From a8f704129669d6a690452b13ccdb7c96e016fd39 Mon Sep 17 00:00:00 2001
From: MaxGraey <maxgraey@gmail.com>
Date: Mon, 31 Aug 2020 19:02:20 +0300
Subject: [PATCH] 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 | 49 +++++++++++-----------------------
 1 file changed, 15 insertions(+), 34 deletions(-)

diff --git a/crates/runtime/src/libcalls.rs b/crates/runtime/src/libcalls.rs
index 4dcb0963de..a7dfe0a723 100644
--- a/crates/runtime/src/libcalls.rs
+++ b/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 {
-        // Preserve the sign of zero.
+    // Nearest is either ceil or floor depending on which is nearest or even.
+    // 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.
-        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
-        }
+        (x.abs() + TOINT_32 - TOINT_32).copysign(x)
     }
 }
 
@@ -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 {
-        // Preserve the sign of zero.
+    // Nearest is either ceil or floor depending on which is nearest or even.
+    // 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.
-        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
-        }
+        (x.abs() + TOINT_64 - TOINT_64).copysign(x)
     }
 }