On Thursday, 31 August 2017 at 17:39:35 UTC, H. S. Teoh wrote:
This is just one example off the top of my head; I'm sure there
are plenty of others that we can come up with once we break
free of the C++ mold of thinking of templates as "copy-n-paste
except substitute X with Y". Another example that just
occurred to me is the various recursive templates in std.meta
for manipulating AliasSeq's. There is actually no need for the
compiler to instantiate anything at all, except perhaps for the
final result. By treating the AliasSeq as an in-compiler data
type with compile-time operations performed on it, the compiler
ought to be able to evaluate the template without needing to
instantiate anything past the first level of recursion.
T
That is a good example and it is one where we could maybe deduce
what is doing on.
However the classification of a template (rather the deduction of
unused intanciations) actually requires us to instantiate the
template.
Because we cannot predict to what it might evaluate given
specific parameters.
For this to work we have to produce all possible instantiation
behaviors of a template, and deduplicate this set.
Which happens to be infinite, in most cases.
Even predicting if the set of instantiations can be finite is
essentially an instance of the halting problem.
I've worked on this for some time until I realized that I was
fighting without gaining ground.
It would end up a myriad of special cases and would be impossible
to maintain.
Considering that the current template system which does not try
anything smart is already fiendishly complicated, it'd rather not
introduce an even more complicated
template-instance-shape-predictor which will probably only work
on the simplest of cases and is not guaranteed to bring wins that
would justify the time it'll take to implement it.
TL;DR
The change to optimize certain groups of templates will require
MASSIVE amounts of work and is unlikely to pay for itself.
