Expand x==null using a disjunction (#2572)

Summary:
Fixes #2563, #2564
Pull Request resolved: #2572

Differential Revision: D10027955

Pulled By: sb98052

fbshipit-source-id: 4023e3c32362cb95f477d4d990dd9b8edb5a963c

src/utils/paths.js

@@ -336,13 +336,24 @@ function pushPathCondition(condition: Value): void {
       let right = condition.args[1];
       if (left instanceof ConcreteValue && right instanceof AbstractValue) [left, right] = [right, left];
       if (left instanceof AbstractValue && (right instanceof UndefinedValue || right instanceof NullValue)) {
-        let op = condition.kind === "!=" ? "!==" : "===";
-        if (op === "!==") pushPathCondition(left);
-        else pushInversePathCondition(left);
-        let leftNeNull = AbstractValue.createFromBinaryOp(realm, op, left, realm.intrinsics.null);
-        if (leftNeNull.mightNotBeFalse()) pushPathCondition(leftNeNull);
-        let leftNeUndefined = AbstractValue.createFromBinaryOp(realm, op, left, realm.intrinsics.undefined);
-        if (leftNeUndefined.mightNotBeFalse()) pushPathCondition(leftNeUndefined);
+        if (condition.kind === "!=") {
+          // x != null => x!==null && x!==undefined
+          pushPathCondition(left);
+          let leftNeNull = AbstractValue.createFromBinaryOp(realm, "!==", left, realm.intrinsics.null);
+          let leftNeUndefined = AbstractValue.createFromBinaryOp(realm, "!==", left, realm.intrinsics.undefined);
+          pushPathCondition(leftNeNull);
+          pushPathCondition(leftNeUndefined);
+        } else if (condition.kind === "==") {
+          // x == null => x===null || x===undefined
+          pushInversePathCondition(left);
+          let leftEqNull = AbstractValue.createFromBinaryOp(realm, "===", left, realm.intrinsics.null);
+          let leftEqUndefined = AbstractValue.createFromBinaryOp(realm, "===", left, realm.intrinsics.undefined);
+          let c;
+          if (!leftEqNull.mightNotBeFalse()) c = leftEqUndefined;
+          else if (!leftEqUndefined.mightNotBeFalse()) c = leftEqNull;
+          else c = AbstractValue.createFromLogicalOp(realm, "||", leftEqNull, leftEqUndefined);
+          pushPathCondition(c);
+        }
         return;
       }
     }

test/serializer/optimizations/simplifyCompNullOrUndefined.js

@@ -0,0 +1,5 @@
+let x = global.__abstract ? __abstract(undefined, "undefined") : undefined;
+
+if (x == null) global.result = x === null;
+
+inspect = () => global.result;

test/serializer/optimizations/simplifyCompNullOrUndefined2.js

@@ -0,0 +1,7 @@
+// does contain result = false
+
+let x = global.__abstract ? __abstract(undefined, "undefined") : undefined;
+
+if (x != null) global.result = x === undefined;
+
+inspect = () => global.result;

test/serializer/optimizations/simplifyCompNullOrUndefined3.js

@@ -0,0 +1,8 @@
+function f(obj) {
+  let y = obj.foo;
+  if (y == null) {
+    return y === undefined;
+  }
+}
+global.__optimize && __optimize(f);
+inspect = () => f({ foo: null });