Rainer Deyke wrote:
On 7/26/2010 18:30, Jim Balter wrote:
Consider some code that calls a pure function that uses a low-overhead
exponential algorithm when the parameter is small, and otherwise calls a
pure function that uses a high-overhead linear algorithm. The calling
code happens not to be CTFEable and thus the test, even though it only
depends on a constant, is not computed at compile time. The compiler
sees two calls, one to each of the two functions, with the same
parameter passed to each, but only one of the two will actually be
called at run time. Trying to evaluate the low-overhead exponential
algorithm with large parameters at compile time would be a lose without
a timeout to terminate the attempt. It might be best if the compiler
only attempts CTFE if the code explicitly requests it.
It seems to me that you're describing something like this:
if (x < some_limit) {
return some_function(x);
} else {
return some_other_function(x);
}
This does not pose a problem, assuming 'some_limit' is a compile-time
constant. If 'x' is known at compile, the test can be performed at
compile time. If 'x' is not known at compile time, neither of the
function invocations can be evaluated at compile time.
The problem does exist in this code:
if (complex_predicate(x)) {
return some_function(x);
} else {
return some_other_function(x);
}
...but only if 'complex_predicate' is not a candidate for CTFE but the
other functions are. (This can happen if 'complex_predicate' performs
any type of output, including debug logging, so the scenario is not
entirely unlikely.)
Actually I'm not entirely sure if CTFE is even necessary for producing
optimal code. The optimizations enabled by CTFE seem like a subset of
those enabled by aggressive inlining combined with other common
optimizations.
I think that's probably true. If the inliner is in the front-end (as is
currently the case in DMD, but not DMC), then CTFE can produce better
code. If functions can be inlined *after* optimisation, CTFE may be a
more efficient way to perform certain optimisations, but otherwise it's
probably not much of a win. From the programmer's point of view, it
makes no difference whether it is done or not.
