LLVM (thus, Rust) floating point support support is incomplete

[thestinger]

• signalling NaN results in undefined behaviour
• floating point environment (rounding mode, exceptions) is not supported

This is not going to be easy to fix since the only sane fix is to improve LLVM.

[pnkfelix]

cc @nikomatsakis since i think this is relevant to a point of discussion from last night's triage meeting.

[steveklabnik]

triage bump: I don't follow LLVM development closely enough to know if anything has changed here, but it's true, if they're deficient, it's going to be hard for us.

[eefriedman]

This is https://bugs.llvm.org//show_bug.cgi?id=8100 on the LLVM side; it's a longstanding issue.

[steveklabnik]

Triage: LLVM bug is still open.

[varkor]

Since https://reviews.llvm.org/D27028 (late January 2017), LLVM has had experimental intrinsics for floating-point environment support (see llvm.experimental.constrained.*). It's probably better to wait until they're stabilised before implementing the corresponding functionality in Rust?

[bstrie]

@varkor , is there an LLVM tracking issue for when (or if) those might be stabilized? cc #55107

[varkor]

@bstrie: I've not found one, unfortunately 😕The process seems to be ad hoc. From what I could tell, the intrinsics should be correct, but not necessarily efficient.