On Thursday, 14 April 2016 at 00:31:34 UTC, Simen Kjaeraas wrote:
@constanttime functions can only call other functions marked
@constanttime, and may not contain conditionals, gotos or
while-loops.
@constanttime functions may contain for and foreach-loops, iff
the number of iterations are known at compile-time, and 'break'
is never used.
The part about conditionals seems a bit harsh, but it's got to
be there for determinism.
It can be relaxed: All alternative branches must take the same
number of cycles, though this can be hard to determine.
Recursion is an interesting problem, though: to reliably detect
it, the compiler has to know the entire call graph, i.e. not
extern functions are allowed. On the other hand, without
conditionals, recursion will always result in an endless loop,
which will immediately be recognized in testing.
