Merge rust-lang/libm#84

84: Implement fmod r=japaric a=P1n3appl3

closes rust-lang/libm#21 
I replaced the `isnanf()` function from `fmodf()` with a call to the core function `is_nan()`. If there's a reason we needed to re-implement it then I can change that back.

Also I couldn't figure out what to do in `src/lib.rs`. Is the `#[cfg(todo]` that I should be removing the one for `mod_euc()`? If so why is the equivalent one for the `f64` version still there?

Co-authored-by: Joseph Ryan <josephryan3.14@gmail.com>
Co-authored-by: Jorge Aparicio <jorge@japaric.io>

library/compiler-builtins/libm/src/math/fmod.rs

@@ -0,0 +1,80 @@
+use core::u64;
+
+#[inline]
+pub fn fmod(x: f64, y: f64) -> f64 {
+    let mut uxi = x.to_bits();
+    let mut uyi = y.to_bits();
+    let mut ex = (uxi >> 52 & 0x7ff) as i64;
+    let mut ey = (uyi >> 52 & 0x7ff) as i64;
+    let sx = uxi >> 63;
+    let mut i;
+
+    if uyi << 1 == 0 || y.is_nan() || ex == 0x7ff {
+        return (x * y) / (x * y);
+    }
+    if uxi << 1 <= uyi << 1 {
+        if uxi << 1 == uyi << 1 {
+            return 0.0 * x;
+        }
+        return x;
+    }
+
+    /* normalize x and y */
+    if ex == 0 {
+        i = uxi << 12;
+        while i >> 63 == 0 {
+            ex -= 1;
+            i <<= 1;
+        }
+        uxi <<= -ex + 1;
+    } else {
+        uxi &= u64::MAX >> 12;
+        uxi |= 1 << 52;
+    }
+    if ey == 0 {
+        i = uyi << 12;
+        while i >> 63 == 0 {
+            ey -= 1;
+            i <<= 1;
+        }
+        uyi <<= -ey + 1;
+    } else {
+        uyi &= u64::MAX >> 12;
+        uyi |= 1 << 52;
+    }
+
+    /* x mod y */
+    while ex > ey {
+        i = uxi - uyi;
+        if i >> 63 == 0 {
+            if i == 0 {
+                return 0.0 * x;
+            }
+            uxi = i;
+        }
+        uxi <<= 1;
+        ex -= 1;
+    }
+    i = uxi - uyi;
+    if i >> 63 == 0 {
+        if i == 0 {
+            return 0.0 * x;
+        }
+        uxi = i;
+    }
+    while uxi >> 52 == 0 {
+        uxi <<= 1;
+        ex -= 1;
+    }
+
+    /* scale result */
+    if ex > 0 {
+        uxi -= 1 << 52;
+        uxi |= (ex as u64) << 52;
+    } else {
+        uxi >>= -ex + 1;
+    }
+    uxi |= (sx as u64) << 63;
+
+    f64::from_bits(uxi)
+}

library/compiler-builtins/libm/src/math/mod.rs

@@ -19,6 +19,7 @@ mod fdim;
 mod fdimf;
 mod floor;
 mod floorf;
+mod fmod;
 mod fmodf;
 mod hypot;
 mod hypotf;
@@ -54,6 +55,7 @@ pub use self::fdim::fdim;
 pub use self::fdimf::fdimf;
 pub use self::floor::floor;
 pub use self::floorf::floorf;
+pub use self::fmod::fmod;
 pub use self::fmodf::fmodf;
 pub use self::hypot::hypot;
 pub use self::hypotf::hypotf;

library/compiler-builtins/libm/test-generator/src/main.rs

@@ -725,7 +725,7 @@ f64_f64! {
 f64f64_f64! {
     // atan2,
     fdim,
-    // fmod,
+    fmod,
     hypot,
     // pow,
 }