cmd/gc: avoid confusing error message "ovf in mpaddxx"

Fixes #6889

LGTM=rsc
R=gri, rsc
CC=golang-codereviews
https://golang.org/cl/85080044

src/cmd/gc/mparith2.c

@@ -300,13 +300,21 @@ mpmulfixfix(Mpint *a, Mpint *b)
 	for(i=0; i<na; i++) {
 		x = *a1++;
 		for(j=0; j<Mpscale; j++) {
-			if(x & 1)
+			if(x & 1) {
+				if(s.ovf) {
+					q.ovf = 1;
+					goto out;
+				}
 				mpaddfixfix(&q, &s, 1);
+				if(q.ovf)
+					goto out;
+			}
 			mplsh(&s, 1);
 			x >>= 1;
 		}
 	}
 
+out:
 	q.neg = a->neg ^ b->neg;
 	mpmovefixfix(a, &q);
 	if(a->ovf)

test/fixedbugs/issue6889.go

+// errorcheck
+
+// 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 6889: confusing error message: ovf in mpaddxx
+
+package main
+
+const (
+	f1  = 1
+	f2  = f1 * 2
+	f3  = f2 * 3
+	f4  = f3 * 4
+	f5  = f4 * 5
+	f6  = f5 * 6
+	f7  = f6 * 7
+	f8  = f7 * 8
+	f9  = f8 * 9
+	f10 = f9 * 10
+	f11 = f10 * 11
+	f12 = f11 * 12
+	f13 = f12 * 13
+	f14 = f13 * 14
+	f15 = f14 * 15
+	f16 = f15 * 16
+	f17 = f16 * 17
+	f18 = f17 * 18
+	f19 = f18 * 19
+	f20 = f19 * 20
+	f21 = f20 * 21
+	f22 = f21 * 22
+	f23 = f22 * 23
+	f24 = f23 * 24
+	f25 = f24 * 25
+	f26 = f25 * 26
+	f27 = f26 * 27
+	f28 = f27 * 28
+	f29 = f28 * 29
+	f30 = f29 * 30
+	f31 = f30 * 31
+	f32 = f31 * 32
+	f33 = f32 * 33
+	f34 = f33 * 34
+	f35 = f34 * 35
+	f36 = f35 * 36
+	f37 = f36 * 37
+	f38 = f37 * 38
+	f39 = f38 * 39
+	f40 = f39 * 40
+	f41 = f40 * 41
+	f42 = f41 * 42
+	f43 = f42 * 43
+	f44 = f43 * 44
+	f45 = f44 * 45
+	f46 = f45 * 46
+	f47 = f46 * 47
+	f48 = f47 * 48
+	f49 = f48 * 49
+	f50 = f49 * 50
+	f51 = f50 * 51
+	f52 = f51 * 52
+	f53 = f52 * 53
+	f54 = f53 * 54
+	f55 = f54 * 55
+	f56 = f55 * 56
+	f57 = f56 * 57
+	f58 = f57 * 58
+	f59 = f58 * 59
+	f60 = f59 * 60
+	f61 = f60 * 61
+	f62 = f61 * 62
+	f63 = f62 * 63
+	f64 = f63 * 64
+	f65 = f64 * 65
+	f66 = f65 * 66
+	f67 = f66 * 67
+	f68 = f67 * 68
+	f69 = f68 * 69
+	f70 = f69 * 70
+	f71 = f70 * 71
+	f72 = f71 * 72
+	f73 = f72 * 73
+	f74 = f73 * 74
+	f75 = f74 * 75
+	f76 = f75 * 76
+	f77 = f76 * 77
+	f78 = f77 * 78
+	f79 = f78 * 79
+	f80 = f79 * 80
+	f81 = f80 * 81
+	f82 = f81 * 82
+	f83 = f82 * 83
+	f84 = f83 * 84
+	f85 = f84 * 85
+	f86 = f85 * 86
+	f87 = f86 * 87
+	f88 = f87 * 88
+	f89 = f88 * 89
+	f90 = f89 * 90
+	f91 = f90 * 91 // ERROR "overflow"
+)