Ralph,
Indeed, some of us are using clang (and other llvm front ends) for JIT on our hetero HPC clusters for amorphous problem spaces. Obviously, I don't work for a National Lab. But I do mod/sim/vis for quantum, nano, and meso-physics. Just wanted you to be aware. Ken From: devel-boun...@open-mpi.org [mailto:devel-boun...@open-mpi.org] On Behalf Of Paul Hargrove Sent: Wednesday, October 31, 2012 12:48 PM To: Open MPI Developers Subject: Re: [OMPI devel] Compile-time MPI_Datatype checking Note that with Apple's latest versions of Xcode (4.2 and higher, IIRC) Clang is now the default C compiler. I am told that Clang is the ONLY bundled compiler for OSX 10.8 (Mountain Lion) unless you take extra steps to install gcc (which is actually llvm-gcc and cross-compiles for OSX 10.7). So, Clang *is* gaining some "market share", though not yet in major HPC systems. -Paul On Wed, Oct 31, 2012 at 11:40 AM, Ralph Castain <r...@open-mpi.org> wrote: 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 <griboz...@gmail.com> wrote: > On Wed, Oct 31, 2012 at 5:04 PM, Jeff Squyres <jsquy...@cisco.com> 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 <griboz...@gmail.com>*/ > _______________________________________________ > devel mailing list > de...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/devel _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel -- Paul H. Hargrove phhargr...@lbl.gov Future Technologies Group Computer and Data Sciences Department Tel: +1-510-495-2352 Lawrence Berkeley National Laboratory Fax: +1-510-486-6900
