Re: Flying Obfu

This was done for speed. XOR is a very fast operation on some processor types (e.g., 8086). Along similar lines, we were taught to use XOR to zero a register, because XORing it with itself was much faster than assigning a constant value to the register. Whether it's actually any faster in Perl than the equivalent swap, I don't know, but it would be faster in some assembly languages. So, three XORs would accomplish the swap faster than the usual three assignments, too. How many times a second does this need to run? You might want to leave it alone, if it's a flight control system, or at least do a benchmark before you change it.

Comment on Re: Flying Obfu

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Re: Re: Flying Obfu by awkmonk (Monk) on Apr 14, 2003 at 16:02 UTC
Not much chance of this one being changed - the paperwork would be horrendous (it's part of the flight control of a Tornado). The person that wrote it clearly didn't do it for speed as they didn't recognise that 2 `if`'s are quicker than an `and`. It appears to have been done primarily to avoid introducing a third variable. No `($a, $b) = ($b, $a)` functionality available. Even so, I still believe it to be a prime example of 'how not to code' in a high risk, real time environment when it should count as 'quite important' that maintenance staff can clearly follow the code. We live and learn I suppose. 'I think the problem lies in the fact that your data doesn't fit my program'.	[reply] [d/l] [select]

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Re: Re: Flying Obfu
by awkmonk (Monk) on Apr 14, 2003 at 16:02 UTC

Not much chance of this one being changed - the paperwork would be horrendous (it's part of the flight control of a Tornado).

The person that wrote it clearly didn't do it for speed as they didn't recognise that 2 if's are quicker than an and.

It appears to have been done primarily to avoid introducing a third variable. No ($a, $b) = ($b, $a) functionality available. Even so, I still believe it to be a prime example of 'how not to code' in a high risk, real time environment when it should count as 'quite important' that maintenance staff can clearly follow the code.

We live and learn I suppose.

'I think the problem lies in the fact that your data doesn't fit my program'.

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