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

1 files changed, 13 insertions, 4 deletions

diff --git a/runtime/entrypoints/entrypoint_utils-inl.h b/runtime/entrypoints/entrypoint_utils-inl.h
index 9a9f42b976..68f2dcbf31 100644
--- a/runtime/entrypoints/entrypoint_utils-inl.h
+++ b/runtime/entrypoints/entrypoint_utils-inl.h
@@ -410,10 +410,19 @@ inline ArtMethod* FindMethodFromCode(uint32_t method_idx, mirror::Object** this_
     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 =