JDK-8160942 : Unused code in GraphKit::record_profiled_receiver_for_speculation
  • Type: Bug
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: 9
  • Priority: P5
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2016-07-07
  • Updated: 2016-09-29
  • Resolved: 2016-08-30
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JDK 9
9 b138Fixed
Related Reports
Relates :  
JDK-8031755 - Type speculation should be used to optimize explicit null checks
Description
JDK-8031755 introduced a "return n" in GraphKit::record_profiled_receiver_for_speculation() which is never executed and an assignment to 'maybe_null' that has no effect:
http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/rev/37023a7f1e1b#l7.99

+  bool maybe_null = true;
+  if (java_bc() == Bytecodes::_checkcast ||
+      java_bc() == Bytecodes::_instanceof ||
+      java_bc() == Bytecodes::_aastore) {
+    ciProfileData* data = method()->method_data()->bci_to_data(bci());
+    bool maybe_null = data == NULL ? true : data->as_BitData()->null_seen();
+  }
+  return record_profile_for_speculation(n, exact_kls, maybe_null);
+  return n;
Comments
Fix is trivial. However, the fact that these two faults (or their kind) are present in code committed to the JDK repository is outright embarrassing. Both errors are trivially detected by any C/C++ compiler and it is evident that the warning level used is not sufficient, i.e. it should be strict. (There should be a work package to rectify the lack of use of contemporary C/C++ compiler diagnostics.)
12-07-2016
I think the code should look like this: bool maybe_null = true; if (java_bc() == Bytecodes::_checkcast || java_bc() == Bytecodes::_instanceof || java_bc() == Bytecodes::_aastore) { ciProfileData* data = method()->method_data()->bci_to_data(bci()); maybe_null = data == NULL ? true : data->as_BitData()->null_seen(); } return record_profile_for_speculation(n, exact_kls, maybe_null);
07-07-2016
ILW = never executed statements / potentially losing type information, affects type speculation, no workaround = LMH = P5
07-07-2016
This issue was reported by Rados��aw Smogura: http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-July/023658.html
07-07-2016