Re: [Boost-users] Meta Template Programming with MPL - Compiler B ug or is the code really so evil slow?

From: James Hughes (JHughes_at_[hidden])
Date: 2006-02-13 09:48:00

> -----Original Message-----
> From: Vinzenz 'evilissimo' Feenstra [mailto:evilissimo_at_[hidden]]
> Sent: 13 February 2006 12:40
> To: boost-users_at_[hidden]
> Subject: [Boost-users] Meta Template Programming with MPL - 
> Compiler Bug
> or is the code really so evil slow?
> 
> 
> 
> Hi,
> 
> I've written a little Meta Template Program which should 
> calculate the 
> first 3 perfect numbers. ( range 1 - 500 )
> 
> The code is compilable if you use the range 1 - 30 really 
> fast and works 
> correctly. But I've tried to compile the code but I can't get 
> it finished.
> 
> Someone tried it on a Machine with 40 GB RAM and it now compiles for 
> more than 12 hours and uses more than 7.1 GB RAM ( the compilation )
> 
> I've tried it by myself with g++ 3.4.x and g++ 4.0.x and with VC++ 7.1
> 
> The code works on all tested compilers if the range would be 
> 1 - 30 but
> 
> If the range is 1 - 500 VC++ says:
> 
> d:\boost\vc71\include\boost-1_33_1\boost\mpl\aux_\preprocessed
> \plain\apply_wrap.hpp(48): 
> fatal error C1204: compiler limit: internal structure overflow.
> 
> ( This seems to be a known bug in VC++ 
> http://support.microsoft.com/default.aspx?scid=kb;en-us;883655 )
> 
> and g++ compiles and compiles and uses more and more RAM.
> 
> 
> Is the code really so evil slow or is it a compiler Bug in g++?
> 
> Maybe anyone has an answer.
> 
> BR
> evilissimo
> 
> 

Without the code its difficult to tell, but this meta programming 
uses recursive definitions of templates, and with a lot of information 
required per template per iteration, you are running out of memory, as 
the error messages indicate. Not a compiler bug I don't expect - you are 
just asking too much. Templates require quite a bit of memory for a single 
instance - multiply that by the 500 you are asking for and things rapidly 
go downhill, which is why 30 works - you haven't hit the point at which you
run out of memory (and hence require swapping  which is a very slow process
in
comparison to memory access). In fact the multiplication factor is probably 
more than 500, it may go up as exponential or perhaps even factorial
depending 
on the code

James