8155028: javac crashes in silly do-while loop

Compiler should short circuit code generation for unreachable code.

Reviewed-by: mcimadamore

langtools/src/jdk.compiler/share/classes/com/sun/tools/javac/jvm/Gen.java

@@ -1086,17 +1086,19 @@ public class Gen extends JCTree.Visitor {
                 genStat(body, loopEnv, CRT_STATEMENT | CRT_FLOW_TARGET);
                 code.resolve(loopEnv.info.cont);
                 genStats(step, loopEnv);
-                CondItem c;
+                if (code.isAlive()) {
-                if (cond != null) {
+                    CondItem c;
-                    code.statBegin(cond.pos);
+                    if (cond != null) {
+                        code.statBegin(cond.pos);
+                        Assert.check(code.state.stacksize == 0);
+                        c = genCond(TreeInfo.skipParens(cond), CRT_FLOW_CONTROLLER);
+                    } else {
+                        c = items.makeCondItem(goto_);
+                    }
+                    code.resolve(c.jumpTrue(), startpc);
                     Assert.check(code.state.stacksize == 0);
-                    c = genCond(TreeInfo.skipParens(cond), CRT_FLOW_CONTROLLER);
+                    code.resolve(c.falseJumps);
-                } else {
-                    c = items.makeCondItem(goto_);
                 }
-                code.resolve(c.jumpTrue(), startpc);
-                Assert.check(code.state.stacksize == 0);
-                code.resolve(c.falseJumps);
             }
             Chain exit = loopEnv.info.exit;
             if (exit != null) {

langtools/test/tools/javac/UnreachableLoopCond.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8155028
+ * @summary javac crashes in silly do-while loop
+ * @compile UnreachableLoopCond.java
+ */
+
+class UnreachableLoopCond {
+
+    public void foo() {
+        Integer i = 100;
+        do {
+            return;
+        } while (i++ < 10);
+    }
+
+    public void goo() {
+        Integer i = 100;
+        do {
+            break;
+        } while (i++ < 10);
+    }
+
+    public void zoo() {
+        Integer i = 100;
+        do {
+            throw new RuntimeException();
+        } while (i++ < 10);
+    }
+
+    public void loo() {
+        Integer i = 100;
+        Integer j = 100;
+        do {
+           do {
+               return;
+           } while (i++ < 10);
+        } while (j++ < 10);
+    }
+
+    public void moo() {
+        Integer i = 100;
+        do {
+            if (true) {
+                return;
+            } else {
+                return;
+            }
+        } while (i++ < 10);
+    }
+
+    public void moo(boolean cond) {
+        Integer i = 100;
+        do {
+            if (cond) {
+                return;
+            } else {
+                return;
+            }
+        } while (i++ < 10);
+    }
+}