runtime: omit nil-channel cases from selectgo's orders

This is the gofrontend version of https://golang.org/cl/245123.
Original CL description:

    Currently, selectgo does an initial pass over the cases array to look
    for entries with nil channels, so they can be easily recognized and
    skipped later on. But this still involves actually visiting the cases.

    This commit changes selectgo to omit cases with nil channels when
    constructing pollorder, so that they'll be skipped over entirely later
    on. It also checks for caseDefault up front, which will facilitate
    changing it to use a "block bool" parameter instead.

    Updates golang/go#40410

This is being brought over to gofrontend as a step toward upgrading to
Go1.16beta1, setting up for more compiler changes related to select handling.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279733

1 files changed, 28 insertions, 40 deletions

diff --git a/libgo/go/runtime/select.go b/libgo/go/runtime/select.go
index 81e00ec5a4e..d5087fbc045 100644
--- a/libgo/go/runtime/select.go
+++ b/libgo/go/runtime/select.go
@@ -41,7 +41,7 @@ func sellock(scases []scase, lockorder []uint16) {
 	var c *hchan
 	for _, o := range lockorder {
 		c0 := scases[o].c
-		if c0 != nil && c0 != c {
+		if c0 != c {
 			c = c0
 			lock(&c.lock)
 		}
@@ -57,11 +57,8 @@ func selunlock(scases []scase, lockorder []uint16) {
 	// the G that calls select runnable again and schedules it for execution.
 	// When the G runs on another M, it locks all the locks and frees sel.
 	// Now if the first M touches sel, it will access freed memory.
-	for i := len(scases) - 1; i >= 0; i-- {
+	for i := len(lockorder) - 1; i >= 0; i-- {
 		c := scases[lockorder[i]].c
-		if c == nil {
-			break
-		}
 		if i > 0 && c == scases[lockorder[i-1]].c {
 			continue // will unlock it on the next iteration
 		}
@@ -138,15 +135,6 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 	pollorder := order1[:ncases:ncases]
 	lockorder := order1[ncases:][:ncases:ncases]
 
-	// Replace send/receive cases involving nil channels with
-	// caseNil so logic below can assume non-nil channel.
-	for i := range scases {
-		cas := &scases[i]
-		if cas.c == nil && cas.kind != caseDefault {
-			*cas = scase{}
-		}
-	}
-
 	var t0 int64
 	if blockprofilerate > 0 {
 		t0 = cputicks()
@@ -166,15 +154,31 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 	}
 
 	// generate permuted order
-	for i := 1; i < ncases; i++ {
-		j := fastrandn(uint32(i + 1))
-		pollorder[i] = pollorder[j]
+	dfli := -1
+	norder := 0
+	for i := range scases {
+		cas := &scases[i]
+
+		// Omit cases without channels from the poll and lock orders.
+		if cas.c == nil {
+			if cas.kind == caseDefault {
+				dfli = i
+			}
+			cas.elem = nil // allow GC
+			continue
+		}
+
+		j := fastrandn(uint32(norder + 1))
+		pollorder[norder] = pollorder[j]
 		pollorder[j] = uint16(i)
+		norder++
 	}
+	pollorder = pollorder[:norder]
+	lockorder = lockorder[:norder]
 
 	// sort the cases by Hchan address to get the locking order.
 	// simple heap sort, to guarantee n log n time and constant stack footprint.
-	for i := 0; i < ncases; i++ {
+	for i := range lockorder {
 		j := i
 		// Start with the pollorder to permute cases on the same channel.
 		c := scases[pollorder[i]].c
@@ -185,7 +189,7 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 		}
 		lockorder[j] = pollorder[i]
 	}
-	for i := ncases - 1; i >= 0; i-- {
+	for i := len(lockorder) - 1; i >= 0; i-- {
 		o := lockorder[i]
 		c := scases[o].c
 		lockorder[i] = lockorder[0]
@@ -209,7 +213,7 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 	}
 
 	if debugSelect {
-		for i := 0; i+1 < ncases; i++ {
+		for i := 0; i+1 < len(lockorder); i++ {
 			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
 				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
 				throw("select: broken sort")
@@ -233,21 +237,16 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 
 loop:
 	// pass 1 - look for something already waiting
-	var dfli int
-	var dfl *scase
 	var casi int
 	var cas *scase
 	var caseReleaseTime int64 = -1
 	var recvOK bool
-	for i := 0; i < ncases; i++ {
-		casi = int(pollorder[i])
+	for _, casei := range pollorder {
+		casi = int(casei)
 		cas = &scases[casi]
 		c = cas.c
 
 		switch cas.kind {
-		case caseNil:
-			continue
-
 		case caseRecv:
 			sg = c.sendq.dequeue()
 			if sg != nil {
@@ -271,17 +270,12 @@ loop:
 			if c.qcount < c.dataqsiz {
 				goto bufsend
 			}
-
-		case caseDefault:
-			dfli = casi
-			dfl = cas
 		}
 	}
 
-	if dfl != nil {
+	if dfli >= 0 {
 		selunlock(scases, lockorder)
 		casi = dfli
-		cas = dfl
 		goto retc
 	}
 
@@ -294,9 +288,6 @@ loop:
 	for _, casei := range lockorder {
 		casi = int(casei)
 		cas = &scases[casi]
-		if cas.kind == caseNil {
-			continue
-		}
 		c = cas.c
 		sg := acquireSudog()
 		sg.g = gp
@@ -355,9 +346,6 @@ loop:
 
 	for _, casei := range lockorder {
 		k = &scases[casei]
-		if k.kind == caseNil {
-			continue
-		}
 		if sg == sglist {
 			// sg has already been dequeued by the G that woke us up.
 			casi = int(casei)
@@ -468,7 +456,7 @@ retc:
 
 	// Check preemption, since unlike gc we don't check on every call.
 	// A test case for this one is BenchmarkPingPongHog in proc_test.go.
-	if dfl != nil && getg().preempt {
+	if dfli >= 0 && getg().preempt {
 		checkPreempt()
 	}