[Mpi-22] Borderline ticket #107: Types of predefined constants in Appendix A

[Erez Haba]

I agree David,  and as I said, it might be nitpicking :)

Most compliers would probably optimize the cast out (Microsoft compiler sure does); however the question still remains; do implementers have to change the definition of their constant variables to meet this table?

As for the second item, the point I'm making is that adding the 'const' qualifier for the types in the tables does not help clarity and it does quite the opposite. Would adding 'const' to MPI_Datatype, MPI_Comm or MPI_Op types in these tables helps?

Which brings me to my third note which is, the use of 'const' in these tables is not consistent. Some types get the const modifiers and some do not.

David, per your comment below, and to the audience, please note that this ticket is not just a trivial text change to the standard, as it introduces types for constants that were not assigned in the standard before.
As you said below David, adding 'const' makes them non-lvalue for the specific C binding.

I know that this ticket has passed 2nd vote, imho, wrongly without giving enough consideration to its content; as it was introduced as text only change. See the large number of abstains (including me). However it's up to us to decide. I don't know what the rules say, and if there is a way to take back this ticket without taking the entire chapter out.

Thoughts??

Thanks,
.Erez

-----Original Message-----
From: mpi-22-bounces_at_[hidden] [mailto:mpi-22-bounces_at_[hidden]] On Behalf Of Dave Goodell
Sent: Monday, August 03, 2009 2:00 PM
To: MPI 2.2
Subject: Re: [Mpi-22] Borderline ticket #107: Types of predefined constants in Appendix A

On Aug 3, 2009, at 12:36 PM, Erez Haba wrote:

> This might be nitpicking but wouldn't that force implementation to
> typecast their definition of error codes? For example,
>
>        #define MPI_ERR_BUFFER 0
>
> Is not a correct definition with the proposed change, because it is
> not typed.

I think that the "Constants with the type const int may also be
implemented as literal integer constants substituted by the
preprocessor" bit on lines 23-24 helps implementors realize that they
don't need to insert a cast here.  Also, the type of the integer
constant 0 is typed as the first integer type in {int, long int, long
long int} that can express the value, which is required to be int in
this particular case.

> Thus, it needs to change to
>
> #define MPI_ERR_BUFFER (int)0   // you don't really need to define
> it as (const int)0
>
> as a result the following statement
>
>        short x = MPI_ERR_BUFFER;
>
> which compiled correctly before, would break with this change.

To the best of my knowledge this is still perfectly legitimate C.
Section 6.5.16.1 of my copy of the C99 standard seems to corroborate
this:

-------8<--------
Semantics

2 In simple assignment (=), the value of the right operand is
converted to the type of the assignment expression and replaces the
value stored in the object designated by the left operand.
-------8<--------

GCC doesn't give a warning for this assignment, even when you add a
bunch of warning flags.  It does warn if the value is something too
large for the short type (like 0xdeadbeef), although of course only
for direct assignments like this when the compiler still knows the
origin of the value.  Does the Microsoft compiler behave differently?

> Second comment.
>
> Adding the 'const' modifier to all types is very confusing imho. I
> understand that it meant to make it clear that the values are
> constants and should not change, but still is very confusing.
>
> For example,
>
>        C Type: void * const
>        MPI_BOTTOM
>
> This means that the value of MPI_BOTTOM is constant and not the
> memory it points to. however at first look you might think that it's
> the later. (if it was, it would break any application that uses it
> because the API's do not take a pointer to const void)

I'm not sure that there's a lot we can do if people can't properly
read cv-qualified C types.  This is a standards document, not a
tutorial.  I don't think that this is intentionally misleading or
difficult to read.

> In any case implementers should define MPI_BOTTOM as
>
>        #define MPI_BOTTOM (void*)0
>
> Rather than
>
>        #define MPI_BOTTOM (void* const)0
>
> As the value is constant and you don't need to add the const modifier.

The reason you don't really need the const here is that casts create
rvalues. So it has the same effect as a cast to the unqualified type.
This, combined with the constant initializer rule in MPI-2.1 section
2.5.4, is one reason that choosing "const int" as the type for these
constants is debatable.

> Note that the 'constness' of these values is already stated in the
> title of the section '19.1.1 Defined Constants'

I don't agree.  The semantic constness of these constants is implied
by the title, but not the specific language implementation constness.
In the standard we could say "X, Y, and Z are constants (their values
never change) with the exact type `int'".  Depending on the exact uses
with the rest of the API this would be perfectly valid, although we
might lose some error checking assistance from C compilers.

The title together with MPI-2.1 section 2.5.4 indicates that most of
these are compile-time constants that may be used for initialization,
so you could say that the constness doesn't matter since these may
never be stored in link-time constant variables.  However using
"const" as a way of indicating that these constants are "non-
lvalue" (and in particular, "non-(modifiable lvalue)") is fine by me.

-Dave

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