Re: cost of -MP

[Noel Yap]

Interesting, non-recursive was slower than recursive.  I suppose I can see this 
happening if there weren't that many directories in the project.  How many 
directories do you have?

Regarding the topic at hand, we don't use -MP, but post-process the output and 
create empty targets for the header files.  For example:

        my_header:

We haven't performed any time comparisons, though.

Noel

Boris Kolpackov wrote:

> Good day,
>
> JFYI:
>
> I was wondering what is the cost of using -MP option for dependency
> generation. In essence, -MP makes gcc output (implicit) PHONY target
> for each header to prevent make from flagging header removal as 
> an error.
>
> I have a project of a moderate size: about a hundred C++ translation
> units. It takes about 12 minutes to rebuild on a single Intel 
> Pentium M 1.6Ghz. I also have both recursive and non-recursive build
> systems for this project. 
>
>
> The alternative to -MP that I am considering is to define a do-nothing
> terminal pattern rule for headers:
>
> %.h:: ;
>
>
> To compare these two approaches I run make on an up-to-date project and
> measure time it took make to figure out everything is up-to-date. Below
> are the results (in seconds):
>
>                   %.h      -MP     r
>
> non-recursive     2.35     3.30    0.40
>
> recursive         1.77     2.51    0.41
>
>
>
> Here is the list of drawbacks with the new approach that I am aware of:
>
>
>  * If some of the headers are generated using pattern rules those rules
>    should come before the do-nothing rule. See also
>
>    http://mail.gnu.org/archive/html/help-make/2004-02/msg00015.html
>
>
>  * Some headers (notably ones from C++ standard library) don't have 
>    suffixes. There are two observations to consider:
>
>    - headers in system directories don't get removed very often
>
>
>    - for cases like C++ std lib we can write something like this:
>
>      /usr/include/%:: ;
>      /usr/local/include/%:: ;
>
>
> comments?
> -boris
>
>
> ------------------------------------------------------------------------
>
> _______________________________________________
> Help-make mailing list
> address@hidden
> http://mail.gnu.org/mailman/listinfo/help-make