new implementations for nearest lib calls

use approach with copysign for handling negative zero


format


refactor for better branch prediction


move copysign back to internal branch


format


fix


use abs instead branches


better comments


switch arms for better branch prediction

crates/runtime/src/libcalls.rs

@@ -67,6 +67,9 @@ use wasmtime_environ::wasm::{
     TableIndex,
 };
 
+const TOINT_32: f32 = 1.0 / f32::EPSILON;
+const TOINT_64: f64 = 1.0 / f64::EPSILON;
+
 /// Implementation of f32.ceil
 pub extern "C" fn wasmtime_f32_ceil(x: f32) -> f32 {
     x.ceil()
@@ -86,25 +89,14 @@ pub extern "C" fn wasmtime_f32_trunc(x: f32) -> f32 {
 #[allow(clippy::float_arithmetic, clippy::float_cmp)]
 pub extern "C" fn wasmtime_f32_nearest(x: f32) -> f32 {
     // Rust doesn't have a nearest function, so do it manually.
-    if x == 0.0 {
+    // Nearest is either ceil or floor depending on which is nearest or even.
-        // Preserve the sign of zero.
+    // This approach exploited round half to even default mode.
+    let i = x.to_bits();
+    let e = i >> 23 & 0xff;
+    if e >= 0x7f_u32 + 23 {
         x
     } else {
-        // Nearest is either ceil or floor depending on which is nearest or even.
+        (x.abs() + TOINT_32 - TOINT_32).copysign(x)
-        let u = x.ceil();
-        let d = x.floor();
-        let um = (x - u).abs();
-        let dm = (x - d).abs();
-        if um < dm
-            || (um == dm && {
-                let h = u / 2.;
-                h.floor() == h
-            })
-        {
-            u
-        } else {
-            d
-        }
     }
 }
 
@@ -162,25 +154,14 @@ pub extern "C" fn wasmtime_f64_trunc(x: f64) -> f64 {
 #[allow(clippy::float_arithmetic, clippy::float_cmp)]
 pub extern "C" fn wasmtime_f64_nearest(x: f64) -> f64 {
     // Rust doesn't have a nearest function, so do it manually.
-    if x == 0.0 {
+    // Nearest is either ceil or floor depending on which is nearest or even.
-        // Preserve the sign of zero.
+    // This approach exploited round half to even default mode.
+    let i = x.to_bits();
+    let e = i >> 52 & 0x7ff;
+    if e >= 0x3ff_u64 + 52 {
         x
     } else {
-        // Nearest is either ceil or floor depending on which is nearest or even.
+        (x.abs() + TOINT_64 - TOINT_64).copysign(x)
-        let u = x.ceil();
-        let d = x.floor();
-        let um = (x - u).abs();
-        let dm = (x - d).abs();
-        if um < dm
-            || (um == dm && {
-                let h = u / 2.;
-                h.floor() == h
-            })
-        {
-            u
-        } else {
-            d
-        }
     }
 }