[dev.ssa] cmd/compile/ssa: implement constant booleans

The removal of if false { ... } blocks in the opt
pass exposed that removePredecessor needed
to do more cleaning, on pain of failing later
consistency checks.

Change-Id: I45d4ff7e1f7f1486fdd99f867867ce6ea006a288
Reviewed-on: https://go-review.googlesource.com/11879
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -464,7 +464,7 @@ func (s *state) expr(n *Node) *ssa.Value {
 		switch n.Val().Ctype() {
 		case CTINT:
 			return s.constInt(n.Type, Mpgetfix(n.Val().U.(*Mpint)))
-		case CTSTR:
+		case CTSTR, CTBOOL:
 			return s.entryNewValue0A(ssa.OpConst, n.Type, n.Val().U)
 		default:
 			s.Unimplementedf("unhandled OLITERAL %v", n.Val().Ctype())

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

@@ -98,15 +98,14 @@ func deadcode(f *Func) {
 
 // There was an edge b->c.  It has been removed from b's successors.
 // Fix up c to handle that fact.
-func removePredecessor(b, c *Block) {
+func (f *Func) removePredecessor(b, c *Block) {
+	work := [][2]*Block{{b, c}}
+
+	for len(work) > 0 {
+		b, c := work[0][0], work[0][1]
+		work = work[1:]
+
 		n := len(c.Preds) - 1
-	if n == 0 {
-		// c is now dead - don't bother working on it
-		if c.Preds[0] != b {
-			b.Fatalf("%s.Preds[0]==%s, want %s", c, c.Preds[0], b)
-		}
-		return
-	}
 
 		// find index of b in c's predecessor list
 		var i int
@@ -120,6 +119,7 @@ func removePredecessor(b, c *Block) {
 		c.Preds[i] = c.Preds[n]
 		c.Preds[n] = nil // aid GC
 		c.Preds = c.Preds[:n]
+
 		// rewrite phi ops to match the new predecessor list
 		for _, v := range c.Values {
 			if v.Op != OpPhi {
@@ -132,4 +132,17 @@ func removePredecessor(b, c *Block) {
 				v.Op = OpCopy
 			}
 		}
+
+		if n == 0 {
+			// c is now dead--recycle its values
+			for _, v := range c.Values {
+				f.vid.put(v.ID)
+			}
+			c.Values = nil
+			// Also kill any successors of c now, to spare later processing.
+			for _, succ := range c.Succs {
+				work = append(work, [2]*Block{c, succ})
+			}
+		}
+	}
 }

src/cmd/compile/internal/ssa/gen/rulegen.go

@@ -232,7 +232,7 @@ func genRules(arch arch) {
 
 			// Modify predecessor lists for no-longer-reachable blocks
 			for succ := range m {
-				fmt.Fprintf(w, "removePredecessor(b, %s)\n", succ)
+				fmt.Fprintf(w, "v.Block.Func.removePredecessor(b, %s)\n", succ)
 			}
 
 			fmt.Fprintf(w, "b.Kind = %s\n", blockName(t[0], arch))

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

@@ -46,7 +46,7 @@ func nilcheckelim(f *Func) {
 			// and the fuse pass will join this block with its successor.
 			b.Kind = BlockPlain
 			b.Control = nil
-			removePredecessor(b, b.Succs[1])
+			f.removePredecessor(b, b.Succs[1])
 			b.Succs = b.Succs[:1]
 		}
 	}

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

@@ -403,7 +403,7 @@ func rewriteBlockgeneric(b *Block) bool {
 			if !(c.(bool)) {
 				goto end915e334b6388fed7d63e09aa69ecb05c
 			}
-			removePredecessor(b, no)
+			v.Block.Func.removePredecessor(b, no)
 			b.Kind = BlockPlain
 			b.Control = nil
 			b.Succs = b.Succs[:1]
@@ -427,7 +427,7 @@ func rewriteBlockgeneric(b *Block) bool {
 			if !(!c.(bool)) {
 				goto end6452ee3a5bb02c708bddc3181c3ea3cb
 			}
-			removePredecessor(b, yes)
+			v.Block.Func.removePredecessor(b, yes)
 			b.Kind = BlockPlain
 			b.Control = nil
 			b.Succs = b.Succs[:1]