Description
The C standard annex F.10 says that the result of x^0 = 1 for all x, even a NaN. For pown it explicitly calls out "for all x not a signalling NaN" (and doesn't say what happens on a signalling NaN); for powf it just says "any x, even a NaN" -- it is unclear whether that is an intentional difference or not since C generally leaves everything about the behavior of signalling NaNs unspecified. I am not sure what the IEEE spec says. This is a somewhat odd case as usually in C, operations behave the same for qNaN and sNaN, but some widely-used libc implementations return a qNaN for sNan^0 but return 1 for qNan^0 (the latter part being required by the standard), therefore clearly distinguishing qNaN and sNaN.
The "easiest" choice for us is to say that our functions return either 1 or a NaN non-deterministically, and if they do return a NaN the usual NaN propagation rules apply -- since one input is an sNaN, the payload if the returned NaN is completely arbitrary (in particular, it can be a qNaN or an sNaN). glibc and musl seem to disagree on the result here (see https://rust.godbolt.org/z/chsbv5v4d), so requiring more consistent behavior would involve convincing one of them that they are wrong.
@tgross35 does that sound reasonable? Should https://doc.rust-lang.org/nightly/std/primitive.f32.html#method.powf talk and friends about this case or is that not worth it?