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, 6 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/ssa/layout.go b/src/cmd/compile/internal/ssa/layout.go
index f784c45c18..55454445c3 100644
--- a/src/cmd/compile/internal/ssa/layout.go
+++ b/src/cmd/compile/internal/ssa/layout.go
@@ -39,7 +39,8 @@ blockloop:
 			break
 		}
 
-		for _, c := range b.Succs {
+		for _, e := range b.Succs {
+			c := e.b
 			indegree[c.ID]--
 			if indegree[c.ID] == 0 {
 				posdegree.remove(c.ID)
@@ -54,9 +55,9 @@ blockloop:
 		var likely *Block
 		switch b.Likely {
 		case BranchLikely:
-			likely = b.Succs[0]
+			likely = b.Succs[0].b
 		case BranchUnlikely:
-			likely = b.Succs[1]
+			likely = b.Succs[1].b
 		}
 		if likely != nil && !scheduled[likely.ID] {
 			bid = likely.ID
@@ -66,7 +67,8 @@ blockloop:
 		// Use degree for now.
 		bid = 0
 		mindegree := f.NumBlocks()
-		for _, c := range order[len(order)-1].Succs {
+		for _, e := range order[len(order)-1].Succs {
+			c := e.b
 			if scheduled[c.ID] {
 				continue
 			}