Add tests against MPFR for ilogb and ilogbf

[tgross35]

No description provided.

[programmerjake]

get_exp follows frexp's rules for determining which exponent to use, which is to pick v = m * 2^e where 0.5 <= |m| < 1.0. ilogb by contrast uses e = floor(log2(|v|)) (without rounding errors) which is equivalent to 1.0 <= |m| < 2.0.
so, you need get_exp().map(|v| v - 1)

[tgross35]

Makes sense, thanks. Updated this and added your explanation as a comment.

[programmerjake]

some microcontrollers have 16-bit c_int (AVR? icr if the rust version does but iirc some C compilers do), so you should probably use c_int here rather than i32

[programmerjake]

or are you assuming the tests never are compiled on 16-bit c_int targets? (since idk if mpfr can compile for those targets)

[tgross35]

Yeah this isn't accurate but the source needs to be fixed for this and other routines that should be c_int

libm/src/math/ilogb.rs

Lines 1 to 5 in ff15e46

	const FP_ILOGBNAN: i32 = -1 - 0x7fffffff;
	const FP_ILOGB0: i32 = FP_ILOGBNAN;
	#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
	pub fn ilogb(x: f64) -> i32 {

I doubt MPFR would build on 16-bit targets, I can't even get it to compile for i586 or any Windows targets (might be possible but I haven't spent a lot of time here yet). I have a branch that serializes test input and MPFR results to a file and can run tests against that, that is my loose plan for how to get coverage on these targets without building mpfr/musl.

[programmerjake]

well, upstream mpfr claims to compile on i686 mingw and debian has a version for "i386", so i expect it to work for those; AVR is more questionable.

[tgross35]

MinGW should be possible, I just ran into a lot of difficulty getting the correct dependencies and paths set up via msys on GHA. I guess I need to revisit i586; I thought there were build failures but it looks like it is our implementation that is dropping the sign #404. Probably related to how we unconditionally use x87 https://github.com/rust-lang/libm/blob/ff15e465d6d4beb81e686fdfdeb40388b474bb3e/src/math/arch/i586.rs.