Re: [cpp-netlib-devel] URI: Spirit Compile Times

[Dean M. B. <mik...@gm...>]

On Wed, Dec 1, 2010 at 3:51 AM, Jeroen Habraken <vex...@gm...> wrote:
>
> On 30 November 2010 17:38, Dean Michael Berris <mik...@gm...> wrote:
>>
>> I see that the URI code has a lot of rules (>5) which can still be
>> reduced into a single rule. Jeroen/Glyn can you try and look at
>> reducing the number of rules and consolidate to just one rule just to
>> see whether it would help reduce the compile times?
>>
>> Thanks in advance and I hope to hear what you guys think about trying
>> that out. I might do that on my own but getting time lately to work on
>> cpp-netlib ironically just got really hard.
>>
>
> There is a best practices documentation addendum here:
> http://boost-spirit.com/home/articles/doc-addendum/best-practices/,
> which states:
>
> "Avoid complex rules. Rules with complex definitions hurt the compiler
> badly. We’ve seen rules that are more than a hundred lines long and
> take a couple of minutes to compile. On some compilers, experience
> shows that the compile time is exponential in relation to the RHS
> expression length. C++ compilers were not designed to handle such big
> expressions and some just couldn’t cope (crashes). It is always best
> to break complex rules into more manageable, easier to digest parts.
> Doing so also makes the rules more readable."
>

Yes, that is true, but there was a mailing list discussion about the
cost of doing assignments to rules which basically invokes the Proto
evaluator. This kills the compiler because the Proto evaluation is a
full-on compiler in itself. Invoking it multiple times is a bigger
problem than having it deal with a sufficiently complex rule (in our
case it's really just a sequencing of optionals, which shouldn't
require too much mumbo-jumbo on the Proto side I imagine).

> and I've always understood is that multiple simple rules are
> preferable to a single complex rule. However, if I find time I'll look
> into combining some rules and testing whether it helps the
> compile-time, it shouldn't be to hard.
>

Yep they were -- especially in the Classic days IIRC. But with Qi and
the Proto stuff, the rule assignments get more expensive than the more
complex rules if I get my Proto correctly.

I'll look forward to your experiments and hopefully we can address
some of the compile time concerns as I go head-on into 0.9.

Thanks for looking into it Jeroen!

-- 
Dean Michael Berris
deanberris.com