Merge rust-lang/libm#61

61: Implement `round` r=japaric a=notriddle

Fixes rust-lang/libm#31 

Co-authored-by: Michael Howell <michael@notriddle.com>

library/compiler-builtins/libm/src/lib.rs

@@ -377,7 +377,6 @@ pub trait F64Ext {
     #[cfg(todo)]
     fn ceil(self) -> Self;
 
-    #[cfg(todo)]
     fn round(self) -> Self;
 
     #[cfg(todo)]
@@ -498,7 +497,6 @@ impl F64Ext for f64 {
         ceil(self)
     }
 
-    #[cfg(todo)]
     #[inline]
     fn round(self) -> Self {
         round(self)

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

@@ -1,7 +1,14 @@
+macro_rules! force_eval {
+    ($e:expr) => {
+        unsafe { ::core::ptr::read_volatile(&$e); }
+    }
+}
+
 mod fabs;
 mod fabsf;
 mod fmodf;
 mod powf;
+mod round;
 mod scalbnf;
 mod sqrtf;
 
@@ -9,6 +16,7 @@ pub use self::fabs::fabs;
 pub use self::fabsf::fabsf;
 pub use self::fmodf::fmodf;
 pub use self::powf::powf;
+pub use self::round::round;
 pub use self::scalbnf::scalbnf;
 pub use self::sqrtf::sqrtf;
 

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

@@ -0,0 +1,35 @@
+use core::f64;
+
+const TOINT: f64 = 1.0 / f64::EPSILON;
+
+pub fn round(mut x: f64) -> f64 {
+    let (f, i) = (x, x.to_bits());
+    let e: u64 = i >> 52 & 0x7ff;
+    let mut y: f64;
+
+    if e >= 0x3ff + 52 {
+        return x;
+    }
+    if i >> 63 != 0 {
+        x = -x;
+    }
+    if e < 0x3ff - 1 {
+        // raise inexact if x!=0
+        force_eval!(x + TOINT);
+        return 0.0 * f;
+    }
+    y = x + TOINT - TOINT - x;
+    if y > 0.5 {
+        y = y + x - 1.0;
+    } else if y <= -0.5 {
+        y = y + x + 1.0;
+    } else {
+        y = y + x;
+    }
+    
+    if i >> 63 != 0 {
+        -y
+    } else {
+        y
+    }
+}

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

@@ -618,7 +618,7 @@ f64_f64! {
     // log10,
     // log1p,
     // log2,
-    // round,
+    round,
     // sin,
     // sinh,
     // sqrt,