Request for reviews (M): 6614597: Performance variability in jvm2008 xml.validation

Vladimir Kozlov Vladimir.Kozlov at Sun.COM
Fri Jan 22 15:19:33 PST 2010


John does not suggest to remove Reason_bimorphic. He suggests next:


   static DeoptReason reason_recorded_per_bytecode_if_any(DeoptReason reason) {
     if (reason_is_recorded_per_bytecode(reason))
       return reason;
     else if (reason == Reason_div0_check) // null check due to divide-by-zero?
       return Reason_null_check;           // recorded per BCI as a null check
+   else if (reason == Reason_bimorphic)  // Piggy back Reason_bimorphic on the
+     return Reason_class_check;          // Reason_class_check per bytecode count.
       return Reason_none;

Which will allow only to remove the next new code in deoptimization.cpp:

+        if (reason == Reason_bimorphic) {
+          // This isn't recorded per bci because of MDO limitations
+          // but lets piggy back the invalidation on the
+          // Reason_class_check count.
+          uint prior_trap_count = trap_mdo->trap_count(Reason_bimorphic);
+          if (prior_trap_count >= (uint)PerBytecodeTrapLimit) {
+            make_not_entrant = true;
+          }
+        } else {

But since reason_recorded_per_bytecode_if_any() is used in other places I am looking if it is OK there also.


John Rose wrote:
> On Jan 22, 2010, at 2:59 PM, Tom Rodriguez wrote:
>> I don't see how this will work right.  The whole point of introducing a new reason was to get a separate counter to distinguish between the monomorphic and bimorphic predicted call cases.  Otherwise you can never make the mono to bi transition.  Maybe I don't understand what changing reason_recorded_per_bytecode_if_any in this way would do.  I'd prefer to have a per bytecode counter for bimorphic but we didn't have any left.
> It's a little better than the previous code, because at least the state changes are distinct now, because of the reasons.
> There is also a small distinction between counter states.  If the per-nmethod counter for Reason_bimorphic is zero, then the per-bytecode count is disregarded, even though it is non-zero due to aliased Reason_class_check failures.  Note that maybe_prior_trap returned by query_update_method_data will be false even though the per-bytecode count (which is a single bit) is non-zero.
> This allows one free bimorphism deopt per nmethod.  If this isn't enough, maybe we can downgrade one of the other one-bit counts to a per-nmethod count.
> -- John

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