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by zahlman 69 days ago
> A meta question is why this persists. It has the right qualities for a "party trick"

This is discussed in a section near the end. But I feel like the discussion of why people care about using XOR to swap two values is missing an underlying discussion of why people would care about swapping values. As shown earlier in the division example, at the point where you would do the swap, typically you can just write the rest of the code with the roles of the values swapped.
2 comments
saulpw 69 days ago
Usually it's used like:

   if max < min:
       min, max = max, min

   [... algorithm that requires min < max]
So your suggestion would be to have two versions of the algorithm in the two branches of the 'if'. This is significantly more complicated and may even be slower depending on lots of factors.
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addaon 69 days ago
> typically you can just write the rest of the code with the roles of the values swapped

Today, yes; most modern instruction sets are pretty orthogonal, and you can use a value in any register symmetrically -- although even today, division instructions (if they exist!) are among the most likely to violate that expectation, along with instructions that work with the stack pointer. But in the XOR heyday, this was less true -- instruction sets were less orthogonal, and registers were more scarced. It's not unreasonable for an OS scheduler tick to do some work to figure out the newly-scheduled task's stack pointer in one register, and need to swap it into an SPR or similar so that the return from interrupt returns to the new location, for example; and this is the exact type of place where the XOR trick occasionally has value.

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