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digitalmars.D.learn - Xor trick?

reply "bearophile" <bearophileHUGS lycos.com> writes:
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
Jan 25 2014
next sibling parent reply "Martin Cejp" <minexew gmail.com> writes:
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.
Jan 25 2014
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
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
Jan 25 2014
prev sibling parent reply "Adam D. Ruppe" <destructionator gmail.com> writes:
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.
Jan 25 2014
parent reply "bearophile" <bearophileHUGS lycos.com> writes:
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
Jan 25 2014
parent "H. S. Teoh" <hsteoh quickfur.ath.cx> writes:
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

-- 
WINDOWS = Will Install Needless Data On Whole System -- CompuMan
Jan 25 2014