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, 4 insertions, 0 deletions

diff --git a/runtime/interpreter/interpreter_common.cc b/runtime/interpreter/interpreter_common.cc
index 18fb0d8518..ecd4de9bd0 100644
--- a/runtime/interpreter/interpreter_common.cc
+++ b/runtime/interpreter/interpreter_common.cc
@@ -592,6 +592,10 @@ static inline bool DoCallCommon(ArtMethod* called_method,
   //
   // (at this point the ArtMethod has already been replaced,
   // so we just need to fix-up the arguments)
+  //
+  // Note that FindMethodFromCode in entrypoint_utils-inl.h was also special-cased
+  // to handle the compiler optimization of replacing `this` with null without
+  // throwing NullPointerException.
   uint32_t string_init_vreg_this = is_range ? vregC : arg[0];
   if (UNLIKELY(string_init)) {
     DCHECK_GT(num_regs, 0u);  // As the method is an instance method, there should be at least 1.