Re: Different multiply operator for pointers and integers

[Richard Henderson <rth at redhat dot com>]

On Fri, Dec 17, 2004 at 04:12:01PM +0100, Laura Tosoratto wrote:
> 3)When an address calculation is performed  (i.e.  for a[i])  I want to 
> use the mult_a operator (at RTL level)
> instead of the mult_i:
> @a[i] = &a + i mult_a sizeof(a[i])
> because I want to leave unchanged the upper 32 bits of the address (the 
> remote part).
> A typical situation is:
> b = a[i+X_PLUS]
> that means that I want to read variable a[i] on the remote node at 
> coordinates x+1 from the local node (machine has SIMD architecture)
> and store it in the local variable b. X_PLUS is a pure remote address 
> (its lower 32 bits are zero).

Why a mult_a operator then?  You don't perform multiplication on
pointers.  You perform multiplication on integers and then add
an integer to a pointer.

As for a special plus_a operator, I'm certain that you could not
reliably do this in gcc.  However...

In what situation do you expect the low half of an object to be
located at the highest address on a node, and the upper half of
the same object to be located at the lowest address on a node?

This would be a highly surprising bit of memory management, it
seems to me.  That sort of thing in my experience only gets used
on 8- and 16-bit microcontrolers in which we want to use, say, a
signed 4-bit displacement from the zero register, or some really
quirky addressing mode like that.

If you never have such an object placement, then you *don't care*
what the wrapping characteristics of pointer arithmetic are.  
Plain old 64-bit integer arithmetic is fine.

So why, exactly, do you want to do anything special with pointer
arithmetic at all?  My guess is that your goals are not clear.


r~