[llvm-dev] [GlobalISel][AArch64] Toward flipping the switch for O0: Please give it a try!

[Kristof Beyls via llvm-dev]

On 18 Dec 2017, at 15:11, Amara Emerson <aemerson at apple.com<mailto:aemerson at apple.com>> wrote:


On Dec 18, 2017, at 12:37 PM, Kristof Beyls <Kristof.Beyls at arm.com<mailto:Kristof.Beyls at arm.com>> wrote:

Hi Amara,

My reasons for preferring the switch to happen after the release branch is based on the following observations:

  *   As far as I can see, the projects and products following top-of-trunk tend to test much more extensively than the testing that happens for llvm.org<http://llvm.org/> releases. I expect that the llvm.org<http://llvm.org/> release testing won’t find potential remaining issues, whereas the projects and products following trunk are far more likely to find remaining issues with GlobalISel. Therefore, I think it’s best to give as much time as possible for these projects/products following top-of-trunk to find issues before there’s an llvm.org<http://llvm.org/> release with GlobalISel enabled-by-default.
  *   For the projects and products that follow top-of-trunk that I know off, if globalisel-by-default would really break things, it’s possible to disable it within that project/product without end users of it needing to know about this. If for the llvm.org<http://llvm.org/> release it would turn out that globalisel has a big issue after release, we’d need to somehow let all users of the release know to disable it by adding a convoluted command line option (‘-mllvm -global-isel=0’?)

Combining the 2 observations above, I think it’s better to do the switch shortly after a release branch is taken, rather than just before it.

I wasn’t suggesting that we proceed with the full release with GISel enabled by default if there are serious issues, I was more saying that we have additional time during the RC period to test further on trunk, and then merge in fixes into the RC. Worst case is that we back out the change completely but I doubt it’ll get to that stage. I do agree that trunk sees more of the testing coverage. If one month isn’t enough, what would have been the cut off point?

Thanks,
Amara

Yeah, I agree that the time needed before the next release happens when switching is debatable.
After looking it up, I see that the final release tag is planned for no earlier than 21 February 2018, so we’d indeed have a bit of time to revert it on the release branch if serious issues were discovered.
Seeing the actual release date is that far out, I don’t have strong objections to flipping the switch now if others want to press on with it.

Thanks,

Kristof
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