cmd/compile: in a Tarjan algorithm, DFS should really be DFS

Replaced incorrect recursion-free rendering of DFS with
something that was correct.  Enhanced test with all
permutations of IF successors to ensure that all possible
DFS traversals are exercised.

Test is improved version of
https://go-review.googlesource.com/#/c/22334

Update 15084.

Change-Id: I6e944c41244e47fe5f568dfc2b360ff93b94079e
Reviewed-on: https://go-review.googlesource.com/22347
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/dom.go

@@ -5,11 +5,13 @@
 package ssa
 
 // mark values
+type markKind uint8
+
 const (
-	notFound    = 0 // block has not been discovered yet
-	notExplored = 1 // discovered and in queue, outedges not processed yet
-	explored    = 2 // discovered and in queue, outedges processed
-	done        = 3 // all done, in output ordering
+	notFound    markKind = 0 // block has not been discovered yet
+	notExplored markKind = 1 // discovered and in queue, outedges not processed yet
+	explored    markKind = 2 // discovered and in queue, outedges processed
+	done        markKind = 3 // all done, in output ordering
 )
 
 // This file contains code to compute the dominator tree
@@ -18,7 +20,7 @@ const (
 // postorder computes a postorder traversal ordering for the
 // basic blocks in f. Unreachable blocks will not appear.
 func postorder(f *Func) []*Block {
-	mark := make([]byte, f.NumBlocks())
+	mark := make([]markKind, f.NumBlocks())
 
 	// result ordering
 	var order []*Block
@@ -96,7 +98,7 @@ func (cfg *Config) scratchBlocksForDom(maxBlockID int) (a, b, c, d, e, f, g, h [
 // dfs performs a depth first search over the blocks starting at the set of
 // blocks in the entries list (in arbitrary order). dfnum contains a mapping
 // from block id to an int indicating the order the block was reached or
-// notFound if the block was not reached.  order contains a mapping from dfnum
+// 0 if the block was not reached.  order contains a mapping from dfnum
 // to block.
 func (f *Func) dfs(entries []*Block, succFn linkedBlocks, dfnum, order, parent []ID) (fromID []*Block) {
 	maxBlockID := entries[0].Func.NumBlocks()
@@ -114,7 +116,7 @@ func (f *Func) dfs(entries []*Block, succFn linkedBlocks, dfnum, order, parent [
 	n := ID(0)
 	s := make([]*Block, 0, 256)
 	for _, entry := range entries {
-		if dfnum[entry.ID] != notFound {
+		if dfnum[entry.ID] != 0 {
 			continue // already found from a previous entry
 		}
 		s = append(s, entry)
@@ -122,18 +124,19 @@ func (f *Func) dfs(entries []*Block, succFn linkedBlocks, dfnum, order, parent [
 		for len(s) > 0 {
 			node := s[len(s)-1]
 			s = s[:len(s)-1]
-
-			n++
-			for _, w := range succFn(node) {
-				// if it has a dfnum, we've already visited it
-				if dfnum[w.ID] == notFound {
-					s = append(s, w)
-					parent[w.ID] = node.ID
-					dfnum[w.ID] = notExplored
-				}
+			if dfnum[node.ID] != 0 {
+				continue // already found from a previous entry
 			}
+			n++
 			dfnum[node.ID] = n
 			order[n] = node.ID
+			for _, w := range succFn(node) {
+				// if it has a dfnum, we've already visited it
+				if dfnum[w.ID] == 0 {
+					s = append(s, w)
+					parent[w.ID] = node.ID // keep overwriting this till it is visited.
+				}
+			}
 		}
 	}
 
@@ -154,8 +157,6 @@ func dominators(f *Func) []*Block {
 
 // postDominators computes the post-dominator tree for f.
 func postDominators(f *Func) []*Block {
-	preds := func(b *Block) []*Block { return b.Preds }
-	succs := func(b *Block) []*Block { return b.Succs }
 
 	if len(f.Blocks) == 0 {
 		return nil
@@ -170,6 +171,10 @@ func postDominators(f *Func) []*Block {
 		}
 	}
 
+	// TODO: postdominators is not really right, and it's not used yet
+	preds := func(b *Block) []*Block { return b.Preds }
+	succs := func(b *Block) []*Block { return b.Succs }
+
 	// infinite loop with no exit
 	if exits == nil {
 		return make([]*Block, f.NumBlocks())
@@ -214,7 +219,7 @@ func (f *Func) dominatorsLT(entries []*Block, predFn linkedBlocks, succFn linked
 			continue
 		}
 
-		if dfnum[w] == notFound {
+		if dfnum[w] == 0 {
 			// skip unreachable node
 			continue
 		}
@@ -236,7 +241,7 @@ func (f *Func) dominatorsLT(entries []*Block, predFn linkedBlocks, succFn linked
 		var sp ID
 		// calculate the semidominator of w
 		for _, v := range predFn(fromID[w]) {
-			if dfnum[v.ID] == notFound {
+			if dfnum[v.ID] == 0 {
 				// skip unreachable predecessor
 				continue
 			}

src/cmd/compile/internal/ssa/dom_test.go

@@ -420,3 +420,48 @@ func TestInfiniteLoop(t *testing.T) {
 	postDoms := map[string]string{}
 	verifyDominators(t, fun, postDominators, postDoms)
 }
+
+func TestDomTricky(t *testing.T) {
+	doms := map[string]string{
+		"4":  "1",
+		"2":  "4",
+		"5":  "4",
+		"11": "4",
+		"15": "4", // the incorrect answer is "5"
+		"10": "15",
+		"19": "15",
+	}
+
+	if4 := [2]string{"2", "5"}
+	if5 := [2]string{"15", "11"}
+	if15 := [2]string{"19", "10"}
+
+	for i := 0; i < 8; i++ {
+		a := 1 & i
+		b := 1 & i >> 1
+		c := 1 & i >> 2
+
+		fun := Fun(testConfig(t), "1",
+			Bloc("1",
+				Valu("mem", OpInitMem, TypeMem, 0, nil),
+				Valu("p", OpConstBool, TypeBool, 1, nil),
+				Goto("4")),
+			Bloc("2",
+				Goto("11")),
+			Bloc("4",
+				If("p", if4[a], if4[1-a])), // 2, 5
+			Bloc("5",
+				If("p", if5[b], if5[1-b])), //15, 11
+			Bloc("10",
+				Exit("mem")),
+			Bloc("11",
+				Goto("15")),
+			Bloc("15",
+				If("p", if15[c], if15[1-c])), //19, 10
+			Bloc("19",
+				Goto("10")))
+		CheckFunc(fun.f)
+		verifyDominators(t, fun, dominators, doms)
+		verifyDominators(t, fun, dominatorsSimple, doms)
+	}
+}

src/cmd/compile/internal/ssa/id.go

@@ -11,7 +11,7 @@ type idAlloc struct {
 	last ID
 }
 
-// get allocates an ID and returns it.
+// get allocates an ID and returns it. IDs are always > 0.
 func (a *idAlloc) get() ID {
 	x := a.last
 	x++