net: reduce allocation size in ReadFromUDP (experiment)

Switch to concrete types. Bring your own object to fill in.

Allocate just enough for the IP byte slice.
The allocation is now just 4 bytes, which puts it in the tiny allocator,
which is much faster.

name                  old time/op    new time/op    delta
WriteToReadFromUDP-8    5.59µs ±10%    6.03µs ± 9%   +7.80%  (p=0.004 n=9+10)

name                  old alloc/op   new alloc/op   delta
WriteToReadFromUDP-8     32.0B ± 0%      4.0B ± 0%  -87.50%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
WriteToReadFromUDP-8      1.00 ± 0%      1.00 ± 0%     ~     (all equal)

Change-Id: Ief506f891b401d28715d22dce6ebda037941924e

src/internal/poll/fd_unix.go

@@ -229,6 +229,60 @@ func (fd *FD) ReadFrom(p []byte) (int, syscall.Sockaddr, error) {
 	}
 }
 
+// ReadFrom wraps the recvfrom network call.
+func (fd *FD) ReadFromInet4(p []byte, from *syscall.SockaddrInet4) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := syscall.RecvfromInet4(fd.Sysfd, p, 0, from)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, err
+	}
+}
+
+// ReadFrom wraps the recvfrom network call.
+func (fd *FD) ReadFromInet6(p []byte, from *syscall.SockaddrInet6) (int, error) {
+	if err := fd.readLock(); err != nil {
+		return 0, err
+	}
+	defer fd.readUnlock()
+	if err := fd.pd.prepareRead(fd.isFile); err != nil {
+		return 0, err
+	}
+	for {
+		n, err := syscall.RecvfromInet6(fd.Sysfd, p, 0, from)
+		if err != nil {
+			if err == syscall.EINTR {
+				continue
+			}
+			n = 0
+			if err == syscall.EAGAIN && fd.pd.pollable() {
+				if err = fd.pd.waitRead(fd.isFile); err == nil {
+					continue
+				}
+			}
+		}
+		err = fd.eofError(n, err)
+		return n, err
+	}
+}
+
 // ReadMsg wraps the recvmsg network call.
 func (fd *FD) ReadMsg(p []byte, oob []byte) (int, int, int, syscall.Sockaddr, error) {
 	if err := fd.readLock(); err != nil {

src/net/fd_posix.go

@@ -63,6 +63,18 @@ func (fd *netFD) readFrom(p []byte) (n int, sa syscall.Sockaddr, err error) {
 	return n, sa, wrapSyscallError(readFromSyscallName, err)
 }
 
+func (fd *netFD) readFromInet4(p []byte, from *syscall.SockaddrInet4) (n int, err error) {
+	n, err = fd.pfd.ReadFromInet4(p, from)
+	runtime.KeepAlive(fd)
+	return n, wrapSyscallError(readFromSyscallName, err)
+}
+
+func (fd *netFD) readFromInet6(p []byte, from *syscall.SockaddrInet6) (n int, err error) {
+	n, err = fd.pfd.ReadFromInet6(p, from)
+	runtime.KeepAlive(fd)
+	return n, wrapSyscallError(readFromSyscallName, err)
+}
+
 func (fd *netFD) readMsg(p []byte, oob []byte) (n, oobn, flags int, sa syscall.Sockaddr, err error) {
 	n, oobn, flags, sa, err = fd.pfd.ReadMsg(p, oob)
 	runtime.KeepAlive(fd)

src/net/udpsock_posix.go

@@ -42,15 +42,22 @@ func (a *UDPAddr) toLocal(net string) sockaddr {
 	return &UDPAddr{loopbackIP(net), a.Port, a.Zone}
 }
 
-func (c *UDPConn) readFrom(b []byte, addr *UDPAddr) (int, *UDPAddr, error) {
-	n, sa, err := c.fd.readFrom(b)
-	switch sa := sa.(type) {
-	case *syscall.SockaddrInet4:
-		*addr = UDPAddr{IP: sa.Addr[0:], Port: sa.Port}
-	case *syscall.SockaddrInet6:
-		*addr = UDPAddr{IP: sa.Addr[0:], Port: sa.Port, Zone: zoneCache.name(int(sa.ZoneId))}
+func (c *UDPConn) readFrom(b []byte, a *UDPAddr) (n int, addr *UDPAddr, err error) {
+	switch c.fd.family {
+	case syscall.AF_INET:
+		var from syscall.SockaddrInet4
+		n, err = c.fd.readFromInet4(b, &from)
+		ipbuf := make([]byte, 4)
+		copy(ipbuf, from.Addr[:])
+		*a = UDPAddr{IP: ipbuf, Port: from.Port}
+	case syscall.AF_INET6:
+		var from syscall.SockaddrInet6
+		n, err = c.fd.readFromInet6(b, &from)
+		ipbuf := make([]byte, 16)
+		copy(ipbuf, from.Addr[:])
+		*a = UDPAddr{IP: ipbuf, Port: from.Port, Zone: zoneCache.name(int(from.ZoneId))}
 	}
-	return n, addr, err
+	return n, a, err
 }
 
 func (c *UDPConn) readMsg(b, oob []byte) (n, oobn, flags int, addr *UDPAddr, err error) {

src/syscall/syscall_unix.go

@@ -290,6 +290,37 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	return
 }
 
+func RecvfromInet4(fd int, p []byte, flags int, from *SockaddrInet4) (n int, err error) {
+	var rsa RawSockaddrAny
+	var socklen _Socklen = SizeofSockaddrAny
+	if n, err = recvfrom(fd, p, flags, &rsa, &socklen); err != nil {
+		return
+	}
+	pp := (*RawSockaddrInet4)(unsafe.Pointer(&rsa))
+	port := (*[2]byte)(unsafe.Pointer(&pp.Port))
+	from.Port = int(port[0])<<8 + int(port[1])
+	for i := 0; i < len(from.Addr); i++ {
+		from.Addr[i] = pp.Addr[i]
+	}
+	return
+}
+
+func RecvfromInet6(fd int, p []byte, flags int, from *SockaddrInet6) (n int, err error) {
+	var rsa RawSockaddrAny
+	var socklen _Socklen = SizeofSockaddrAny
+	if n, err = recvfrom(fd, p, flags, &rsa, &socklen); err != nil {
+		return
+	}
+	pp := (*RawSockaddrInet6)(unsafe.Pointer(&rsa))
+	port := (*[2]byte)(unsafe.Pointer(&pp.Port))
+	from.Port = int(port[0])<<8 + int(port[1])
+	from.ZoneId = pp.Scope_id
+	for i := 0; i < len(pp.Addr); i++ {
+		from.Addr[i] = pp.Addr[i]
+	}
+	return
+}
+
 func SendtoInet4(fd int, p []byte, flags int, to SockaddrInet4) (err error) {
 	ptr, n, err := to.sockaddr()
 	if err != nil {