suggestion to reduce unnecessary rebuilding

[Ilya Shlyakhter]

I want to make a suggestion that can speed up make by reducing
unnecessary rebuilding:

Before making a target, if that target already exists, move it to a
temporary backup file.
After the target is re-made, diff the newly made target against the backup.
If it didn't change, reset the date of the newly made target to the
date of the backup.
In either case, delete the backup.

Then, if a source file is changed in an insignificant way (e.g. by
adding comments),
that source file is still recompiled, but at least the downstream
files that depend on it are not.

Instead of diff, for specific types of files specific programs can be
used that can
ignore insignificant differences (e.g. zdiff for compressed files).

ilya