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Subject: Re: [Boost-build] Is there any way to prevent Boost.Build fromrecursively scanning header files for #include directives?
From: Lee Winter (lee.j.i.winter_at_[hidden])
Date: 2009-04-27 10:46:22

On Mon, Apr 27, 2009 at 7:06 AM, Johan Nilsson
<r.johan.nilsson_at_[hidden]> wrote:
> Lin Luo wrote:
>> Hi there,
>> We would like to know that is there a way to limit the header files
>> that Boost.Build recursively scans for #include directives to a
>> particular directory or set of directories?

> I've mentioned something like this in the past. The general opinion seems to
> be that dependency scanning only takes an insignificant amount of time,
> which I really don't agree with (especially not for rebuilds after minor,
> local changes).

Has the Pre-Scanned Headers (PSH) approach been considered? It is
similar in concept to pre-compiled headers (PCH), but it is much
simpler because it does not have all the contextural complexity of

Essentially a PSH file is a cache of the most recent scan. After a
source file has been scanned the list of included files is saved in
the respective PSH file.

When a source file is a candidate for scanning the timestamp on its
respective PSH file is checked. If the PSH file is up to date the
scan is unnecessary because the results of the scan are already known
and available in the contents of the PSH file. If the PSH file is out
of date then it is deleted. If the PSH file does not exist (possibly
because it has been deleted) then the souce file is scanned and a new
PSH file created.

Asa bebeficial side effect the collection of PSH files can also be
useful for finding potential complexity reductions in the inclusion
tree. Lakos mentions this as a potential performance improvement in
his book about large scale software design. But there he was not
concerned with finding dependencies, but more about the time it takes
the compioler to process the headers. The value of the collection of
PSH files is that it is hard to deal with a large body of code
manually due to the sheer amount of text to handle. Having the
inclusion tree available directly makes it much easier to identify the
tough/hot spots that are worth optimizing.

Just a thought.

Lee Winter
NP Engineering
Nashua, New Hampshire

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