Do you know how to perform the xor trick (http://en.wikipedia.org/wiki/XOR_swap_algorithm ) on two pointers in D?

This is a try:


void foo(T)(ref T x, ref T y) pure nothrow {
    x ^= y;
    y ^= x;
    x ^= y;
}
void main() {
    int* p, q;
    foo(p, q);
}


Currently that gives:

test.d(2): Error: 'x' is not of integral type, it is a int*
test.d(2): Error: 'y' is not of integral type, it is a int*
test.d(3): Error: 'y' is not of integral type, it is a int*
test.d(3): Error: 'x' is not of integral type, it is a int*
test.d(4): Error: 'x' is not of integral type, it is a int*
test.d(4): Error: 'y' is not of integral type, it is a int*
test.d(8): Error: template instance test.foo!(int*) error instantiating

Bye,
bearophile

That's what you get for trying to be a smartass!

Seriously though, why would you want to do this? It's not actually faster or anything, you know.

On Sunday, 26 January 2014 at 00:04:08 UTC, bearophile wrote:
> Do you know how to perform the xor trick (http://en.wikipedia.org/wiki/XOR_swap_algorithm ) on two pointers in D?

You don't; it is undefined behavior and could lead to crashes. Suppose another thread triggers a GC run right after the first xor. Then there may be no valid pointers to *x and the GC frees it, so then after the swap, *y points to something entirely different.

> test.d(2): Error: 'x' is not of integral type, it is a int*

You could cast it to size_t, then the compiler will let you do the operation, but casting pointers to and from integers means you take matters of correctness into your own hands.

Adam D. Ruppe:

> You could cast it to size_t, then the compiler will let you do the operation, but casting pointers to and from integers means you take matters of correctness into your own hands.

Right, so I have to carry around size_t instead of pointers.

Bye and thank you,
bearophile

On 01/25/2014 04:08 PM, Martin Cejp wrote:
> That's what you get for trying to be a smartass!
>
> Seriously though, why would you want to do this? It's not actually
> faster or anything, you know.

Yeah, surprisingly, it is not faster for integers either. It was probably faster for older CPUs.

Ali

On Sun, Jan 26, 2014 at 12:14:40AM +0000, bearophile wrote:
> Adam D. Ruppe:
> 
> >You could cast it to size_t, then the compiler will let you do the operation, but casting pointers to and from integers means you take matters of correctness into your own hands.
> 
> Right, so I have to carry around size_t instead of pointers.
[...]

And you can't use the GC since the GC won't be able to find your xor'd pointers.


T

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