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Subject: Re: [OMPI devel] Compile-time MPI_Datatype checking
From: Ralph Castain (rhc_at_[hidden])
Date: 2012-10-31 14:40:54


If it's only on for Clang, I very much doubt anyone will care - I'm unaware of any of our users that currently utilize that compiler, and certainly not on the clusters in the national labs (gcc, Intel, etc. - but I've never seen them use Clang).

Not saying anything negative about Clang - just noting it isn't much used in our current community that I've heard.

On Oct 31, 2012, at 11:19 AM, Dmitri Gribenko <gribozavr_at_[hidden]> wrote:

> On Wed, Oct 31, 2012 at 5:04 PM, Jeff Squyres <jsquyres_at_[hidden]> wrote:
>> On Oct 31, 2012, at 9:38 AM, Dmitri Gribenko wrote:
>>
>>>> The rationale here is that correct MPI applications should not need to add any extra compiler files to compile without warnings.
>>>
>>> I would disagree with this. Compiler warnings are most useful when
>>> they are on by default. Only a few developers will turn on a warning
>>> because warnings are hard to discover and enabling a warning requires
>>> an explicit action from the developer.
>>
>> Understood, but:
>>
>> a) MPI explicitly allows this kind of deliberate mismatch. It does not make sense to warn for things that are correct in MPI.
>
> I don't think it is MPI. It is the C standard that allows one to
> store any pointer in void* and char*. But C standard also considers
> lots of other weird things to be valid, see below.
>
>> b) Warnings are significantly less useful if the user looks at them and says, "the compiler is wrong; I know that MPI says that this deliberate mismatch in my code is ok."
>
> Well, one can also argue that since the following is valid C, the
> warning in question should not be implemented at all:
>
> long *b = malloc(sizeof(int));
> MPI_Recv(b, 1, MPI_INT, ...);
>
> But this code is clearly badly written, so we are left with a question
> about where to draw the line.
>
>> c) as such, these warnings are really only useful for the application where type/MPI_Datatype matching is expected/desired.
>
> Compilers already warn about valid C code. Silencing many warnings
> relies on conventions that are derived from best practices of being
> explicit about something unusual. For example:
>
> $ cat /tmp/aaa.c
> void foo(void *a) {
> for(int i = a; i < 10; i++)
> {
> if(i = 5)
> return;
> }
> }
> $ clang -fsyntax-only -std=c99 /tmp/aaa.c
> /tmp/aaa.c:2:11: warning: incompatible pointer to integer conversion
> initializing 'int' with an expression of type 'void *'
> [-Wint-conversion]
> for(int i = a; i < 10; i++)
> ^ ~
> /tmp/aaa.c:4:10: warning: using the result of an assignment as a
> condition without parentheses [-Wparentheses]
> if(i = 5)
> ~~^~~
> /tmp/aaa.c:4:10: note: place parentheses around the assignment to
> silence this warning
> if(i = 5)
> ^
> ( )
> /tmp/aaa.c:4:10: note: use '==' to turn this assignment into an
> equality comparison
> if(i = 5)
> ^
> ==
> 2 warnings generated.
>
> According to C standard this is valid C code, but clang emits two
> warnings on this.
>
>> Can these warnings be enabled as part of the warnings rollup -Wall option? That would be an easy way to find/enable these warnings.
>
> IIRC, -Wall warning set is frozen in clang. -Wall is misleading in
> that it does not turn on all warnings implemented in the compiler.
> Clang has -Weverything to really turn on all warnings. But
> -Weverything is very noisy (by design, not to be fixed) unless one
> also turns off all warnings that are not interesting for the project
> with -Wno-foo.
>
> I don't think it is possible to disable this warning by default
> because off-by-default warnings are discouraged in Clang. There is no
> formal policy, but the rule of thumb is: either make the warning good
> enough for everyone or don't implement it; if some particular app does
> something strange, it can disable this warning.
>
>>> The pattern you described is an important one, but most MPI
>>> applications will have matching buffer types/type tags.
>>
>> I agree that most applications *probably* don't do this. But significant developer in this community (i.e., Sandia) has at least multiple applications that *do* do it. I can't ignore that. :-(
>
> Here are a few approaches to solving this in order of preference:
>
> 0. Is this really a concern for Sandia? (I.e., do they target Clang?)
>
> 1. Ask the developer to silence the warning with a cast to 'void *' or
> -Wno-type-safety. Rationale: compilers already do warn about valid
> but suspicious code.
>
> 2. Turn off checking for char* just like for void*. Rationale: C
> standard allows char* to alias a pointer of any type. Note that char*
> is special in this regard (strict aliasing rules).
>
> 3. Turn off annotations by default in mpi.h.
>
> Dmitri
>
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr_at_[hidden]>*/
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