wip: add fastfastpath

name                    old time/op  new time/op  delta
SelectUncontended-6     15.2ns ±11%  11.3ns ±12%  -25.92%  (p=0.000 n=10+10)
SelectSyncContended-6   1.16µs ± 6%  1.34µs ± 2%  +15.99%  (p=0.000 n=10+9)
SelectAsyncContended-6   124ns ± 3%    90ns ± 1%  -27.51%  (p=0.000 n=10+9)
SelectNonblock-6        3.16ns ± 6%  3.25ns ± 4%   +2.96%  (p=0.020 n=10+9)
SelectProdCons-6         460ns ± 7%   280ns ± 4%  -39.14%  (p=0.000 n=10+10)
GoroutineSelect-6       1.49ms ± 3%  1.50ms ± 3%     ~     (p=0.684 n=10+10)

Change-Id: I5a02ef5f458c88ac26079abfec3b6a0f95d6e023

Author: CAFxX

src/runtime/select.go

@@ -7,6 +7,7 @@ package runtime
 // This file contains the implementation of Go select statements.
 
 import (
+	"runtime/internal/atomic"
 	"unsafe"
 )
 
@@ -173,12 +174,16 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 		cas := scases[n]
 		switch cas.kind {
 		case caseRecv:
-			// TODO: lift/outline the fast path checks from chanrecv
+			if empty(cas.c) && atomic.Load(&cas.c.closed) == 0 {
+				continue
+			}
 			if selected, received := chanrecv(cas.c, cas.elem, false); selected {
 				return n, received
 			}
 		case caseSend:
-			// TODO: lift/outline the fast path checks from chansend
+			if cas.c.closed == 0 && full(cas.c) {
+				continue
+			}
 			if chansend(cas.c, cas.elem, false, cas.pc) {
 				return n, false
 			}