David Abrahams:
Vesa Karvonen:You are right, top-down "tree unrolling" isn't probably practical. However, I think that it should be possible to do the "tree unrolling" in a bottom-up style rather than top-down fashion. In other words, the "iteration tree" would look something like this:
> BTW, the same "acceleration" technique can be used with templates to
> overcome template recursion depth limitations. Probably most people
> know about it already, though I haven't explicitly checked. > (Plain unrolling is not as effective: Theta(k*n) < Theta(pow(k,n)).)
Well, if I understand your suggestion correctly, it's one thing I
thought of. Basically you're doing "tree unrolling" instead of
"linear unrolling", right? In the TMP case you don't have the
advantage of auto-detecting the depth limit, though ;-)
0 -> 1 -> 4 -> ...
/ \ / \
2 3 5 8
/ \ / \
6 7 9 10
Above, each node represents a single iteration and the numbering represents the order in which the iterations would be done. (The numbering might also look different - take it as a hint rather than the only/correct/best choice.) The idea is that from one "top-level" iteration to another, the template would try iterating bigger and bigger [sub-]trees. I really seriously Do Not have the time to [try to] implement such a while_<pred,op,state> template, but out of interest I probably will, unless someone else demonstrates it earlier.
> IMO, it is not a very good argument to say that more powerful tools > are unnecessary I am not making that argument.
Sorry, I guess my interpretation of your argument was too quick.
I have been thinking about making a lexical analyzer generator (using a syntax for regular expressions and using the preprocessor or run-time (initialization) code to generate the transition tables (once)), which is a lot easier to implement than a parser generator. I'd rather start with a simpler task, as getting something nearly as complicated done with the preprocessor is enough of a challenge.Ever thought about writing Yacc with just the preprocessor (<wink>, I think)?
-Vesa Karvonen
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