Re: cost of -MP

[Boris Kolpackov]

Noel Yap <address@hidden> writes:
 
> OK, but then, if you're following the newer way of generating dependency 
> files, the dependency file will get regenerated right after the object file 
> is rebuilt and the no-longer-existing header file would no longer be 
> included in it.  It'll be a one time hit, if at all since presumably some 
> other dependency would've changed, that shouldn't affect the build time all 
> that much.

It doesn't affect the build time very much, it affects the time before make 
starts the first command (or says that nothing needs to be done). In large
non-recursive builds this could be pretty annoying. Imagine every time you
say `make' you have to wait for 30 seconds before the first compiler execution
starts.

Recursive builds have the same problem except that they keep printing 
"entering this", "leaving that" which makes you feel that something is
going on. With non-recursive build there are no such messages which
makes a two seconds delay quite obvious.

As of why approach that I suggest is faster than what -MP does - I don't
really know. I can speculate that it takes make some time to capture them
as targets.

-boris

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