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/interpreter/interpreter_common.cc b/runtime/interpreter/interpreter_common.cc
index 18fb0d8..ecd4de9 100644
--- a/runtime/interpreter/interpreter_common.cc
+++ b/runtime/interpreter/interpreter_common.cc
@@ -592,6 +592,10 @@
//
// (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.