runtime: apply minor tweaks to channels

Remove complicated PRNG algorithm
(argument is limited by uint16 and can't be <= 1).
Do not require chansend/chanrecv selgen to be bumped with CAS.

R=rsc, ken
CC=golang-dev
https://golang.org/cl/4816041

src/pkg/runtime/chan.c

@@ -6,6 +6,7 @@
 #include "type.h"
 
 #define	MAXALIGN	7
+#define	NOSELGEN	1
 
 static	int32	debug	= 0;
 
@@ -87,7 +88,6 @@ static	SudoG*	dequeue(WaitQ*, Hchan*);
 static	void	enqueue(WaitQ*, SudoG*);
 static	SudoG*	allocsg(Hchan*);
 static	void	freesg(Hchan*, SudoG*);
-static	uint32	fastrandn(uint32);
 static	void	destroychan(Hchan*);
 
 Hchan*
@@ -215,7 +215,7 @@ runtime·chansend(Hchan *c, byte *ep, bool *pres)
 
 	mysg.elem = ep;
 	mysg.g = g;
-	mysg.selgen = g->selgen;
+	mysg.selgen = NOSELGEN;
 	g->param = nil;
 	g->status = Gwaiting;
 	enqueue(&c->sendq, &mysg);
@@ -243,7 +243,7 @@ asynch:
 		}
 		mysg.g = g;
 		mysg.elem = nil;
-		mysg.selgen = g->selgen;
+		mysg.selgen = NOSELGEN;
 		g->status = Gwaiting;
 		enqueue(&c->sendq, &mysg);
 		runtime·unlock(c);
@@ -322,7 +322,7 @@ runtime·chanrecv(Hchan* c, byte *ep, bool *selected, bool *received)
 
 	mysg.elem = ep;
 	mysg.g = g;
-	mysg.selgen = g->selgen;
+	mysg.selgen = NOSELGEN;
 	g->param = nil;
 	g->status = Gwaiting;
 	enqueue(&c->recvq, &mysg);
@@ -354,7 +354,7 @@ asynch:
 		}
 		mysg.g = g;
 		mysg.elem = nil;
-		mysg.selgen = g->selgen;
+		mysg.selgen = NOSELGEN;
 		g->status = Gwaiting;
 		enqueue(&c->recvq, &mysg);
 		runtime·unlock(c);
@@ -854,7 +854,7 @@ selectgo(Select **selp)
 		sel->order[i] = i;
 	for(i=1; i<sel->ncase; i++) {
 		o = sel->order[i];
-		j = fastrandn(i+1);
+		j = runtime·fastrand1()%(i+1);
 		sel->order[i] = sel->order[j];
 		sel->order[j] = o;
 	}
@@ -1151,7 +1151,9 @@ loop:
 	q->first = sgp->link;
 
 	// if sgp is stale, ignore it
-	if(!runtime·cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 1)) {
+	if(sgp->selgen != NOSELGEN &&
+		(sgp->selgen != sgp->g->selgen ||
+		!runtime·cas(&sgp->g->selgen, sgp->selgen, sgp->selgen + 2))) {
 		//prints("INVALID PSEUDOG POINTER\n");
 		freesg(c, sgp);
 		goto loop;
@@ -1220,21 +1222,3 @@ freesg(Hchan *c, SudoG *sg)
 		c->free = sg;
 	}
 }
-
-static uint32
-fastrandn(uint32 n)
-{
-	uint32 max, r;
-
-	if(n <= 1)
-		return 0;
-
-	r = runtime·fastrand1();
-	if(r < (1ULL<<31)-n)  // avoid computing max in common case
-		return r%n;
-
-	max = (1ULL<<31)/n * n;
-	while(r >= max)
-		r = runtime·fastrand1();
-	return r%n;
-}