[dev.regabi] go/types: use 512 bits as max. integer precision

This is a port of CL 288633 to go/types. It differs from that CL
in the implementation of opName, which now uses ast Exprs.

Additionally, a couple tests had to be updated:
 + TestEvalArith is updated to not overflow.
 + stmt0.src is updated to have an error positioned on the '<<'
   operator.

Change-Id: I628357c33a1e7b0bb5bb7de5736f1fb10ce404e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/290630
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/eval_test.go

@@ -76,7 +76,7 @@ func TestEvalArith(t *testing.T) {
 		`false == false`,
 		`12345678 + 87654321 == 99999999`,
 		`10 * 20 == 200`,
-		`(1<<1000)*2 >> 100 == 2<<900`,
+		`(1<<500)*2 >> 100 == 2<<400`,
 		`"foo" + "bar" == "foobar"`,
 		`"abc" <= "bcd"`,
 		`len([10]struct{}{}) == 2*5`,

src/go/types/expr.go

@@ -84,11 +84,6 @@ func (check *Checker) op(m opPredicates, x *operand, op token.Token) bool {
 func (check *Checker) overflow(x *operand, op token.Token, opPos token.Pos) {
 	assert(x.mode == constant_)
 
-	what := "" // operator description, if any
-	if int(op) < len(op2str) {
-		what = op2str[op]
-	}
-
 	if x.val.Kind() == constant.Unknown {
 		// TODO(gri) We should report exactly what went wrong. At the
 		//           moment we don't have the (go/constant) API for that.
@@ -105,15 +100,37 @@ func (check *Checker) overflow(x *operand, op token.Token, opPos token.Pos) {
 	}
 
 	// Untyped integer values must not grow arbitrarily.
-	const limit = 4 * 512 // 512 is the constant precision - we need more because old tests had no limits
-	if x.val.Kind() == constant.Int && constant.BitLen(x.val) > limit {
-		check.errorf(atPos(opPos), _InvalidConstVal, "constant %s overflow", what)
+	const prec = 512 // 512 is the constant precision
+	if x.val.Kind() == constant.Int && constant.BitLen(x.val) > prec {
+		check.errorf(atPos(opPos), _InvalidConstVal, "constant %s overflow", opName(x.expr))
 		x.val = constant.MakeUnknown()
 	}
 }
 
+// opName returns the name of an operation, or the empty string.
+// For now, only operations that might overflow are handled.
+// TODO(gri) Expand this to a general mechanism giving names to
+//           nodes?
+func opName(e ast.Expr) string {
+	switch e := e.(type) {
+	case *ast.BinaryExpr:
+		if int(e.Op) < len(op2str2) {
+			return op2str2[e.Op]
+		}
+	case *ast.UnaryExpr:
+		if int(e.Op) < len(op2str1) {
+			return op2str1[e.Op]
+		}
+	}
+	return ""
+}
+
+var op2str1 = [...]string{
+	token.XOR: "bitwise complement",
+}
+
 // This is only used for operations that may cause overflow.
-var op2str = [...]string{
+var op2str2 = [...]string{
 	token.ADD: "addition",
 	token.SUB: "subtraction",
 	token.XOR: "bitwise XOR",
@@ -763,8 +780,17 @@ func (check *Checker) shift(x, y *operand, e ast.Expr, op token.Token) {
 
 	if x.mode == constant_ {
 		if y.mode == constant_ {
+			// if either x or y has an unknown value, the result is unknown
+			if x.val.Kind() == constant.Unknown || y.val.Kind() == constant.Unknown {
+				x.val = constant.MakeUnknown()
+				// ensure the correct type - see comment below
+				if !isInteger(x.typ) {
+					x.typ = Typ[UntypedInt]
+				}
+				return
+			}
 			// rhs must be within reasonable bounds in constant shifts
-			const shiftBound = 1023 - 1 + 52 // so we can express smallestFloat64
+			const shiftBound = 1023 - 1 + 52 // so we can express smallestFloat64 (see issue #44057)
 			s, ok := constant.Uint64Val(yval)
 			if !ok || s > shiftBound {
 				check.invalidOp(y, _InvalidShiftCount, "invalid shift count %s", y)

src/go/types/stdlib_test.go

@@ -175,7 +175,6 @@ func TestStdFixed(t *testing.T) {
 		"issue16369.go",  // go/types handles this correctly - not an issue
 		"issue18459.go",  // go/types doesn't check validity of //go:xxx directives
 		"issue18882.go",  // go/types doesn't check validity of //go:xxx directives
-		"issue20232.go",  // go/types handles larger constants than gc
 		"issue20529.go",  // go/types does not have constraints on stack size
 		"issue22200.go",  // go/types does not have constraints on stack size
 		"issue22200b.go", // go/types does not have constraints on stack size

src/go/types/testdata/builtins.src

@@ -514,7 +514,7 @@ func panic1() {
 	panic("foo")
 	panic(false)
 	panic(1<<10)
-	panic(1 /* ERROR overflows */ <<1000)
+	panic(1 << /* ERROR constant shift overflow */ 1000)
 	_ = panic /* ERROR used as value */ (0)
 
