go.tools/go/ssa: support 3-operand x[lo:hi:max] slices.

+ very basic test ($GOROOT/test/slice3.go needs unsafe.Pointer)

LGTM=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/60030043

go/ssa/builder.go

@@ -569,7 +569,7 @@ func (b *builder) expr0(fn *Function, e ast.Expr) Value {
 		}
 
 	case *ast.SliceExpr:
-		var low, high Value
+		var low, high, max Value
 		var x Value
 		switch fn.Pkg.typeOf(e.X).Underlying().(type) {
 		case *types.Array:
@@ -586,10 +586,14 @@ func (b *builder) expr0(fn *Function, e ast.Expr) Value {
 		if e.Low != nil {
 			low = b.expr(fn, e.Low)
 		}
+		if e.Slice3 {
+			max = b.expr(fn, e.Max)
+		}
 		v := &Slice{
 			X:    x,
 			Low:  low,
 			High: high,
+			Max:  max,
 		}
 		v.setPos(e.Lbrack)
 		v.setType(fn.Pkg.typeOf(e))

go/ssa/interp/interp.go

@@ -214,7 +214,7 @@ func visitInstr(fr *frame, instr ssa.Instruction) continuation {
 		fr.env[instr] = fr.get(instr.Tuple).(tuple)[instr.Index]
 
 	case *ssa.Slice:
-		fr.env[instr] = slice(fr.get(instr.X), fr.get(instr.Low), fr.get(instr.High))
+		fr.env[instr] = slice(fr.get(instr.X), fr.get(instr.Low), fr.get(instr.High), fr.get(instr.Max))
 
 	case *ssa.Return:
 		switch len(instr.Results) {

go/ssa/interp/ops.go

@@ -231,29 +231,44 @@ func zero(t types.Type) value {
 	panic(fmt.Sprint("zero: unexpected ", t))
 }
 
-// slice returns x[lo:hi].  Either or both of lo and hi may be nil.
-func slice(x, lo, hi value) value {
+// slice returns x[lo:hi:max].  Any of lo, hi and max may be nil.
+func slice(x, lo, hi, max value) value {
+	var Len, Cap int
+	switch x := x.(type) {
+	case string:
+		Len = len(x)
+	case []value:
+		Len = len(x)
+		Cap = cap(x)
+	case *value: // *array
+		a := (*x).(array)
+		Len = len(a)
+		Cap = cap(a)
+	}
+
 	l := 0
 	if lo != nil {
 		l = asInt(lo)
 	}
+
+	h := Len
+	if hi != nil {
+		h = asInt(hi)
+	}
+
+	m := Cap
+	if max != nil {
+		m = asInt(max)
+	}
+
 	switch x := x.(type) {
 	case string:
-		if hi != nil {
-			return x[l:asInt(hi)]
-		}
-		return x[l:]
+		return x[l:h]
 	case []value:
-		if hi != nil {
-			return x[l:asInt(hi)]
-		}
-		return x[l:]
+		return x[l:h:m]
 	case *value: // *array
 		a := (*x).(array)
-		if hi != nil {
-			return []value(a)[l:asInt(hi)]
-		}
-		return []value(a)[l:]
+		return []value(a)[l:h:m]
 	}
 	panic(fmt.Sprintf("slice: unexpected X type: %T", x))
 }

go/ssa/interp/testdata/coverage.go

@@ -624,3 +624,15 @@ func init() {
 		panic(r)
 	}
 }
+
+// Test of 3-operand x[lo:hi:max] slice.
+func init() {
+	s := []int{0, 1, 2, 3}
+	lenCapLoHi := func(x []int) [4]int { return [4]int{len(x), cap(x), x[0], x[len(x)-1]} }
+	if got := lenCapLoHi(s[1:3]); got != [4]int{2, 3, 1, 2} {
+		panic(got)
+	}
+	if got := lenCapLoHi(s[1:3:3]); got != [4]int{2, 2, 1, 2} {
+		panic(got)
+	}
+}

go/ssa/print.go

@@ -201,6 +201,10 @@ func (v *Slice) String() string {
 	if v.High != nil {
 		b.WriteString(relName(v.High, v))
 	}
+	if v.Max != nil {
+		b.WriteString(":")
+		b.WriteString(relName(v.Max, v))
+	}
 	b.WriteString("]")
 	return b.String()
 }

go/ssa/ssa.go

@@ -723,8 +723,8 @@ type MakeSlice struct {
 //
 type Slice struct {
 	register
-	X         Value // slice, string, or *array
-	Low, High Value // either may be nil
+	X              Value // slice, string, or *array
+	Low, High, Max Value // each may be nil
 }
 
 // The FieldAddr instruction yields the address of Field of *struct X.