cmd/gc: round floats with a large negative exponent towards zero

Fixes #6902

LGTM=iant
R=iant, rsc
CC=golang-codereviews
https://golang.org/cl/78730049

src/cmd/gc/mparith1.c

@@ -427,7 +427,8 @@ mpatoflt(Mpflt *a, char *as)
 			mppow10flt(&b, ex-dp);
 			mpmulfltflt(a, &b);
 		} else {
-			if((short)(dp-ex) != dp-ex) {
+			// 4 approximates least_upper_bound(log2(10)).
+			if(dp-ex >= (1<<(8*sizeof(dp)-3)) || (short)(4*(dp-ex)) != 4*(dp-ex)) {
 				mpmovecflt(a, 0.0);
 			}
 			else {

test/fixedbugs/issue6902.go

+// run
+
+// Copyright 2014 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.
+
+// Issue 6902: confusing printing of large floating point constants
+
+package main
+
+import (
+	"os"
+)
+
+var x = -1e-10000
+
+func main() {
+	if x != 0 {
+		os.Exit(1)
+	}
+}