Enable _Float16 "half-float" Functionality Only When Available.

[johnoneil]

Description:

We're cross-compiling impeller for Android, but x86 Android targets don't have half float (_Float16) _ support. This change disables half float support in impeller/geometry/half.h for platforms where the _Float16 builtin is lacking.

As can be seen in half.h, Float16 support is enabled for all non-windows platforms, which doesn't not allow our use case.

Issues

I can't seem to find a way to file an issue describing this, but

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[jonahwilliams]

We actually only use the half types on iOS and macOS platforms, everywhere else we end up using floats for compatibility reasons. You could ifdef out the Half precision support for non-apple platforms, but you'd also need to turn off the tests in the non-apple configurations as well.

[johnoneil]

You could ifdef out the Half precision support for non-apple platforms

Thanks for this Jonah. I'll take that approach. Will update this in a few hours.

[flutter-dashboard]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[jonahwilliams]

You also need to update the ifdef here: https://github.com/flutter/engine/blob/main/impeller/geometry/geometry_unittests.cc#L1623-L1654

[johnoneil]

Updated PR to green.
If there's anything further you'd like on this just let me know.

[jonahwilliams]

LGTM

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