Re: lto

[Jan Hubicka <jh at suse dot cz>]

> IP RA as currenly implemented in IRA does propagate info only down in 
> topological order.  But a good IP RA (e.g. Minimal cost inter-procedural 
> regiter allocator http://citeseer.ist.psu.edu/kurlander96minimum.html) 
> needs to propagate info up and down.
> 
> But I am quite skeptical about IP RA.  To be sucessfull it needs that 
> the called function is small and uses few registers.  Such functions 
> should be just inlined.  It solves IP RA besides other problems.  
> Intel/Sun/Pathscale compilers make very aggressive function inlining 
> during LTO.  Therefore the code generated by the compilers with LTO is 
> much bigger (e.g. LTO in pathscale results in 30% bigger code for x86_64 
> SPECINT2000 and by the way improves the code by 4% and makes the 
> compiler 50% slower).

It seems to me that you can afford such a tradeoff if you are targetting
primarily CPU houngry software.  We need to handle the other cases (ie
system utilities or openoffice, say) where the code size is about major
bottleneck, so it seems to me that we don't want to make such an extreme
tradeoffs (though our current implementation of inliner heruistics will
do precisely that too)

Honza
> 
> In overall I don't think that IP RA is important thing and worth to be 
> implemented.