go/callgraph/vta: optimize scc computation

Optimizes how sccs are internally computed by reducing the
amount of maps created and map lookups performed. Between
4-10% faster on benchmarks of callgraph construction.

Updates golang/go#57357

Change-Id: I0ecd92c32358a56d1cae67beb352ec15b7b4367e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/460435
Run-TryBot: Tim King <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
gopls-CI: kokoro <[email protected]>
Reviewed-by: Zvonimir Pavlinovic <[email protected]>

go/callgraph/callgraph_test.go

@@ -27,16 +27,15 @@ import (
 // Current results were on an i7 macbook on go version devel go1.20-2730.
 // Number of nodes, edges, and reachable function are expected to vary between
 // go versions. Timing results are expected to vary between machines.
-//
 // BenchmarkStatic-12	 53 ms/op     6 MB/op	12113 nodes	 37355 edges	1522 reachable
 // BenchmarkCHA-12    	 86 ms/op	 16 MB/op	12113 nodes	131717 edges	7640 reachable
 // BenchmarkRTA-12		110 ms/op	 12 MB/op	 6566 nodes	 42291 edges	5099 reachable
 // BenchmarkPTA-12	   1427 ms/op	600 MB/op	 8714 nodes	 28244 edges	4184 reachable
-// BenchmarkVTA-12		603 ms/op	 87 MB/op	12112 nodes	 44857 edges	4918 reachable
-// BenchmarkVTA2-12		774 ms/op	115 MB/op	 4918 nodes	 20247 edges	3841 reachable
-// BenchmarkVTA3-12		928 ms/op	134 MB/op	 3841 nodes	 16502 edges	3542 reachable
-// BenchmarkVTAAlt-12	395 ms/op	 61 MB/op	 7640 nodes  29629 edges	4257 reachable
-// BenchmarkVTAAlt2-12	556 ms/op	 82 MB/op	 4257 nodes	 18057 edges	3586 reachable
+// BenchmarkVTA-12		562 ms/op	 78 MB/op	12112 nodes	 44857 edges	4918 reachable
+// BenchmarkVTA2-12		743 ms/op	103 MB/op	 4918 nodes	 20247 edges	3841 reachable
+// BenchmarkVTA3-12		837 ms/op	119 MB/op	 3841 nodes	 16502 edges	3542 reachable
+// BenchmarkVTAAlt-12	356 ms/op	 56 MB/op	 7640 nodes  29629 edges	4257 reachable
+// BenchmarkVTAAlt2-12	490 ms/op	 75 MB/op	 4257 nodes	 18057 edges	3586 reachable
 //
 // Note:
 // * Static is unsound and may miss real edges.

go/callgraph/vta/propagation.go

@@ -20,61 +20,60 @@ import (
 // with ids X and Y s.t. X < Y, Y comes before X in the topological order.
 func scc(g vtaGraph) (map[node]int, int) {
 	// standard data structures used by Tarjan's algorithm.
-	var index uint64
+	type state struct {
+		index   int
+		lowLink int
+		onStack bool
+	}
+	states := make(map[node]*state, len(g))
 	var stack []node
-	indexMap := make(map[node]uint64)
-	lowLink := make(map[node]uint64)
-	onStack := make(map[node]bool)
 
-	nodeToSccID := make(map[node]int)
+	nodeToSccID := make(map[node]int, len(g))
 	sccID := 0
 
 	var doSCC func(node)
 	doSCC = func(n node) {
-		indexMap[n] = index
-		lowLink[n] = index
-		index = index + 1
-		onStack[n] = true
+		index := len(states)
+		ns := &state{index: index, lowLink: index, onStack: true}
+		states[n] = ns
 		stack = append(stack, n)
 
 		for s := range g[n] {
-			if _, ok := indexMap[s]; !ok {
+			if ss, visited := states[s]; !visited {
 				// Analyze successor s that has not been visited yet.
 				doSCC(s)
-				lowLink[n] = min(lowLink[n], lowLink[s])
-			} else if onStack[s] {
+				ss = states[s]
+				ns.lowLink = min(ns.lowLink, ss.lowLink)
+			} else if ss.onStack {
 				// The successor is on the stack, meaning it has to be
 				// in the current SCC.
-				lowLink[n] = min(lowLink[n], indexMap[s])
+				ns.lowLink = min(ns.lowLink, ss.index)
 			}
 		}
 
 		// if n is a root node, pop the stack and generate a new SCC.
-		if lowLink[n] == indexMap[n] {
-			for {
-				w := stack[len(stack)-1]
+		if ns.lowLink == index {
+			var w node
+			for w != n {
+				w = stack[len(stack)-1]
 				stack = stack[:len(stack)-1]
-				onStack[w] = false
+				states[w].onStack = false
 				nodeToSccID[w] = sccID
-				if w == n {
-					break
-				}
 			}
 			sccID++
 		}
 	}
 
-	index = 0
 	for n := range g {
-		if _, ok := indexMap[n]; !ok {
+		if _, visited := states[n]; !visited {
 			doSCC(n)
 		}
 	}
 
 	return nodeToSccID, sccID
 }
 
-func min(x, y uint64) uint64 {
+func min(x, y int) int {
 	if x < y {
 		return x
 	}