pure functions calling impure functions at compile-time

[Don Clugston]

On 23/05/12 11:41, bearophile wrote:
> Simen Kjaeraas:
>
>> Should this be filed as a bug, or is the plan that only pure functions be
>> ctfe-able? (or has someone already filed it, perhaps)
>
> It's already in Bugzilla, see issue 7994 and 6169.

It's just happening because the purity checking is currently being done 
in a very unsophisticated way.

> But I think there is a semantic hole in some of the discussions about
> this problem. Is a future compile-time JIT allowed to perform
> purity-derived optimizations in CTFE?

Some, definitely. Eg.  foo(n) + foo(n)

can be changed to 2*foo(n), where n is an integer, regardless of what 
foo does.

It does need to be a bit conservative, but I think the issues aren't 
CTFE specific. Eg, something like this currently gives an assert at runtime:

pure nothrow void check(int n) pure nothrow
{
   assert(n == 4);
}

void main()
{
     check(3);
}

even though check() can do nothing other than cause an error, it still 
cannot be optimized away. But you can get rid of all subsequent calls to 
it, because they'll produce the same error every time.