this is a great change.
Regarding the default off flag, Even with this flag to on, Polly will not influence the default behavior of clang/LLVM/... So the only thing that is tested by default is that our unit tests work on all platforms and that the build system does not cause troubles.
Now, I still think that we should not force Polly on our release testers. So off sounds like the right default for this patch.
Obviously, I would very much appreciate them to test Polly, but as Hans suggested we should kindly ask this on the release testers mailing list.
I agree that we should probably leave the default for Polly to be off since this is the first attempt to add it to the test release script and release testers may not be familiar with Polly. We can revisit turning it on by default in the future.
The next LLVM release is coming up, we just verified that Polly can compile all of AOSP, and Polly has been shipping in debian and ubuntu since over a year. We should consider to flip the switch now. Pengxuan, would you like to propose such a patch?