compile time regressions (was: merging for 3.4)

[Jan Hubicka]

> Mike Stump wrote:-
> 
> > No, no, you have it backwards...  PCH is going to expose all that slow 
> > code that should be fast, that has to be fast, so that we can fix it.  
> > By being 12x faster, or just 2x faster, there is _much_ less room to be 
> > slow, not more, really.
> 
> But slow code in things that PCH elides won't get exposed by the very
> nature of PCH.  e.g. drastic CPP slowdowns (not gonna happen!), or say
> the parser.

I don't think so.  It will make parser to do about comparable amount of
work to backend, so it will bring us into more realistic picture.  Right
now for certain class of C++ code it is almost uninteresting to optimize
backend becuase parsing is so slow.  Not so for Java, fortran and many C
programs.

There is lots of time wasted in all parts of GCC (yes, CPP is now
probably the very notable exception :)

And PCH is not improtant only because of precompiled headers.  It is
first step towards whole program optimization that will bring all sorts
of new performance problem, so I am not affraid that we will once feel
that GCC is fast.

Honza
> 
> Neil.