Re: Apple's implementation of precompiled headers

[Daniel Berlin <dan at cgsoftware dot com>]

Stan Shebs <shebs@apple.com> writes:

> Daniel Egger wrote:
> > 
>> On Fre, 2001-09-28 at 23:23, Stan Shebs wrote:
>> 
>> > Our approach to precompiled headers has been to write GCC's trees
>> > and related data into a file, using a format that can be mapped in
>> > and randomly accessed. Changes to GCC are not large, mostly to
>> > replace the low-level memory allocation machinery, and to add hooks
>> > to both "freeze" and "thaw" GCC's internal state.
>> 
>> So you are serializing GCC's trees after parsing the headers into a
>> file. When this file is available at a recompilation you somehow figure
>> out whether the #include lines in the code have changed and whether
>> the (recursivly) included files themselves are more recent than your
>> serialized file. Correct?
>
> Basically yes.  There are many more data structures than trees that
> have to be recorded, as it turns out, and we're discovering more as
> we go along.

Yes, particularly for C++.
Fun, isn't it?

trees point to binding levels which point to ...

>
>> DICE on the Amiga had a somewhat different approach to reduce the time
>> needed to compile files: A given file was checked for it's includes and
>> those had were followed and written into a massive header file which
>> (if existed) was read into the preprocessor instead of traversing the
>> included files themselves. Although the files stayed in ASCII format
>> and were rather huge this resulted (when cleverly used) in a drastic
>> compile time reduction. Only drawback: There was no decent check for
>> changed include files and this probably would have ledd to inferior
>> performance since the "not having to traverse dotzends of files on
>> a slow filesystem" was the warrant of the speed improvement.
>
> I don't think that's much of an issue for today's systems; a large
> flat include file today takes nearly as much time as a complex of
> includes.  Easy test is to to take the .i file and run it through
> again, compare times.
>
> Stan

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