Date: Tue, 23 Jan 2001 10:32:21 -0500
From: Nicholas Nevin - Sun HPC High Performance Computing <Nicholas.Nevin@XXXXXXXXXXXX>
To: Rolf Rabenseifner <Rabenseifner@XXXXXXXXXXXXXXXXXXXX>
Cc: mpi-core@XXXXXXXXXXX
Subject: Re: Buffer overlap with collective communications
User-Agent: Mutt/1.3.9i
Sender: owner-mpi-core@XXXXXXXXXXXXX

On Tue, Jan 23, 2001 at 10:49:40AM +0100, Rolf Rabenseifner wrote:
> Hi,
> 
> I have a request that asks about the rules about
> an overlap of sendbuf and recvbuf in collective
> communications.
> He stated, that he cannot see, where it is forbidden
> in the MPI 1.1 standard.
> He uses MPI_BOTTOM and datatypes with absolute addresses. 
>  
> Where does MPI-1.1 forbid the overlap of sendbuf and recvbuf
> in collective communications? 
>  
> Any idea to solve this problem?
> 
> Best regards
> Rolf 
> 

Refer him to section 2.2 of the MPI 1.1 standard. I believe the following
makes overlap of the buffers illegal.

    Unless specified otherwise, an argument of type OUT or type INOUT
    cannot be aliased with any other argument passed to an MPI procedure.
    An example of argument aliasing in C appears below. If we define a C
    procedure like this,
    
    void copyIntBuffer( int *pin, int *pout, int len ) 
    {   int i; 
        for (i=0; i<len; ++i) *pout++ = *pin++; 
    } 

    then a call to it in the following code fragment has aliased arguments. 

    int a[10]; 
    copyIntBuffer( a, a+3, 7); 

    Although the C language allows this, such usage of MPI procedures is
    forbidden unless otherwise specified. Note that Fortran prohibits
    aliasing of arguments. 


-nick