Re: Apple's implementation of precompiled headers

[Devang Patel <dpatel at apple dot com>]

On Saturday, September 29, 2001, at 04:36  AM, 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?

Yes and No,  because how/when to decide if it is 'changed' ?

-  content is changed
-  content is same but different compiler options
-  content is same but different macro settings
-  content is same but different time stamp
-  content is same but different file system location
- ...

Any suggestions ?

It is easy to be conservative but when total build time is 'in-hours' 
instead
of 'in-minutes',  sometime it hurts to be very conservative.

Devang