go.tools/go/ssa: fix incorrect indentation in SSA printout.

Very long instructions caused the printf width spec to go
negative, which causes right-padding, often several lines'
worth.

Also: print the basic block comment once on the RHS. It's too
verbose to print it each time we mention the block.

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/97490046

go/ssa/example_test.go

@@ -85,21 +85,21 @@ func main() {
 	// # Package: main
 	// # Synthetic: package initializer
 	// func init():
-	// .0.entry:                                                               P:0 S:2
+	// 0:                                                                entry P:0 S:2
 	// 	t0 = *init$guard                                                   bool
-	// 	if t0 goto 2.init.done else 1.init.start
+	// 	if t0 goto 2 else 1
-	// .1.init.start:                                                          P:1 S:1
+	// 1:                                                           init.start P:1 S:1
 	// 	*init$guard = true:bool
 	// 	t1 = fmt.init()                                                      ()
-	// 	jump 2.init.done
+	// 	jump 2
-	// .2.init.done:                                                           P:2 S:0
+	// 2:                                                            init.done P:2 S:0
 	// 	return
 	//
 	// # Name: main.main
 	// # Package: main
 	// # Location: hello.go:8:6
 	// func main():
-	// .0.entry:                                                               P:0 S:0
+	// 0:                                                                entry P:0 S:0
 	// 	t0 = new [1]interface{} (varargs)                       *[1]interface{}
 	// 	t1 = &t0[0:int]                                            *interface{}
 	// 	t2 = make interface{} <- string ("Hello, World!":string)    interface{}

go/ssa/func.go

@@ -31,7 +31,7 @@ func (b *BasicBlock) Parent() *Function { return b.parent }
 // It is not guaranteed unique within the function.
 //
 func (b *BasicBlock) String() string {
-	return fmt.Sprintf("%d.%s", b.Index, b.Comment)
+	return fmt.Sprintf("%d", b.Index)
 }
 
 // emit appends an instruction to the current basic block.
@@ -575,16 +575,19 @@ func WriteFunction(buf *bytes.Buffer, f *Function) {
 		buf.WriteString("\t(external)\n")
 	}
 
-	// NB. column calculations are confused by non-ASCII characters.
+	// NB. column calculations are confused by non-ASCII
+	// characters and assume 8-space tabs.
 	const punchcard = 80 // for old time's sake.
+	const tabwidth = 8
 	for _, b := range f.Blocks {
 		if b == nil {
 			// Corrupt CFG.
 			fmt.Fprintf(buf, ".nil:\n")
 			continue
 		}
-		n, _ := fmt.Fprintf(buf, ".%s:", b)
+		n, _ := fmt.Fprintf(buf, "%d:", b.Index)
-		fmt.Fprintf(buf, "%*sP:%d S:%d\n", punchcard-1-n-len("P:n S:n"), "", len(b.Preds), len(b.Succs))
+		bmsg := fmt.Sprintf("%s P:%d S:%d", b.Comment, len(b.Preds), len(b.Succs))
+		fmt.Fprintf(buf, "%*s%s\n", punchcard-1-n-len(bmsg), "", bmsg)
 
 		if false { // CFG debugging
 			fmt.Fprintf(buf, "\t# CFG: %s --> %s --> %s\n", b.Preds, b, b.Succs)
@@ -593,18 +596,23 @@ func WriteFunction(buf *bytes.Buffer, f *Function) {
 			buf.WriteString("\t")
 			switch v := instr.(type) {
 			case Value:
-				l := punchcard
+				l := punchcard - tabwidth
 				// Left-align the instruction.
 				if name := v.Name(); name != "" {
 					n, _ := fmt.Fprintf(buf, "%s = ", name)
 					l -= n
 				}
-				// TODO(adonovan): append instructions directly to w.
 				n, _ := buf.WriteString(instr.String())
 				l -= n
-				// Right-align the type.
+				// Right-align the type if there's space.
 				if t := v.Type(); t != nil {
-					fmt.Fprintf(buf, " %*s", l-10, relType(t, pkgobj))
+					buf.WriteByte(' ')
+					ts := relType(t, pkgobj)
+					l -= len(ts) + len("  ") // (spaces before and after type)
+					if l > 0 {
+						fmt.Fprintf(buf, "%*s", l, "")
+					}
+					buf.WriteString(ts)
 				}
 			case nil:
 				// Be robust against bad transforms.

go/ssa/print.go

@@ -89,12 +89,11 @@ func (v *Phi) String() string {
 			b.WriteString(", ")
 		}
 		// Be robust against malformed CFG.
-		blockname := "?"
+		block := -1
 		if v.block != nil && i < len(v.block.Preds) {
-			blockname = v.block.Preds[i].String()
+			block = v.block.Preds[i].Index
 		}
-		b.WriteString(blockname)
+		fmt.Fprintf(&b, "%d: ", block)
-		b.WriteString(": ")
 		edgeVal := "<nil>" // be robust
 		if edge != nil {
 			edgeVal = relName(edge, v)
@@ -272,21 +271,21 @@ func (v *Extract) String() string {
 
 func (s *Jump) String() string {
 	// Be robust against malformed CFG.
-	blockname := "?"
+	block := -1
 	if s.block != nil && len(s.block.Succs) == 1 {
-		blockname = s.block.Succs[0].String()
+		block = s.block.Succs[0].Index
 	}
-	return fmt.Sprintf("jump %s", blockname)
+	return fmt.Sprintf("jump %d", block)
 }
 
 func (s *If) String() string {
 	// Be robust against malformed CFG.
-	tblockname, fblockname := "?", "?"
+	tblock, fblock := -1, -1
 	if s.block != nil && len(s.block.Succs) == 2 {
-		tblockname = s.block.Succs[0].String()
+		tblock = s.block.Succs[0].Index
-		fblockname = s.block.Succs[1].String()
+		fblock = s.block.Succs[1].Index
 	}
-	return fmt.Sprintf("if %s goto %s else %s", relName(s.Cond, s), tblockname, fblockname)
+	return fmt.Sprintf("if %s goto %d else %d", relName(s.Cond, s), tblock, fblock)
 }
 
 func (s *Go) String() string {