Re: Better location streaming

[Jan Hubicka <hubicka@ucw.cz>]

> > 
> > This looks all very hackish with no immediate benefit mostly because
> > of the use of lto_output_string.  I think what you should do instead
> > is split up lto_output_string_with_length into the piece that streams
> > the string itself to the string-stream and returns an index into it
> > and the piece streaming the index to the specified stream.  Then you
> > can simply bitpack that index and the two int line / column fields.
> 
> Hmm, I plan to optimize string streaming (since we always stream one uleb to
> set it is non-NULL that can be easilly handled by assigining NULL string index
> 0).  How precisely you however suggest to bitpack line/column and string offset
> together?
> 
> The point is to make location info occupy not even whole byte most of time.
> Adding a simple stats claims that for tramp3d 30% of time location is
> undefined, 15% of time file changes, 39% of time line changes and 44% of time
> column changes (on tramp3d).  So assuming one byte for each uleb (that is
> optimistic, of course) one need 4 bits for the changed flags + less than a byte
> for the data.  Situation is similar for combine.c where unknown id 10%, file
> change is 0.5% and line change is 30%.

Note that for tramp3d the function sections reduce in size by about 16%, decl
section by about 10%. So there is some immediate benefit.

Honza