.M. Maclaren via mpi-forum
<mpi-forum@lists.mpi-forum.org
<mailto:mpi-forum@lists.mpi-forum.org>> wrote:
>>
>>> On Jul 30 2019, Jeff Squyres (jsquyres) via mpi-forum wrote:
>>>
>>> B. C11 _Generic polymorphism kinda sucks, *and* we're in a
transition period where not all C compilers are C11-capable. Hence,
we're exposing up to *3* C bindings per MPI procedure to
applications (including explicitly exposing the "_x" variant where
relevant).
>>
>> This following point may have already been debated, so please
excuse me if
>> has. Let's skip the politics, for the sake of all our blood
pressures.
>>
>> The executive summary is that the failure mode pointed out by Joseph
>> Schuchart is going to be a very, very serious problem for
BigCount users,
>> and continue for the forseeable future. I would guess until at
least
>> 2025, and possibly long after that.
>>
>> Speaking as someone who may be teaching MPI programming again,
with an
>> emphasis on reliability and portability, I would almost
certainly add
>> warnings that would be, roughly: "Don't touch BigCount if you
can find a
>> way round it; and be paranoid about it if you use it." I
already do that
>> about I/O attributes, for similar reasons. That isn't good.
>>
>> I don't know how you would document that, but the MPI standard
already
>> has gotchas that aren't easy to find, and adding another one
isn't good,
>> either.
>>
>> The explanation:
>>
>> C99 was not received favourably by most of the C-using community
(I don't
>> mean compilers here). I tracked a dozen important, active
projects, and
>> it was 2010/11 (yes, over a decade) before even half of them
converted
>> from C90 to C99 as a base. I last checked a few years ago, but
quite a
>> few C99 features were still not reliably available in compilers;
I know
>> that many of the ones I found still aren't. Courses are another
problem,
>> because they rarely include warnings about gotchas caused by
standards
>> differences (and there are lots between C90 and C99).
>>
>> I haven't tracked C11 as carefully, but the evidence I have seen
is that it
>> received even less interest and acceptance than C99, so people
are going
>> to be using C99 compilers for a LONG time yet. There is also
the problem
>> that C is not a language that is upwards compatible between
versions, but
>> C++ takes more notice, so C++ compilers' C support (which is
arguably more
>> important than direct C support, because people call MPI using
C++'s C
>> interface) is often in conflict. This is almost certainly a
case where
>> that will be true, but it may not affect these interfaces - I
can't say.
>>
>> The result is that C code (and, worse, libraries) often require
specific
>> versions (i.e. not just 'any standard later than'). I agree that
it looks
>> likely that generic interfaces are going to be one of the more
widely
>> implemented parts of C99, but don't discount the problem of
people using
>> multiple libraries where other ones constrain the C version for
other
>> reasons.
>>
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http://jeffhammond.github.io/
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