Re: how to generate code for (a,b):=(b,a)

>When you don't have any free registers, it should be possible to achieve
>the same result with a series of xors instead of moves. For example, two
>registers can be exchanged with the sequence:
> xor a,b
> xor b,a
> xor a,b


wilson@cs.utexas.edu writes:
> This is very likely to confuse a conservative garbage collector if
> these are pointer variables. I'd hope that compiler writers wouldn't
> use such optimizations casually, especially if they're not clearly
> better than the alternatives.


That's not a problem in this case unless the GC is running as a
preemptive thread and can get control in the middle of the xors though
right? Before and after the sequence, a and b contain valid pointers;
only in the middle to wierd split pointers appear. I don't see any
reason why Boehm's GC would have a problem with this construct.


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Tim Hollebeek |
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Princeton University | email: tim@wfn-shop.princeton.edu
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