Re: Dependence based on whether files has been changed

[Eric Melski]

On 05/16/2011 11:00 AM, Philip Guenther wrote:
> On Mon, May 16, 2011 at 9:17 AM, Peng Yu<address@hidden>  wrote:
> > Scons has the support for updating target only if the dependent file
> > contents are changed.
> > http://www.scons.org/doc/0.98.5/HTML/scons-user/c779.html
> >
> > I know that it may not always easy and possible to duplicate feature
> > of other tools. But GNU make is still faster than scons and sometime
> > much faster than scons (especially if there are many dependences in a
> > Makefile). Considering this I'd still rather use make than scons.
> >
> > Therefore, I'm wondering if there is any plan to add the features
> > based on file contents in GNU make in the future.
>
> Maybe part of the reason that GNU make is faster than scons is that
> scons is actually checking the contents of all the source files to see
> whether they changed while GNU make only has to compare the timestamps
> on the files.  If so, then putting the "check contents of the files"
> feature into GNU make will make it just as slow.

SCons' performance problems have almost nothing to do with computing md5 
hashes of the input files, and everything to do with a bad graph 
traversal implementation.  To the best of my understanding, it rescans 
the entire graph each time it needs to pick another target to update, on 
the off chance that the update of a previous target has added 
dependencies to the graph (similar in intent to makefile remaking in GNU 
make, but less efficient).

> I'll note that it's possible to write makefile rules to do content
> comparison of files.  I know I've posted at least one version of
> example rules to do that to the list, though I don't use them myself
> and make no claims about performance.  Check the archive for details.

There's also a Mr. Make article covering the same:

http://www.cmcrossroads.com/ask-mr-make/6795-rebuilding-when-a-files-checksum-changes

br,

Eric Melski
Architect
Electric Cloud, Inc.