cmd/gc: Error out on division by constant zero.

Fixes #4264.

R=cldorian, rsc
CC=golang-dev
https://golang.org/cl/6845113

src/cmd/gc/typecheck.c

@@ -640,6 +640,13 @@ reswitch:
 				n->op = OCMPIFACE;
 			}
 		}
+
+		if((op == ODIV || op == OMOD) && isconst(r, CTINT))
+		if(mpcmpfixc(r->val.u.xval, 0) == 0) {
+			yyerror("division by zero");
+			goto error;
+		} 
+
 		n->type = t;
 		goto ret;
 

test/64bit.go

@@ -594,6 +594,19 @@ const binaryConstR = "func test%vBinaryR%v(a, add, sub, mul, div, mod, and, or,
 	"}\n" +
 	"\n"
 
+const binaryConstR0 = "func test%vBinaryR%v(a, add, sub, mul, div, mod, and, or, xor, andnot %v, dodiv bool) {\n" +
+	"	const b %v = %v;\n" +
+	"	const typ = `%s`;\n" +
+	"	if n, op, want := a + b, `+`, add; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a - b, `-`, sub; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a * b, `*`, mul; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a & b, `&`, and; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a | b, `|`, or; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a ^ b, `^`, xor; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"	if n, op, want := a &^ b, `&^`, andnot; n != want { ok=false; println(typ, `var`, a, op, `const`, b, `=`, n, `should be`, want); }\n" +
+	"}\n" +
+	"\n"
+
 const shiftConstL = "func test%vShiftL%v(b uint64, left, right %v) {\n" +
 	"	const a %v = %v;\n" +
 	"	const typ = `%s`;\n" +
@@ -621,12 +634,20 @@ const shiftConstR = "func test%vShiftR%v(a, left, right %v) {\n" +
 func constTests() {
 	for i, a := range int64Values {
 		fmt.Fprintf(bout, binaryConstL, "Int64", i, "int64", "int64", a, "int64")
-		fmt.Fprintf(bout, binaryConstR, "Int64", i, "int64", "int64", a, "int64")
+		if a.hi == 0 && a.lo == 0 {
+			fmt.Fprintf(bout, binaryConstR0, "Int64", i, "int64", "int64", a, "int64")
+		} else {
+			fmt.Fprintf(bout, binaryConstR, "Int64", i, "int64", "int64", a, "int64")
+		}
 		fmt.Fprintf(bout, shiftConstL, "Int64", i, "int64", "int64", a, "int64")
 	}
 	for i, a := range uint64Values {
 		fmt.Fprintf(bout, binaryConstL, "Uint64", i, "uint64", "uint64", a, "uint64")
-		fmt.Fprintf(bout, binaryConstR, "Uint64", i, "uint64", "uint64", a, "uint64")
+		if a.hi == 0 && a.lo == 0 {
+			fmt.Fprintf(bout, binaryConstR0, "Uint64", i, "uint64", "uint64", a, "uint64")
+		} else {
+			fmt.Fprintf(bout, binaryConstR, "Uint64", i, "uint64", "uint64", a, "uint64")
+		}
 		fmt.Fprintf(bout, shiftConstL, "Uint64", i, "uint64", "uint64", a, "uint64")
 	}
 	for i, a := range shiftValues {

test/fixedbugs/bug410.go

@@ -18,7 +18,7 @@ func zzz () {
     for s := range arr {
         x := make([]byte, 10)
         for i := 0; i < 100 ; i++ {
-            x[i] ^= k[i-arr[s].num%0]
+            x[i] ^= k[i-arr[s].num%3]
         }
     }
 }

test/fixedbugs/issue4264.go

+// errorcheck
+
+// Copyright 2013 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 4264: reject int division by const 0
+
+package main
+
+func main() {
+	var x int
+	var y float64
+	var z complex128
+
+	println(x/0) // ERROR "division by zero"
+	println(y/0)
+	println(z/0)
+}
\ No newline at end of file