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

Dave Lloyd wrote:
> Currently, the whole system is stopped in a synchronised way to GC, so
> traces only occur at procedure calls in all threads. But this is more
> for convenience with the threading mechanisms available than required by
> the method.


Based on what you've said, this seems absolutely essential. It is
also essential that the sequence of 3 XORs cannot be split across a
procedure call. Otherwise the collector could stop a thread in the
middle of the XOR sequence, in which case one of the pointer values
would be very hard to recover. Your descriptor mechanism sounds too
weak to describe such a situation, but I could be mistaken.


If you add a conservative collector to a conventional compiler, the
problem is that both of these properties are hard to come by. This
has nothing to do with C vs whatever; it has to do purely with the
extent to of compiler cooperation. And we were only arguing that C
compilers should optionally exhibit a small amount of such
cooperation.
--
Hans-Juergen Boehm
boehm@mti.sgi.com
--

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