ART: Optimize out redundant NewInstances of String
NewInstance of String creates an empty String object before it is
replaced by the result of a StringFactory call (String.<init>). If
the empty object is never used prior to the call, it can be safely
removed (replaced with null in this case).
We do not remove the instruction if:
- it has a real use (comparison, instanceof, check-cast), or
- we are compiling with --debuggable and there is an environment use.
If removed and execution deoptimizes before the StringFactory call,
the interpreter will see String.<init> being called on a null object.
Since the verifier guarantees that the call was made on new-instance
in the input bytecode (b/26579108), the interpreter can safely assume
that it was optimized out rather than throw NullPointerException.
Results (all without --debuggable):
- boot.oat: 563/563 removed
- Google Maps: 480/480 removed
- Google Docs: 819/819 removed
Change-Id: I9fdfc50e9dea6299a7327f94327cdfd2b2538079
diff --git a/runtime/entrypoints/entrypoint_utils-inl.h b/runtime/entrypoints/entrypoint_utils-inl.h
index 9a9f42b..68f2dcb 100644
--- a/runtime/entrypoints/entrypoint_utils-inl.h
+++ b/runtime/entrypoints/entrypoint_utils-inl.h
@@ -410,10 +410,19 @@
DCHECK(self->IsExceptionPending()); // Throw exception and unwind.
return nullptr; // Failure.
} else if (UNLIKELY(*this_object == nullptr && type != kStatic)) {
- // Maintain interpreter-like semantics where NullPointerException is thrown
- // after potential NoSuchMethodError from class linker.
- ThrowNullPointerExceptionForMethodAccess(method_idx, type);
- return nullptr; // Failure.
+ if (UNLIKELY(resolved_method->GetDeclaringClass()->IsStringClass() &&
+ resolved_method->IsConstructor())) {
+ // Hack for String init:
+ //
+ // We assume that the input of String.<init> in verified code is always
+ // an unitialized reference. If it is a null constant, it must have been
+ // optimized out by the compiler. Do not throw NullPointerException.
+ } else {
+ // Maintain interpreter-like semantics where NullPointerException is thrown
+ // after potential NoSuchMethodError from class linker.
+ ThrowNullPointerExceptionForMethodAccess(method_idx, type);
+ return nullptr; // Failure.
+ }
} else if (access_check) {
mirror::Class* methods_class = resolved_method->GetDeclaringClass();
bool can_access_resolved_method =