Java: Improve customNullGuard performance.

Author: aschackmull

java/ql/lib/semmle/code/java/dataflow/NullGuards.qll

@@ -242,16 +242,20 @@ Guard nullGuard(SsaVariable v, boolean branch, boolean isnull) {
 }
 
 /**
- * A return statement that on a return value of `retval` allows the conclusion that the
- * parameter `p` either is null or non-null as specified by `isnull`.
+ * A return statement in a non-overridable method that on a return value of
+ * `retval` allows the conclusion that the parameter `p` either is null or
+ * non-null as specified by `isnull`.
  */
 private predicate validReturnInCustomNullGuard(
   ReturnStmt ret, Parameter p, boolean retval, boolean isnull
 ) {
   exists(Method m |
     ret.getEnclosingCallable() = m and
     p.getCallable() = m and
-    m.getReturnType().(PrimitiveType).hasName("boolean")
+    m.getReturnType().(PrimitiveType).hasName("boolean") and
+    not p.isVarargs() and
+    p.getType() instanceof RefType and
+    not m.isOverridable()
   ) and
   exists(SsaImplicitInit ssa | ssa.isParameterDefinition(p) |
     nullGuardedReturn(ret, ssa, isnull) and
@@ -267,21 +271,22 @@ private predicate nullGuardedReturn(ReturnStmt ret, SsaImplicitInit ssa, boolean
   )
 }
 
+pragma[nomagic]
+private Method returnStmtGetEnclosingCallable(ReturnStmt ret) {
+  ret.getEnclosingCallable() = result
+}
+
 /**
  * Gets a non-overridable method with a boolean return value that performs a null-check
  * on the `index`th parameter. A return value equal to `retval` allows us to conclude
  * that the argument either is null or non-null as specified by `isnull`.
  */
 private Method customNullGuard(int index, boolean retval, boolean isnull) {
   exists(Parameter p |
-    result.getReturnType().(PrimitiveType).hasName("boolean") and
-    not result.isOverridable() and
     p.getCallable() = result and
-    not p.isVarargs() and
-    p.getType() instanceof RefType and
     p.getPosition() = index and
     forex(ReturnStmt ret |
-      ret.getEnclosingCallable() = result and
+      returnStmtGetEnclosingCallable(ret) = result and
       exists(Expr res | res = ret.getResult() |
         not res.(BooleanLiteral).getBooleanValue() = retval.booleanNot()
       )