RFR(XS): 8167300: Local scheduling failures during gcm should be fatal

[Roland Westrelin]

Thanks for looking at this, Vladimir.

> First, I can find who can set failure reason in LCM. Can you point me?

It's not set in lcm.cpp. PhaseCFG::schedule_local() returns false and
the caller (PhaseCFG::global_code_motion()) sets the failure reason.

> Second, is not better for debugging to add assert in a place where we 
> record failure instead of waiting when we exit compilation?

Are you suggesting this?

diff --git a/src/share/vm/opto/lcm.cpp b/src/share/vm/opto/lcm.cpp
--- a/src/share/vm/opto/lcm.cpp
+++ b/src/share/vm/opto/lcm.cpp
@@ -1147,6 +1147,8 @@
       // If this is the first failure, the sentinel string will "stick"
       // to the Compile object, and the C2Compiler will see it and retry.
       C->record_failure(C2Compiler::retry_no_subsuming_loads());
+    } else {
+      assert(false, "graph should be schedulable");
     }
     // assert( phi_cnt == end_idx(), "did not schedule all" );
     return false;


> Third, should we do the same for late schedule failure?:
>    C->record_method_not_compilable("late schedule failed: incorrect graph");

Ok. But why don't we do the same for all the other
record_method_not_compilable() calls in gcm.cpp that don't cause the
compilation to be reattempted:

C->record_method_not_compilable("late schedule failed: LCA == NULL");
C->record_method_not_compilable("late schedule failed: incorrect graph");
C->record_method_not_compilable("early schedule failed");

?

Roland.