query reg. gcc

[Dale Johannesen]

On Dec 22, 2004, at 8:09 PM, Deepak Soi wrote:

> Paul,
>             First of all, thanks for the fast reply. What I feel  is 
> ,I can't use precompiled headers in my project. As i have a number of 
> modules in our whole project, and each module has different header 
> requirements. However with in a module , the headers required by all 
> .cpp files are almost same.So in case I set aside few headers to be 
> pre-compiled for whole project, it won't work. Also, second thing is i 
> don't know about the which header files will be reqd. by each module 
> in advance , without scanning the whole project, so i can't choose 
> which headers to precompile.
>                                                                        
>                    Whereas if we can cache the header-file  which is 
> once loaded in to memory, then later if its reqd, we won't load it 
> again, during the compilation of next .cpp file, it can be of help- in 
> saving the compilation time of the next .cpp file. What you suggest? 
> Can it be an additional functionality? Do we have something to support 
> it right now in gcc ?

You don't say what OS you're on, but in most cases, a new subprocess is
invoked for each file that's compiled, and it retains no in-memory data 
from
the previous file.  So there's no way to do what you're asking.  
Precompiled
headers put the data to be retained on disk, and that's the best you 
can do.
(Incidentally, the multiple subprocesses are partly responsible for the 
slowdown
you're seeing, although it's probably a minor effect compared to the 
header
files.  You might try -combine, which gets rid of the multiple 
subprocesses but
not the rereading of header files.  Be prepared for it to run out of 
memory on
a big project, though.)

It is probably worth the effort to figure out subset(s) of your header 
files that can
profitably be made into precompiled headers.  It sounds like one pch for
each of your modules would work well.