cmd/internal/gc: avoid spurious div-zero errors

Set overflowing integer constants to 1 rather than 0 to avoid
spurious div-zero errors in subsequent constant expressions.

Also: Exclude new test case from go/types test since it's
running too long (go/types doesn't have an upper constant
size limit at the moment).

Fixes #7746.

Change-Id: I3768488ad9909a3cf995247b81ee78a8eb5a1e41
Reviewed-on: https://go-review.googlesource.com/9165
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

Author: griesemer

src/cmd/internal/gc/const.go

@@ -774,7 +774,7 @@ func evconst(n *Node) {
 		ODIV<<16 | CTRUNE:
 		if mpcmpfixc(rv.U.Xval, 0) == 0 {
 			Yyerror("division by zero")
-			Mpmovecfix(v.U.Xval, 1)
+			mpsetovf(v.U.Xval)
 			break
 		}
 
@@ -784,7 +784,7 @@ func evconst(n *Node) {
 		OMOD<<16 | CTRUNE:
 		if mpcmpfixc(rv.U.Xval, 0) == 0 {
 			Yyerror("division by zero")
-			Mpmovecfix(v.U.Xval, 1)
+			mpsetovf(v.U.Xval)
 			break
 		}
 

src/cmd/internal/gc/mparith2.go

@@ -13,7 +13,7 @@ import (
 /// implements fix arithmetic
 
 func mpsetovf(a *Mpint) {
-	a.Val.SetUint64(0)
+	a.Val.SetUint64(1) // avoid spurious div-zero errors
 	a.Ovf = true
 }
 

src/go/types/stdlib_test.go

@@ -146,6 +146,7 @@ func TestStdFixed(t *testing.T) {
 		"bug459.go",    // possibly incorrect test - see issue 6703 (pending spec clarification)
 		"issue3924.go", // possibly incorrect test - see issue 6671 (pending spec clarification)
 		"issue6889.go", // gc-specific test
+		"issue7746.go", // large constants - consumes too much memory
 	)
 }
 

test/fixedbugs/issue7746.go

@@ -0,0 +1,133 @@
+// 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.
+
+package main
+
+const (
+	c0   = 1 << 100
+	c1   = c0 * c0
+	c2   = c1 * c1
+	c3   = c2 * c2 // ERROR "overflow"
+	c4   = c3 * c3
+	c5   = c4 * c4
+	c6   = c5 * c5
+	c7   = c6 * c6
+	c8   = c7 * c7
+	c9   = c8 * c8
+	c10  = c9 * c9
+	c11  = c10 * c10
+	c12  = c11 * c11
+	c13  = c12 * c12
+	c14  = c13 * c13
+	c15  = c14 * c14
+	c16  = c15 * c15
+	c17  = c16 * c16
+	c18  = c17 * c17
+	c19  = c18 * c18
+	c20  = c19 * c19
+	c21  = c20 * c20
+	c22  = c21 * c21
+	c23  = c22 * c22
+	c24  = c23 * c23
+	c25  = c24 * c24
+	c26  = c25 * c25
+	c27  = c26 * c26
+	c28  = c27 * c27
+	c29  = c28 * c28
+	c30  = c29 * c29
+	c31  = c30 * c30
+	c32  = c31 * c31
+	c33  = c32 * c32
+	c34  = c33 * c33
+	c35  = c34 * c34
+	c36  = c35 * c35
+	c37  = c36 * c36
+	c38  = c37 * c37
+	c39  = c38 * c38
+	c40  = c39 * c39
+	c41  = c40 * c40
+	c42  = c41 * c41
+	c43  = c42 * c42
+	c44  = c43 * c43
+	c45  = c44 * c44
+	c46  = c45 * c45
+	c47  = c46 * c46
+	c48  = c47 * c47
+	c49  = c48 * c48
+	c50  = c49 * c49
+	c51  = c50 * c50
+	c52  = c51 * c51
+	c53  = c52 * c52
+	c54  = c53 * c53
+	c55  = c54 * c54
+	c56  = c55 * c55
+	c57  = c56 * c56
+	c58  = c57 * c57
+	c59  = c58 * c58
+	c60  = c59 * c59
+	c61  = c60 * c60
+	c62  = c61 * c61
+	c63  = c62 * c62
+	c64  = c63 * c63
+	c65  = c64 * c64
+	c66  = c65 * c65
+	c67  = c66 * c66
+	c68  = c67 * c67
+	c69  = c68 * c68
+	c70  = c69 * c69
+	c71  = c70 * c70
+	c72  = c71 * c71
+	c73  = c72 * c72
+	c74  = c73 * c73
+	c75  = c74 * c74
+	c76  = c75 * c75
+	c77  = c76 * c76
+	c78  = c77 * c77
+	c79  = c78 * c78
+	c80  = c79 * c79
+	c81  = c80 * c80
+	c82  = c81 * c81
+	c83  = c82 * c82
+	c84  = c83 * c83
+	c85  = c84 * c84
+	c86  = c85 * c85
+	c87  = c86 * c86
+	c88  = c87 * c87
+	c89  = c88 * c88
+	c90  = c89 * c89
+	c91  = c90 * c90
+	c92  = c91 * c91
+	c93  = c92 * c92
+	c94  = c93 * c93
+	c95  = c94 * c94
+	c96  = c95 * c95
+	c97  = c96 * c96
+	c98  = c97 * c97
+	c99  = c98 * c98
+	c100 = c99 * c99
+)
+
+func main() {
+	println(c1 / c1)
+	println(c2 / c2)
+	println(c3 / c3)
+	println(c4 / c4)
+	println(c5 / c5)
+	println(c6 / c6)
+	println(c7 / c7)
+	println(c8 / c8)
+	println(c9 / c9)
+	println(c10 / c10)
+	println(c20 / c20)
+	println(c30 / c30)
+	println(c40 / c40)
+	println(c50 / c50)
+	println(c60 / c60)
+	println(c70 / c70)
+	println(c80 / c80)
+	println(c90 / c90)
+	println(c100 / c100)
+}