JS: improve switch elimination

js/js.inliner/src/org/jetbrains/kotlin/js/inline/clean/EmptyStatementElimination.kt

@@ -75,11 +75,13 @@ internal class EmptyStatementElimination(private val root: JsStatement) {
                 for (case in x.cases) {
                     processStatements(case.statements)
                 }
-                if (x.cases.lastOrNull() is JsDefault &&
-                    x.cases.dropLast(1).all { it.statements.isEmpty() }) {
+                if (x.cases.lastOrNull() is JsDefault && x.cases.dropLast(1).all { it.statements.isEmpty() }
+                    || x.cases.all { it.statements.isEmpty() }
+                ) {
                     hasChanges = true
-                    val conditionStatement = JsAstUtils.asSyntheticStatement(x.expression)
-                    ctx.replaceMe(JsBlock(listOf(conditionStatement) + x.cases.last().statements))
+                    val replacement = mutableListOf(JsAstUtils.asSyntheticStatement(x.expression))
+                    x.cases.lastOrNull()?.apply { replacement.addAll(statements) }
+                    ctx.replaceMe(JsBlock(replacement))
                 }
             }
 

js/js.tests/test/org/jetbrains/kotlin/js/test/optimizer/EmptyStatementEliminationTest.kt

@@ -24,4 +24,6 @@ class EmptyStatementEliminationTest : BasicOptimizerTest("empty-statement-elimin
     @Test fun ifWithEmptyThenAndNoElse() = box()
 
     @Test fun emptyBlockEliminated() = box()
+
+    @Test fun switchElimination() = box()
 }

js/js.translator/testData/js-optimizer/empty-statement-elimination/switchElimination.optimized.js

@@ -0,0 +1,53 @@
+var log = '';
+
+function test1(v) {
+    switch (v) {
+        case 1:
+        case 2:
+            log += 'A';
+            break;
+    }
+    log += 1;
+}
+
+function test2(v) {
+    loop: while(--v) {
+        switch (v) {
+            case 1:
+            case 2:
+                log += 'B';
+                break loop
+        }
+        log += 2;
+    }
+}
+
+function test3(v) {
+}
+
+function test4(v) {
+    log += 'D';
+    log += 4;
+}
+
+function test5(v) {
+    loop: while (--v) {
+        log += 'E';
+        break loop;
+    }
+}
+
+function test6(v) {
+    log += 'F' + v;
+}
+
+function box() {
+    test1(1);
+    test1(3);
+    test2(4);
+    test4(3);
+    test5(3);
+    test6(6);
+
+    return log === 'A112BD4EF6' ? 'OK' : 'fail: ' + log;
+}

js/js.translator/testData/js-optimizer/empty-statement-elimination/switchElimination.original.js

@@ -0,0 +1,68 @@
+var log = '';
+
+function test1(v) {
+    switch (v) {
+        case 1:
+        case 2:
+            log += 'A';
+            break;
+    }
+    log += 1;
+}
+
+function test2(v) {
+    loop: while(--v) {
+        switch (v) {
+            case 1:
+            case 2:
+                log += 'B';
+                break loop
+        }
+        log += 2;
+    }
+}
+
+function test3(v) {
+    switch (v) {
+        case 1:
+        case 2:
+    }
+}
+
+function test4(v) {
+    switch (v) {
+        case 1:
+        case 2:
+        default:
+            log += 'D';
+    }
+    log += 4;
+}
+
+function test5(v) {
+    loop: while(--v) {
+        switch (v) {
+            case 1:
+            case 2:
+            default:
+                log += 'E';
+                break loop
+        }
+        log += 5;
+    }
+}
+
+function test6(v) {
+    switch (log += 'F' + v) {}
+}
+
+function box() {
+    test1(1);
+    test1(3);
+    test2(4);
+    test4(3);
+    test5(3);
+    test6(6);
+
+    return log === 'A112BD4EF6' ? 'OK' : 'fail: ' + log;
+}