cmd/compile: enable constant-time CFG editing

Provide indexes along with block pointers for Preds
and Succs arrays.  This allows us to splice edges in
and out of those arrays in constant time.

Fixes worst-case O(n^2) behavior in deadcode and fuse.

benchmark                     old ns/op      new ns/op     delta
BenchmarkFuse1-8              2065           2057          -0.39%
BenchmarkFuse10-8             9408           9073          -3.56%
BenchmarkFuse100-8            105238         76277         -27.52%
BenchmarkFuse1000-8           3982562        1026750       -74.22%
BenchmarkFuse10000-8          301220329      12824005      -95.74%
BenchmarkDeadCode1-8          1588           1566          -1.39%
BenchmarkDeadCode10-8         4333           4250          -1.92%
BenchmarkDeadCode100-8        32031          32574         +1.70%
BenchmarkDeadCode1000-8       590407         468275        -20.69%
BenchmarkDeadCode10000-8      17822890       5000818       -71.94%
BenchmarkDeadCode100000-8     1388706640     78021127      -94.38%
BenchmarkDeadCode200000-8     5372518479     168598762     -96.86%

Change-Id: Iccabdbb9343fd1c921ba07bbf673330a1c36ee17
Reviewed-on: https://go-review.googlesource.com/22589
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 27 insertions, 37 deletions

diff --git a/src/cmd/compile/internal/ssa/deadcode.go b/src/cmd/compile/internal/ssa/deadcode.go
index ae990026f5..5ccf39027f 100644
--- a/src/cmd/compile/internal/ssa/deadcode.go
+++ b/src/cmd/compile/internal/ssa/deadcode.go
@@ -25,7 +25,8 @@ func reachableBlocks(f *Func) []bool {
 		if b.Kind == BlockFirst {
 			s = s[:1]
 		}
-		for _, c := range s {
+		for _, e := range s {
+			c := e.b
 			if !reachable[c.ID] {
 				reachable[c.ID] = true
 				p = append(p, c) // push
@@ -69,7 +70,7 @@ func liveValues(f *Func, reachable []bool) []bool {
 		v := q[len(q)-1]
 		q = q[:len(q)-1]
 		for i, x := range v.Args {
-			if v.Op == OpPhi && !reachable[v.Block.Preds[i].ID] {
+			if v.Op == OpPhi && !reachable[v.Block.Preds[i].b.ID] {
 				continue
 			}
 			if !live[x.ID] {
@@ -100,9 +101,12 @@ func deadcode(f *Func) {
 		if reachable[b.ID] {
 			continue
 		}
-		for _, c := range b.Succs {
-			if reachable[c.ID] {
-				c.removePred(b)
+		for i := 0; i < len(b.Succs); {
+			e := b.Succs[i]
+			if reachable[e.b.ID] {
+				b.removeEdge(i)
+			} else {
+				i++
 			}
 		}
 	}
@@ -115,16 +119,9 @@ func deadcode(f *Func) {
 		if b.Kind != BlockFirst {
 			continue
 		}
-		c := b.Succs[1]
-		b.Succs[1] = nil
-		b.Succs = b.Succs[:1]
+		b.removeEdge(1)
 		b.Kind = BlockPlain
 		b.Likely = BranchUnknown
-
-		if reachable[c.ID] {
-			// Note: c must be reachable through some other edge.
-			c.removePred(b)
-		}
 	}
 
 	// Splice out any copies introduced during dead block removal.
@@ -217,35 +214,28 @@ func deadcode(f *Func) {
 	f.Blocks = f.Blocks[:i]
 }
 
-// removePred removes the predecessor p from b's predecessor list.
-func (b *Block) removePred(p *Block) {
-	var i int
-	found := false
-	for j, q := range b.Preds {
-		if q == p {
-			i = j
-			found = true
-			break
-		}
-	}
-	// TODO: the above loop could make the deadcode pass take quadratic time
-	if !found {
-		b.Fatalf("can't find predecessor %v of %v\n", p, b)
-	}
+// removeEdge removes the i'th outgoing edge from b (and
+// the corresponding incoming edge from b.Succs[i].b).
+func (b *Block) removeEdge(i int) {
+	e := b.Succs[i]
+	c := e.b
+	j := e.i
+
+	// Adjust b.Succs
+	b.removeSucc(i)
 
-	n := len(b.Preds) - 1
-	b.Preds[i] = b.Preds[n]
-	b.Preds[n] = nil // aid GC
-	b.Preds = b.Preds[:n]
+	// Adjust c.Preds
+	c.removePred(j)
 
-	// rewrite phi ops to match the new predecessor list
-	for _, v := range b.Values {
+	// Remove phi args from c's phis.
+	n := len(c.Preds)
+	for _, v := range c.Values {
 		if v.Op != OpPhi {
 			continue
 		}
-		v.Args[i].Uses--
-		v.Args[i] = v.Args[n]
-		v.Args[n] = nil // aid GC
+		v.Args[j].Uses--
+		v.Args[j] = v.Args[n]
+		v.Args[n] = nil
 		v.Args = v.Args[:n]
 		phielimValue(v)
 		// Note: this is trickier than it looks. Replacing