 	var s []byte
@@ -538,7 +538,7 @@ func print1() {
 	print(2.718281828)
 	print(false)
 	print(1<<10)
-	print(1 /* ERROR overflows */ <<1000)
+	print(1 << /* ERROR constant shift overflow */ 1000)
 	println(nil /* ERROR untyped nil */ )
 
 	var s []int
@@ -564,7 +564,7 @@ func println1() {
 	println(2.718281828)
 	println(false)
 	println(1<<10)
-	println(1 /* ERROR overflows */ <<1000)
+	println(1 << /* ERROR constant shift overflow */ 1000)
 	println(nil /* ERROR untyped nil */ )
 
 	var s []int
@@ -695,7 +695,7 @@ func Alignof1() {
 	_ = unsafe.Alignof(42)
 	_ = unsafe.Alignof(new(struct{}))
 	_ = unsafe.Alignof(1<<10)
-	_ = unsafe.Alignof(1 /* ERROR overflows */ <<1000)
+	_ = unsafe.Alignof(1 << /* ERROR constant shift overflow */ 1000)
 	_ = unsafe.Alignof(nil /* ERROR "untyped nil */ )
 	unsafe /* ERROR not used */ .Alignof(x)
 
@@ -783,7 +783,7 @@ func Sizeof1() {
 	_ = unsafe.Sizeof(42)
 	_ = unsafe.Sizeof(new(complex128))
 	_ = unsafe.Sizeof(1<<10)
-	_ = unsafe.Sizeof(1 /* ERROR overflows */ <<1000)
+	_ = unsafe.Sizeof(1 << /* ERROR constant shift overflow */ 1000)
 	_ = unsafe.Sizeof(nil /* ERROR untyped nil */ )
 	unsafe /* ERROR not used */ .Sizeof(x)
 

src/go/types/testdata/const0.src

@@ -350,8 +350,14 @@ const _ = unsafe.Sizeof(func() {
 })
 
 // untyped constants must not get arbitrarily large
-const (
-	huge = 1<<1000
-	_ = huge * huge * /* ERROR constant multiplication overflow */ huge
-	_ = huge << 1000 << /* ERROR constant shift overflow */ 1000
-)
+const prec = 512 // internal maximum precision for integers
+const maxInt = (1<<(prec/2) - 1) * (1<<(prec/2) + 1) // == 1<<prec - 1
+
+const _ = maxInt + /* ERROR constant addition overflow */ 1
+const _ = -maxInt - /* ERROR constant subtraction overflow */ 1
+const _ = maxInt ^ /* ERROR constant bitwise XOR overflow */ -1
+const _ = maxInt * /* ERROR constant multiplication overflow */ 2
+const _ = maxInt << /* ERROR constant shift overflow */ 2
+const _ = 1 << /* ERROR constant shift overflow */ prec
+
+const _ = ^ /* ERROR constant bitwise complement overflow */ maxInt

src/go/types/testdata/const1.src

@@ -43,7 +43,12 @@ const (
 
 const (
 	smallestFloat32 = 1.0 / (1<<(127 - 1 + 23))
-	smallestFloat64 = 1.0 / (1<<(1023 - 1 + 52))
+	// TODO(gri) The compiler limits integers to 512 bit and thus
+	//           we cannot compute the value (1<<(1023 - 1 + 52))
+	//           without overflow. For now we match the compiler.
+	//           See also issue #44057.
+	// smallestFloat64 = 1.0 / (1<<(1023 - 1 + 52))
+	smallestFloat64 = 4.940656458412465441765687928682213723651e-324
 )
 
 const (
@@ -53,7 +58,12 @@ const (
 
 const (
 	maxFloat32 = 1<<127 * (1<<24 - 1) / (1.0<<23)
-	maxFloat64 = 1<<1023 * (1<<53 - 1) / (1.0<<52)
+	// TODO(gri) The compiler limits integers to 512 bit and thus
+	//           we cannot compute the value 1<<1023
+	//           without overflow. For now we match the compiler.
+	//           See also issue #44057.
+	// maxFloat64 = 1<<1023 * (1<<53 - 1) / (1.0<<52)
+	maxFloat64 = 1.797693134862315708145274237317043567981e+308
 )
 
 const (
@@ -271,7 +281,9 @@ const (
 	_ = assert(float64(smallestFloat32) == smallestFloat32)
 	_ = assert(float64(smallestFloat32/2) == smallestFloat32/2)
 	_ = assert(float64(smallestFloat64) == smallestFloat64)
-	_ = assert(float64(smallestFloat64/2) == 0)
+	// TODO(gri) With the change to the declaration of smallestFloat64
+	//           this now fails to be true. See issue #44058.
+	// _ = assert(float64(smallestFloat64/2) == 0)
 )
 
 const (

src/go/types/testdata/stmt0.src

@@ -90,7 +90,7 @@ func assignments1() {
 
 	// assignments to _
 	_ = nil /* ERROR "use of untyped nil" */
-	_ = 1 /* ERROR overflow */ <<1000
+	_ = 1  << /* ERROR constant shift overflow */ 1000
 	(_) = 0
 }