On 7/12/2016 6:13 AM, John Colvin wrote:
On Tuesday, 12 July 2016 at 10:19:04 UTC, Walter Bright wrote:
On 7/12/2016 2:40 AM, John Colvin wrote:
For the previous statement to be false, you must define cases where
casting away immutability *is* defined.
@system programming is, by definition, operating outside of the
language guarantees of what happens. It's up to you, the systems
programmer, to know what you're doing there.
This is so, so wrong. There's a world of difference between "you have to
get this right or you're in trouble" and "the compiler (and especially
the optimiser) is free to assume that what you're doing never happens".
Undefined behaviour, as used in practice by modern optimising compilers,
is in the second camp. You might have a different definition, but it's
not the one everyone else is using and not the one that our two fastest
backends understand.
Given the definition of undefined behaviour that everyone else
understands, do you actually mean "modifying immutable data by any means
is undefined behaviour" instead of "casting away immutable is undefined
behaviour"?
What's the difference?
Anyhow, if you cast away immutability, and the data exists in rom, you
still can't write to it (you'll get a seg fault). If it is in mutable
memory, you can change it, but other threads may be caching or reading
the value while you do that, i.e. synchronization issues. The optimizer
may be taking advantage of immutability in its semantic transformations.
As a systems programmer, you'd have to account for that.
