Re: Content-based dependencies

[Yang Zhang]

Mike Shal wrote:
> On 5/6/09, Yang Zhang <address@hidden> wrote:
> > Is there a way to have GNU make cache copies of source files, so that when
> > rules are triggered by timestamp comparisons between targets and sources, it
> > will also compare the current source with the cached source to make sure
> > that it actually changed before executing the actions (only executing if
> > they differ)?
>
> This sounded familiar - I found some older threads on the issue:
>
> http://www.mail-archive.com/address@hidden/msg05559.html
> http://www.mail-archive.com/address@hidden/msg04985.html
>
> Also the solution here may work for you:
> http://www.kolpackov.net/pipermail/notes/2004-September.txt
>
> Ultimately what is your goal here? Are you trying to avoid the "change
> a comment in a header rebuilds everything" issue, or do you have some
> other timestamp problems? Keep in mind that doing a diff or using a
> checksum to determine out-of-dateness isn't exactly cheap - you may be
> trying to optimize a corner case to the detriment of the general case.

That's close to what I'm trying to do.  I'm using code generation tools 
that produce sets of files (e.g., lzz produces .h and .cc files).  In 
that example, usually only the .cc file changes, but the .h file also 
gets touched, causing all the other .cc files that depend on it to get 
recompiled as well.
--
Yang Zhang
http://www.mit.edu/~y_z/