add support for quadruple precision floating point

[thestinger]

This currently requires linking against a library like libquadmath (or
libgcc), because compiler-rt barely has any support for this and most
hardware does not yet have 128-bit precision floating point. For this
reason, it's currently hidden behind a feature gate.

When compiler-rt is updated to trunk, some tests can be added for
constant evaluation since there will be support for the comparison
operators.

Closes #13381

[huonw]

I'm in favour, but I would guess a change like this is theoretically meant to go through an RFC. (That said, since it's small change & is feature gated, I would personally be ok with merging and then going through an RFC to un-feature-gate.)

[alexcrichton]

I'm a little worried about adding a new primitive type to the language without an RFC and without any documentation. The implementation seems ok with me, but this is quite a change to the language which hasn't had much discussion.

[thestinger]

It has actually been discussed a long time ago since it was Graydon's reason for having float in the language (it was going to be f128 on architectures with hardware support for it). I didn't make an RFC simply because it doesn't seem very controversial and will need to be feature gated for other reasons anyway. I can close this and make an RFC if you think that's necessary.

[alexcrichton]

We decided in today's meeting to move forward with this, r=me with a rebase.

[alexcrichton]

Or no, before merging, I had one concern. This feature is enabled in libstd, and looks like it's only used by the visitor. By enabling this feature, are we pulling in runtime support which may not be available on all platforms? (my worry is just libstd, not f128 usage in general)

[thestinger]

@alexcrichton: No runtime support is required to enable it in libstd and define some stuff like constants in an f128 module. Runtime support starts being required when the non-constant operators are used, as that will call into compiler-rt/libgcc_s.

At the moment, compiler-rt only has portable implementations of the comparison operators. I don't think any change needing more than those will make it through the OS X and Android bots.

[alexcrichton]

OK, sounds good to me!