cmd/compile: add sliceBound

Add a constant for the magic -1 for slice bounds.
Use it.
Enforce more aggressively that bounds must be
slice, ddd, or non-negative.
Remove ad hoc check in plive.go.
Check bounds before constructing an array type
when typechecking.

All changes are manual.

Passes toolstash -cmp.

Change-Id: I9fd9cc789d7d4b4eea3b30b24037a254d3788add
Reviewed-on: https://go-review.googlesource.com/21348
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -249,7 +249,7 @@ func dowidth(t *Type) {
 
 			w = t.Bound * t.Elem().Width
 			t.Align = t.Elem().Align
-		} else if t.Bound == -1 {
+		} else if t.IsSlice() {
 			w = int64(sizeof_Array)
 			checkwidth(t.Elem())
 			t.Align = uint8(Widthptr)

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

@@ -277,7 +277,7 @@ func (p *importer) typ() *Type {
 
 	case arrayTag, sliceTag:
 		t = p.newtyp(TARRAY)
-		t.Bound = -1
+		t.Bound = sliceBound
 		if i == arrayTag {
 			t.Bound = p.int64()
 		}
@@ -285,7 +285,7 @@ func (p *importer) typ() *Type {
 
 	case dddTag:
 		t = p.newtyp(TDDDFIELD)
-		t.Bound = -1
+		t.Bound = sliceBound
 		t.Type = p.typ()
 
 	case structTag:

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

@@ -918,11 +918,6 @@ func onebitwalktype1(t *Type, xoffset *int64, bv Bvec) {
 		*xoffset += t.Width
 
 	case TARRAY:
-		// The value of t.bound is -1 for slices types and >=0 for
-		// for fixed array types. All other values are invalid.
-		if t.Bound < -1 {
-			Fatalf("onebitwalktype1: invalid bound, %v", t)
-		}
 		if t.IsSlice() {
 			// struct { byte *array; uintgo len; uintgo cap; }
 			if *xoffset&int64(Widthptr-1) != 0 {

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

@@ -70,7 +70,10 @@ const (
 	NTYPE
 )
 
-const dddBound = -100 // arrays declared as [...]T start life with Bound=dddBound
+const (
+	sliceBound = -1   // slices have Bound=sliceBound
+	dddBound   = -100 // arrays declared as [...]T start life with Bound=dddBound
+)
 
 // Types stores pointers to predeclared named types.
 //
@@ -250,7 +253,7 @@ func typArray(elem *Type, bound int64) *Type {
 func typSlice(elem *Type) *Type {
 	t := typ(TARRAY)
 	t.Type = elem
-	t.Bound = -1
+	t.Bound = sliceBound
 	return t
 }
 
@@ -582,6 +585,7 @@ func (t *Type) isDDDArray() bool {
 	if t.Etype != TARRAY {
 		return false
 	}
+	t.checkBound()
 	return t.Bound == dddBound
 }
 
@@ -878,12 +882,20 @@ func (t *Type) IsChan() bool {
 	return t.Etype == TCHAN
 }
 
+// checkBound enforces that Bound has an acceptable value.
+func (t *Type) checkBound() {
+	if t.Bound != sliceBound && t.Bound < 0 && t.Bound != dddBound {
+		Fatalf("bad TARRAY bounds %d %s", t.Bound, t)
+	}
+}
+
 func (t *Type) IsSlice() bool {
-	// TODO(josharian): Change this to t.Bound == -1.
-	return t.Etype == TARRAY && t.Bound < 0
+	t.checkBound()
+	return t.Etype == TARRAY && t.Bound == sliceBound
 }
 
 func (t *Type) IsArray() bool {
+	t.checkBound()
 	return t.Etype == TARRAY && t.Bound >= 0
 }
 
@@ -918,6 +930,7 @@ func (t *Type) NumElem() int64 {
 	if t.Etype != TARRAY {
 		panic("NumElem on non-TARRAY")
 	}
+	t.checkBound()
 	return t.Bound
 }
 

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

@@ -369,17 +369,18 @@ OpSwitch:
 				return n
 			}
 
-			t = typArray(r.Type, v.U.(*Mpint).Int64())
-
 			if doesoverflow(v, Types[TINT]) {
 				Yyerror("array bound is too large")
 				n.Type = nil
 				return n
-			} else if t.IsSlice() {
+			}
+			bound := v.U.(*Mpint).Int64()
+			if bound < 0 {
 				Yyerror("array bound must be non-negative")
 				n.Type = nil
 				return n
 			}
+			t = typArray(r.Type, bound)
 		}
 
 		n.Op = OTYPE
@@ -2974,7 +2975,8 @@ func typecheckcomplit(n *Node) *Node {
 				if t.IsArray() && length > t.Bound {
 					setlineno(l)
 					Yyerror("array index %d out of bounds [0:%d]", length-1, t.Bound)
-					t.Bound = -1 // no more errors
+					// suppress any further errors out of bounds errors for the same type by pretending it is a slice
+					t.Bound = sliceBound
 				}
 			}