net: reduce allocation size in ReadFromUDP

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 for IPv4,
which puts it in the tiny allocator, which is much faster.

name                  old time/op    new time/op    delta
WriteToReadFromUDP-8    13.7µs ± 1%    13.4µs ± 2%   -2.49%  (p=0.000 n=10+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)

Windows is temporarily stubbed out.

Updates #43451

Change-Id: Ief506f891b401d28715d22dce6ebda037941924e
Reviewed-on: https://go-review.googlesource.com/c/go/+/331490
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

2 files changed, 35 insertions, 0 deletions

diff --git a/src/syscall/net_js.go b/src/syscall/net_js.go
index 17799148bd..2998c2159c 100644
--- a/src/syscall/net_js.go
+++ b/src/syscall/net_js.go
@@ -92,6 +92,14 @@ func Recvfrom(fd int, p []byte, flags int) (n int, from Sockaddr, err error) {
 	return 0, nil, ENOSYS
 }
 
+func RecvfromInet4(fd int, p []byte, flags int, from *SockaddrInet4) (n int, err error) {
+	return 0, ENOSYS
+}
+
+func RecvfromInet6(fd int, p []byte, flags int, from *SockaddrInet6) (n int, err error) {
+	return 0, ENOSYS
+}
+
 func Sendto(fd int, p []byte, flags int, to Sockaddr) error {
 	return ENOSYS
 }
diff --git a/src/syscall/syscall_unix.go b/src/syscall/syscall_unix.go
index 60aeae14de..101e39c7d0 100644
--- a/src/syscall/syscall_unix.go
+++ b/src/syscall/syscall_unix.go
@@ -292,6 +292,33 @@ 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])
+	from.Addr = pp.Addr
+	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
+	from.Addr = pp.Addr
+	return
+}
+
 func SendtoInet4(fd int, p []byte, flags int, to SockaddrInet4) (err error) {
 	ptr, n, err := to.sockaddr()
 	if err != nil {