Add optimization for generator bodies that don't introduce new members (#1013)

If a generator object literal doesn't add any object members, we
can simply use the parent in its place.

Co-authored-by: Kushal Pisavadia <kushi.p@gmail.com>

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorObjectLiteralNode.java

@@ -73,6 +73,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = "checkObjectCannotHaveParameters()")
   protected VmDynamic evalDynamic(VirtualFrame frame, VmDynamic parent) {
     var data = executeChildren(frame, parent, parent.getLength());
+    if (data.hasNoMembers()) {
+      return parent;
+    }
     var result = new VmDynamic(frame.materialize(), parent, data.members(), data.length());
     return data.storeGeneratorFrames(result);
   }
@@ -81,6 +84,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   protected VmTyped evalTyped(VirtualFrame frame, VmTyped parent) {
     VmUtils.checkIsInstantiable(parent.getVmClass(), getParentNode());
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return parent;
+    }
     assert data.hasNoGeneratorFrames();
     return new VmTyped(frame.materialize(), parent, parent.getVmClass(), data.members());
   }
@@ -88,6 +94,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = "checkListingCannotHaveParameters()")
   protected VmListing evalListing(VirtualFrame frame, VmListing parent) {
     var data = executeChildren(frame, parent, parent.getLength());
+    if (data.hasNoMembers()) {
+      return parent;
+    }
     var result = new VmListing(frame.materialize(), parent, data.members(), data.length());
     return data.storeGeneratorFrames(result);
   }
@@ -95,6 +104,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = "checkMappingCannotHaveParameters()")
   protected VmMapping evalMapping(VirtualFrame frame, VmMapping parent) {
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return parent;
+    }
     var result = new VmMapping(frame.materialize(), parent, data.members());
     return data.storeGeneratorFrames(result);
   }
@@ -118,6 +130,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = {"parent == getDynamicClass()", "checkObjectCannotHaveParameters()"})
   protected VmDynamic evalDynamicClass(VirtualFrame frame, VmClass parent) {
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return VmDynamic.empty();
+    }
     var result =
         new VmDynamic(frame.materialize(), parent.getPrototype(), data.members(), data.length());
     return data.storeGeneratorFrames(result);
@@ -126,6 +141,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = {"parent == getMappingClass()", "checkMappingCannotHaveParameters()"})
   protected VmMapping evalMappingClass(VirtualFrame frame, VmClass parent) {
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return VmMapping.empty();
+    }
     var result = new VmMapping(frame.materialize(), parent.getPrototype(), data.members());
     return data.storeGeneratorFrames(result);
   }
@@ -133,6 +151,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   @Specialization(guards = {"parent == getListingClass()", "checkListingCannotHaveParameters()"})
   protected VmListing evalListingClass(VirtualFrame frame, VmClass parent) {
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return VmListing.empty();
+    }
     var result =
         new VmListing(frame.materialize(), parent.getPrototype(), data.members(), data.length());
     return data.storeGeneratorFrames(result);
@@ -142,6 +163,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   protected VmTyped evalTypedObjectClass(VirtualFrame frame, VmClass parent) {
     VmUtils.checkIsInstantiable(parent, getParentNode());
     var data = executeChildren(frame, parent, 0);
+    if (data.hasNoMembers()) {
+      return parent.getPrototype();
+    }
     assert data.hasNoGeneratorFrames();
     return new VmTyped(frame.materialize(), parent.getPrototype(), parent, data.members());
   }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/ObjectData.java

@@ -55,6 +55,10 @@ public final class ObjectData {
     return length;
   }
 
+  boolean hasNoMembers() {
+    return members.isEmpty();
+  }
+
   boolean hasNoGeneratorFrames() {
     return generatorFrames.isEmpty();
   }

pkl-core/src/test/files/LanguageSnippetTests/input/generators/generatorNoMembers.pkl

@@ -0,0 +1,46 @@
+// should only result in one trace in the output; generator object literals that don't add any
+// members should be optimized away.
+res1 {
+  prop = trace("hello")
+}
+
+res2 = (res1) {
+  when (false) {
+    prop2 = 2
+  }
+}
+
+res3 = (res1) {
+  for (_ in List()) {
+    "hello"
+  }
+}
+
+res4 = (res1) {
+  ...List()
+}
+
+res5 = (res1) {
+  when (true) {
+  } else {
+    prop3 = 3
+  }
+}
+
+res6 = (Dynamic) {
+  when (true) {}
+}
+
+res7 = (Mapping) {
+  when (true) {}
+}
+
+class Person {}
+
+res8 = (Person) {
+  when (true) {}
+}
+
+res9 = (Listing) {
+  when (true) {}
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/generatorNoMembers.err

@@ -0,0 +1,20 @@
+res1 {
+  prop = "hello"
+}
+res2 {
+  prop = "hello"
+}
+res3 {
+  prop = "hello"
+}
+res4 {
+  prop = "hello"
+}
+res5 {
+  prop = "hello"
+}
+res6 {}
+res7 {}
+res8 {}
+res9 {}
+pkl: TRACE: "hello" = "hello" (file:///$snippetsDir/input/generators/generatorNoMembers.pkl)