From a4355be058c80df56860abe24ae2d649dbf4d208 Mon Sep 17 00:00:00 2001
From: Felix Lange <fjl@twurst.com>
Date: Thu, 9 Jul 2020 17:45:49 +0200
Subject: [PATCH] common/math: use math/bits intrinsics for Safe* (#21316)

This is a resubmit of ledgerwatch/turbo-geth#556. The performance
benefit of this change is negligible, but it does remove a TODO.
---
 common/math/integer.go | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/common/math/integer.go b/common/math/integer.go
index 7eff4d3b0001d..17a9fb1375d13 100644
--- a/common/math/integer.go
+++ b/common/math/integer.go
@@ -18,6 +18,7 @@ package math
 
 import (
 	"fmt"
+	"math/bits"
 	"strconv"
 )
 
@@ -78,22 +79,20 @@ func MustParseUint64(s string) uint64 {
 	return v
 }
 
-// NOTE: The following methods need to be optimised using either bit checking or asm
-
-// SafeSub returns subtraction result and whether overflow occurred.
+// SafeSub returns x-y and checks for overflow.
 func SafeSub(x, y uint64) (uint64, bool) {
-	return x - y, x < y
+	diff, borrowOut := bits.Sub64(x, y, 0)
+	return diff, borrowOut != 0
 }
 
-// SafeAdd returns the result and whether overflow occurred.
+// SafeAdd returns x+y and checks for overflow.
 func SafeAdd(x, y uint64) (uint64, bool) {
-	return x + y, y > MaxUint64-x
+	sum, carryOut := bits.Add64(x, y, 0)
+	return sum, carryOut != 0
 }
 
-// SafeMul returns multiplication result and whether overflow occurred.
+// SafeMul returns x*y and checks for overflow.
 func SafeMul(x, y uint64) (uint64, bool) {
-	if x == 0 || y == 0 {
-		return 0, false
-	}
-	return x * y, y > MaxUint64/x
+	hi, lo := bits.Mul64(x, y)
+	return lo, hi != 0
 }