Xcode 7 to Xcode 8 Storyboard Constraint Warnings

Select all the Scenes(View Controllers) in the Storyboard, show the Size inspector, change the Simulated Size to Freeform, then set Width Height = 600.

Then, those tons of warnings disappear.

I had the same issue when updating to Xcode 8. My storyboard looked just like you describe. Check your warnings/errors. If they all say something like "set vertical hugging priority to X" or "set compression resistance to Y" then you should be able to fix your issue by setting the priorities as suggested.

Click a warning, then in the attribute inspector under the tab with the ruler icon (not sure what that tab is called) you can set the priorities under the Content Hugging Priority and Content Compression Resistance Priority headers. As you update these properties the warnings should start to change to incorrect frame warnings, at which point you can just update the frames.

This might not be a quick fix, but changing a few priorities for some constraints is easier than redoing all the constraints. I hope this helps!

Select the view controllers one by one and press "Update Frames" each time.