Pointer semantics in CTFE
Don Clugston
dac at nospam.com
Thu May 24 23:50:21 PDT 2012
The current implementation of CTFE strictly enforces C pointer
semantics. One of the restrictions is that you cannot perform ordering
comparisons between unrelated pointers.
This is important for repeatability: if it was permitted, the results
would be arbitrary and might vary unpredictably with subtle changes in
the code, or change between compiler releases.
But, there's an interesting case from bug 7898: the 'inside' operation.
If p and q are pointers to the same array, then (r >= p && r <= q) is
true if r points inside that array, and false if it does not.
This seems to be a perfectly reasonable operation: it is completely
repeatable and safe, regardless of what r points to. But there doesn't
seem to be any way to rewrite it to avoid the disallowed comparisons.
I could write code to allow this special case in CTFE. There's a bit of
work to make sure that all the valid cases are detected, because there
are quite a lot of ways to rewrite it, but it's not too terrible.
But I dunno, I don't like this sort of thing much. Feels a bit clunky.
OTOH it seems like necessary functionality, and I can't see any other
way of doing it.
Opinions?
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