* Richard Treumann <treumann@xxxxxxxxxx> [2008-01-08 15:40:20]:
> The MPI rules presumably would allow the block of code to access buf[0] and
> buf[2] before and after the call to MPI_Send and the MPI_Send will not
> change buf[0] or buf[2]. The practical effect of labeling buf[] const will
> be correct as long as the MPI rules are followed and there is no code in
> the vicinity that references buf[1].
I'm a little bit worried about the fact that const buf[] (in C) spans the
whole array, while the MPI_Datatype only includes elements 0 and 2; (And
even if char[] gets degraded to char * and only affects the first array
element, the problem remains when switching D1 and D2 in the example
below).
This difference could(or could not) lead to problems, even if the user
follows the MPI rules. (Assuming Richard's interpretation of the const
keyword is 100% accurate)
Given 2 mpi datatypes:
D1: byte, 1byte-hole byte
D2: 1byte-hole, byte
Using these definitions, we only pass buf (and not buf[1]) to the MPI send
and receive functions
so:
char buf[3];
1: some_nonconst_irecv_func (buf, D1);
2: read buf[1]
3: some_const_send_func (buf, D2);
4: modify buf[1]
5: some_const_send_func (buf, D2);
6: some_complete_func ();
7: read buf[0], buf[2]
Even though the user doesn't access buf[0] or buf[2] between the receive
start and the receive complete, the compiler might.
Assume an architecture that doesn't do non-aligned memory accesses,
or one where byte access (write/read) is much slower than native-word
accesses.
In that case, the read on line 2 might actually read buf[0] and buf[1]
into a register. In the same fashion, the modify on line 4 might write
buf[0] and buf[1] (maybe even using the buf[0] value stored in a register)
Even more so, since buf was only passed to const functions between line 2
and 7, the compiler might assume on line 7 that it still has the correct
value of buf[0] in a register.
In that case, even though the user program conforms to the MPI rules,
the resulting binary would be flawed.
That being said, I've done some tests and couldn't trick gcc into
this behaviour. Maybe because gcc thinks it isn't slower to do byte
access on the 32 bit x86 machine (which doesn't have a problem with
unaligned memory access) i'm using right now,
--or--
maybe because it has a rule that prohibits this behaviour.
If it is the first, then we might have a problem om some architectures;
If it is the second, than the const shouldn't pose a problem.
As for me, I'm thinking that this is what the restrict/and or volatile
keyword is for (and that the const definition given in the previous mail
is too severe). Unless I'm wrong, restrict indicates that the data isn't
aliased somewhere else and that the compiler can safely assume the data
won't be modified by a function call that doesn't have some (non-const)
reference to the data as a parameter. Except when using restrict, using a
const parameter doesn't buy you anything at all except some ease of mind
that a well-behaved compiler will inform you when you're trying to modify
the const parameter within the function body.
Besides, the fact that a function call is made using a const or nonconst
parameter is a non-issue all together if one allows
``The value of buf[1] can change asynchronously any time between
MPI_Irecv and MPI_Wait without any need for a function call.''
Greetings,
Dries
PS. I've tested using the following program:
int nonconstfunc (void * b);
int constfunc (const void * b);
char b[4];
int main (int argc, char ** args)
{
b[0]=1; b[1]=2; b[2]=3; b[3]=4;
nonconstfunc(b);
b[0]+=1; /* cause read of B0 */
constfunc(b);
b[0]=b[0]+b[2]; /* cause read of B2 */
b[0]*=2;
constfunc(b);
}
which compiles into (gcc4, -O3, -march=prescott):
movl $b, (%esp)
call nonconstfunc
addb $1, b
movl $b, (%esp)
call constfunc
movzbl b+2, %eax
addb b, %al
movsbl %al,%eax
addl %eax, %eax
movb %al, b
movl $b, (%esp)
call constfunc
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