cmd/compile: avoid converting huge floats to integers

Fixes #13471.

Change-Id: I232ad1729343d020254e313cfff182695ad6fc54
Reviewed-on: https://go-review.googlesource.com/17401Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -379,8 +379,13 @@ func toint(v Val) Val {
 
 	case CTFLT:
 		i := new(Mpint)
-		if mpmovefltfix(i, v.U.(*Mpflt)) < 0 {
-			Yyerror("constant %v truncated to integer", Fconv(v.U.(*Mpflt), obj.FmtSharp))
+		if f := v.U.(*Mpflt); mpmovefltfix(i, f) < 0 {
+			msg := "constant %v truncated to integer"
+			// provide better error message if mpmovefltfix failed because f was too large
+			if f.Val.IsInt() {
+				msg = "constant %v overflows integer"
+			}
+			Yyerror(msg, Fconv(f, obj.FmtSharp))
 		}
 		v.U = i
 

src/cmd/compile/internal/gc/mparith2.go

@@ -31,6 +31,12 @@ func mpmovefixfix(a, b *Mpint) {
 }
 
 func mpmovefltfix(a *Mpint, b *Mpflt) int {
+	// avoid converting huge floating-point numbers to integers
+	// (2*Mpprec is large enough to permit all tests to pass)
+	if b.Val.MantExp(nil) > 2*Mpprec {
+		return -1
+	}
+
 	if _, acc := b.Val.Int(&a.Val); acc == big.Exact {
 		return 0
 	}

src/go/types/stdlib_test.go

@@ -151,6 +151,7 @@ func TestStdFixed(t *testing.T) {
 		"issue11326.go",  // large constants
 		"issue11326b.go", // large constants
 		"issue11362.go",  // canonical import path check
+		"issue13471.go",  // large constants - remove once issue 11327 is fixed
 	)
 }
 

test/fixedbugs/issue13471.go

+// errorcheck
+
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Tests for golang.org/issue/13471
+
+package main
+
+func main() {
+	const _ int64 = 1e646456992 // ERROR "1.00000e\+646456992 overflows integer"
+	const _ int32 = 1e64645699  // ERROR "1.00000e\+64645699 overflows integer"
+	const _ int16 = 1e6464569   // ERROR "1.00000e\+6464569 overflows integer"
+	const _ int8 = 1e646456     // ERROR "1.00000e\+646456 overflows integer"
+	const _ int = 1e64645       // ERROR "1.00000e\+64645 overflows integer"
+
+	const _ uint64 = 1e646456992 // ERROR "1.00000e\+646456992 overflows integer"
+	const _ uint32 = 1e64645699  // ERROR "1.00000e\+64645699 overflows integer"
+	const _ uint16 = 1e6464569   // ERROR "1.00000e\+6464569 overflows integer"
+	const _ uint8 = 1e646456     // ERROR "1.00000e\+646456 overflows integer"
+	const _ uint = 1e64645       // ERROR "1.00000e\+64645 overflows integer"
+
+	const _ rune = 1e64645 // ERROR "1.00000e\+64645 overflows integer"
+}