VM options files are a problem

The idea.sh script iterates over a number of options files:

for vm_opts_file in "$IDEA_VM_OPTIONS" "$HOME/.IntelliJIdea14/idea$BITS.vmoptions" "$IDE_BIN_HOME/idea$BITS.vmoptions"; do

which is fine except that all options are cumulative and the final file always exists. Why is this a problem?

-XXMaxPermSize=350m

This option does nothing when running on JDK8 (which I always do, except…) and is not permitted on JDK9 (…which I am starting to use). So whilst the error message is an annoyance using JDK8, it prevents me from using IDEA with JDK9.

There should be a way other than editing or deleting the distribution file, not to have the distribution file read.