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
7 files changed