Re: Pre Compiled Headers

[Mike Stump <mrs at apple dot com>]

On Feb 13, 2007, at 1:34 PM, Brendon Costa wrote:
> Sorry for the long email

Complex questions are better long, as that allows for better answers,  
which should be more useful to you.

> This was based on an idea from Mike in Nov 2006 when i needed to  
> ensure that the FUNCTION_DECL nodes were not collected by the  
> garbage collector while i was still using them. You mentioned that  
> i could do the above to get the garbage collector to think that the  
> nodes are still being used. I also assumed that it could be used  
> for working around the PCH engine.

To be clear, I said:

> > Actually, no, it won't work, precompiled headers will fail to work  
> > because you'd need to modify the PCH writer to write your data,  
> > because you didn't mark your data structures with GTY and didn't  
> > use gc to allocate them.

back then.  I did mean for you to take that as a direct statement  
that while you could defeat collection, it still would not work for pch.

> I should just re-write my code to use the garbage collector to  
> manage my data structures,

Yup.

> though it is a big job

Your code will be cleaner after you fix it up.

> Now i think i have resolved myself to the fact that if i want to  
> support
> PCH files, then I will need to modify my code to use the GTY  
> markers and
> GCC garbage collector.

Yup.

> * Does GCC use the PCH feature to speed up compilation of its  
> standard libraries?

libstdcxx is the only one that I know of that does.

> * If so is there a simple way of disabling this at configure time etc?

From configure help:

  --enable-libstdcxx-pch  build pre-compiled libstdc++ headers
                          [default=$is_hosted]

> * Is it possible to use the GTY(()) flags inside macros?

Only copy preexisting styles.  You won't go wrong.  In short, most  
things you can dream up, won't work.  5 of the most trivial and basic  
things work.  In general, no, sorry.

> #define DECLARE_ARRAY(DataTypeName, DataType) \
> struct DataTypeName##ArrayStruct \
> { \

You can see vec.[ch] for an example that most closely matches this.

> * Is it possible to explicitly free garbage collected memory if i  
> know i
> will not be needing it any more

Yes, ggc_free.[1]

1 - Some people think that all the savings you'd get from this aren't  
work the pain of doing it, if you have to track down any over zealous  
ggc_free calls.