KT-3002 optimized ==/!=0

Optimize byte-code generation for integer comparisons != and ==.
Work done in Hackergarten Vienna.
Co-authored-by: Kilian Matt <kilian.matt@gmail.com>

compiler/backend/src/org/jetbrains/jet/codegen/ExpressionCodegen.java

@@ -2362,6 +2362,14 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
             return genCmpWithNull(left, leftType, opToken);
         }
 
+        if (isIntZero(left, leftType)) {
+            return genCmpWithZero(right, rightType, opToken);
+        }
+
+        if (isIntZero(right, rightType)) {
+            return genCmpWithZero(left, leftType, opToken);
+        }
+
         if (isPrimitive(leftType) != isPrimitive(rightType)) {
             gen(left, leftType);
             StackValue.valueOf(v, leftType);
@@ -2384,6 +2392,27 @@ public class ExpressionCodegen extends JetVisitor<StackValue, StackValue> implem
                 genEqualsForExpressionsOnStack(v, opToken, leftType, rightType, leftJetType.isNullable(), rightJetType.isNullable());
     }
 
+    private boolean isIntZero(JetExpression expr, Type exprType) {
+        CompileTimeConstant<?> exprValue = bindingContext.get(BindingContext.COMPILE_TIME_VALUE, expr);
+        return isIntPrimitive(exprType) && exprValue != null && exprValue.getValue().equals(0);
+    }
+
+    private StackValue genCmpWithZero(JetExpression exp, Type expType, IElementType opToken) {
+        v.iconst(1);
+        gen(exp, expType);
+        Label ok = new Label();
+        if (JetTokens.EQEQ == opToken || JetTokens.EQEQEQ == opToken) {
+            v.ifeq(ok);
+        }
+        else {
+            v.ifne(ok);
+        }
+        v.pop();
+        v.iconst(0);
+        v.mark(ok);
+        return StackValue.onStack(Type.BOOLEAN_TYPE);
+    }
+
     private StackValue genCmpWithNull(JetExpression exp, Type expType, IElementType opToken) {
         v.iconst(1);
         gen(exp, boxType(expType));

compiler/tests/org/jetbrains/jet/codegen/ControlStructuresTest.java

@@ -234,6 +234,22 @@ public class ControlStructuresTest extends CodegenTestCase {
         blackBoxFile("regressions/kt237.jet");
     }
 
+    public void testCompareToZero() throws Exception {
+        createEnvironmentWithMockJdkAndIdeaAnnotations(ConfigurationKind.JDK_ONLY);
+        loadText("fun foo(a: Int, b: Int): Boolean = a == 0 && b != 0 && 0 == a && 0 != b");
+        String text = generateToText();
+        /*
+         * Check that the we generate optimized byte-code!
+         */
+        assertTrue(text.contains("IFEQ"));
+        assertTrue(text.contains("IFNE"));
+        assertFalse(text.contains("IF_ICMPEQ"));
+        assertFalse(text.contains("IF_ICMPNE"));
+        final Method main = generateFunction();
+        assertEquals(true, main.invoke(null, 0, 1));
+        assertEquals(false, main.invoke(null, 1, 0));
+    }
+
     public void testCompareToNull() throws Exception {
         createEnvironmentWithMockJdkAndIdeaAnnotations(ConfigurationKind.JDK_ONLY);
         loadText("fun foo(a: String?, b: String?): Boolean = a == null && b !== null && null == a && null !== b");