A few weeks ago I submitted, then reverted, a patch to default the behavior of ffp-contract to "on". There was discussion on cfe-dev and while the opinions weren't unanimous, there was large support for this choice. This is associated with the default for the option -ffp-model=precise, which should choose -ffp-contract=on.
After the patch was submitted, the LNT build bots reported performance regressions in about 20 tests on a couple different architecture including x86 and aarch64. It seemed to be about the same 20 tests failing across the different architecture. Later the patch was reverted due to the performance regressions. Andy Kaylor told me he thought the regressions were due to the Loop Unroller making non-optimal decisions when -ffp-contract=on.
A couple questions about this:
I'd like to learn how to dig into the LNT performance regressions, any tips or guidance about this?
I'd like to learn how to get the information from the LNT build bot.
1. Do I need to sit there watching the url, else the history will scroll past and be removed?
2. Is there a way to run the LNT tests from my bash shell?
3. There are SHA codes in the LNT url, but they don't seem to correspond to the SHA when I say "git log"
4. If 20 LNT tests have performance regressions but 5000 LNT tests are the same or better, is there a process to approve the change and incur performance regression, possibly opening a bug report to improve the 20 tests that regressed?
Thanks for any insight. --Melanie