Re: Faster compilation speed

[Noel Yap <yap_noel at yahoo dot com>]

--- Neil Booth <neil@daikokuya.co.uk> wrote:
> Heh, if only life were this easy.  If you actually
> think about what CPP
> does, you'd realize this is a no-go.  Two immediate
> issues:
> 
> 1) #include can take a macro as argument

Yes, what I suggest certainly won't work for this
situation.

OTOH, how many times is this really used?  Would it be
such a sin to say that one cannot do the preprocessing
I suggested if one has macros for #include arguments?

> 2) #include can appear in preprocessor conditional
> blocks.  You
>    only know whether they are processed if you know
> the correct value
>    of the #if.  This often depends on macro
> expansions, and correct
>    processing of prior includes.  Of course,
> #defines appear in
>    conditional blocks too, so this is kind of
> important to get right.

I don't see this as too big a problem.  Just output a
file like:
#if COND
/* contents of header file
#endif

In fact, doing it this way has the advantage that
several builds, not necessarily agreeing on the value
of COND, can use the file.

> There are no easy shortcuts here: to preprocess
> something properly,
> you have to do *everything* the preprocessor does
> "normally".  There
> are no shortcuts, not even trivial ones.

I think one needn't preprocess everything perfectly in
order to gain significant advantages.  Would you say
that what I suggest is better than what we have now?

If an ideal solution is being worked on, I'd opt for
that.  OTOH, I think this solution has been in the
works for at least a couple of years now.  I think the
--preprocess-includes option should be very simple to
do.

> We *do* do too many stats and opens though; when I
> get time I'll post
> my ideas about this.

I'm sure my ideas are far from ideal so I'm looking
forward to yours.

Noel

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