So is this an object lesson for why optimizing for benchmarks is
a bad
idea?
If you're inclined to believe this, you could find a confirming
instance here, but there are other lessons that could be drawn. If
you go back to the original thread, you'll see this from Toon Moene:
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg00256.html
It didn't have to be a benchmark-only optimization.
It isn't a benchmark only optimization. Only the perfect nest
conversion was targeted for the benchmarks, because it was necessary.
The rest uses standard spatial optimality metrics to decide whether it
makes sense to interchange loops or not, and *that* works great on
fortran applications (except for a few other random bugs).