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

>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;


It's worse than that. It's true that this typically only happens in
the presence of threads---I should have said that---but the GC doesn't
have to be a thread unto itself for this to happen. If the GC is
invoked by whatever thread forces a GC, other threads may be suspended
in an unsafe state where their pointers are not all identifiable.


Boehm's GC is designed to work with some thread packages, with some
thread-package-specific code to glue it into the system. If the C/C++
compiler starts mangling pointers, it severely restricts the ability
to use threading.
--
| Paul R. Wilson, Comp. Sci. Dept., U of Texas @ Austin (wilson@cs.utexas.edu)
| Papers on memory allocators, garbage collection, memory hierarchies,
| persistence and Scheme interpreters and compilers available via ftp from
| ftp.cs.utexas.edu, in pub/garbage (or http://www.cs.utexas.edu/users/wilson/)
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