unix: use bits.OnesCount64 instead of local copy

The local copy of bits.OnesCount64 was added for compatibility with Go
1.8 and earlier in CL 86477. Go 1.8 is no longer supported and go.mod
requires Go 1.12, so drop the local copy and use bits.OnesCount64.

Change-Id: Ieb77f0cef5f8f206b74ca737449efdcfe1949d44
Reviewed-on: https://go-review.googlesource.com/c/sys/+/192357
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

Author: tklauser

unix/affinity_linux.go

@@ -7,6 +7,7 @@
 package unix
 
 import (
+	"math/bits"
 	"unsafe"
 )
 
@@ -79,50 +80,7 @@ func (s *CPUSet) IsSet(cpu int) bool {
 func (s *CPUSet) Count() int {
 	c := 0
 	for _, b := range s {
-		c += onesCount64(uint64(b))
+		c += bits.OnesCount64(uint64(b))
 	}
 	return c
 }
-
-// onesCount64 is a copy of Go 1.9's math/bits.OnesCount64.
-// Once this package can require Go 1.9, we can delete this
-// and update the caller to use bits.OnesCount64.
-func onesCount64(x uint64) int {
-	const m0 = 0x5555555555555555 // 01010101 ...
-	const m1 = 0x3333333333333333 // 00110011 ...
-	const m2 = 0x0f0f0f0f0f0f0f0f // 00001111 ...
-
-	// Unused in this function, but definitions preserved for
-	// documentation purposes:
-	//
-	//   const m3 = 0x00ff00ff00ff00ff // etc.
-	//   const m4 = 0x0000ffff0000ffff
-	//
-	// Implementation: Parallel summing of adjacent bits.
-	// See "Hacker's Delight", Chap. 5: Counting Bits.
-	// The following pattern shows the general approach:
-	//
-	//   x = x>>1&(m0&m) + x&(m0&m)
-	//   x = x>>2&(m1&m) + x&(m1&m)
-	//   x = x>>4&(m2&m) + x&(m2&m)
-	//   x = x>>8&(m3&m) + x&(m3&m)
-	//   x = x>>16&(m4&m) + x&(m4&m)
-	//   x = x>>32&(m5&m) + x&(m5&m)
-	//   return int(x)
-	//
-	// Masking (& operations) can be left away when there's no
-	// danger that a field's sum will carry over into the next
-	// field: Since the result cannot be > 64, 8 bits is enough
-	// and we can ignore the masks for the shifts by 8 and up.
-	// Per "Hacker's Delight", the first line can be simplified
-	// more, but it saves at best one instruction, so we leave
-	// it alone for clarity.
-	const m = 1<<64 - 1
-	x = x>>1&(m0&m) + x&(m0&m)
-	x = x>>2&(m1&m) + x&(m1&m)
-	x = (x>>4 + x) & (m2 & m)
-	x += x >> 8
-	x += x >> 16
-	x += x >> 32
-	return int(x) & (1<<7 - 1)
-}