make: compilation firewall

[Michael]

I have a medium sized project and have been working to lessen total
compilation time.  The biggest outstanding problem is some auto-
generated code.  We have a program that I'll call 'generate,' which
takes a proprietary internal format and generates C++ code (you can
imagine lex or yacc if that helps).  We have a rule along the lines:

out.h out.cc: in.our_format generate
     generate --input in.our_format --output out

In turn out.h is #included in a lot of downstream files, so whenever
this rule runs, it causes many, many .cc files to recompile, and takes
about 5-10 minutes.

We're doing some refactoring and maintenance on 'generate.'  If we
make any change (even changing a comment), recompiling takes the full
time.

One possibility is to eliminate the dependance on generate, but then
if we make a real change, either on purpose or by accident, we won't
find it.

What I'd like to do is something like this:

out.h out.cc: in.our_format generate
     generate -i in.our_format -o tmp
     if (tmp.h != out.h OR tmp.cc != out.cc) {
         cp tmp.h out.h
         cp tmp.cc out.cc
     }

The idea would be that whenever generate is run, 2 diffs are run (one
on .h and one on .cc), and only if they've changed does it do the full
recompile.

Of course, this doesn't work as written.  Has anyone done anything
like this, and if so how?

Michael