Re: [RFC] callgraph and unit at time compilation

[Jan Hubicka <jh at suse dot cz>]

> > Solving this for only one front end will force us to solve the same
> > problem for all the front ends.  Better design something that we can use
> > in the future to do real unit-at-a-time optimizations.
> 
> If we want to do real unit-at-a-time optimizations, we need to write 
> things out to disk (be it AST's or other information).  If we don't do 
> this, it's just going to be either non-scalable, or non-workable.  If we 
> aren't writing AST's due to whatever the heck politics, we are going to mainly 
> stuck writing annotations and summaries (summaries of various flow 
> info, side-effects, etc)
> If Jan was to  work on this part of the infrastructure, which would 
> consist of some sort of 
> database, be it an  existing reuse of one, or something else, with an 
> interface that let us  store annotations on things like trees, refs, and 
> basic blocks, as well as storing some generic graph type with 
> annotations on edges and vertices, it would make the rest of the basic 
> unit-at-a-time infrastructure much easier to do right.

Agreed in this.  I think PCH branch is slowly targetting this position
(being able to write the tree structures to disk and read them back), so
I don't want to conflick here.  However I think I can do bit here by
keeping things in memory right now but keeping local and global analysis
independent
(ie I first build grapha and later optimize.  Fact is that right now I
need tree reprezentation for first inlining pass, but that is mostly
temporary until I can get more of data into separate place)

Honza
> IMHO.
> If we approach real unit-at-a-time optimizations from the perspective that 
> we can keep everything in memory all the time, or redo everything all the 
> time (rather than store some info somewhere on disk), we're just going to end up with 
> something that is likely slow, huge (in memory footprint), unmaintainable, 
> and non-scaling. 
> We shouldn't build a house of cards when we have a chance to lay a real 
> foundation.
> 
>  > > > Diego.
> > 
> >