8290834: Improve potentially confusing documentation on collection of profiling information

Reviewed-by: thartmann, kvn, dlong

Author: TheShermanTanker

src/hotspot/share/c1/c1_globals.hpp

@@ -314,7 +314,7 @@
           "Use CHA and exact type results at call sites when updating MDOs")\
                                                                             \
   product(bool, C1UpdateMethodData, true,                                   \
-          "Update MethodData*s in Tier1-generated code")                    \
+          "Update MethodData*s in Tier 3 C1 generated code")                \
                                                                             \
   develop(bool, PrintCFGToFile, false,                                      \
           "print control flow graph to a separate file during compilation")

src/hotspot/share/compiler/compilationPolicy.hpp

@@ -34,11 +34,20 @@ class CompileTask;
 class CompileQueue;
 /*
  *  The system supports 5 execution levels:
- *  * level 0 - interpreter
+ *  * level 0 - interpreter (Profiling is tracked by a MethodData object, or MDO in short)
  *  * level 1 - C1 with full optimization (no profiling)
  *  * level 2 - C1 with invocation and backedge counters
- *  * level 3 - C1 with full profiling (level 2 + MDO)
- *  * level 4 - C2
+ *  * level 3 - C1 with full profiling (level 2 + All other MDO profiling information)
+ *  * level 4 - C2 with full profile guided optimization
+ *
+ * The MethodData object is created by both the interpreter or either compiler to store any
+ * profiling information collected on a method (ciMethod::ensure_method_data() for C1 and C2
+ * and CompilationPolicy::create_mdo() for the interpreter). Both the interpreter and code
+ * compiled by C1 at level 3 will constantly update profiling information in the MDO during
+ * execution. The information in the MDO is then used by C1 and C2 during compilation, via
+ * the compiler interface (ciMethodXXX).
+ * See ciMethod.cpp and ciMethodData.cpp for information transfer from an MDO to the compilers
+ * through the compiler interface.
  *
  * Levels 0, 2 and 3 periodically notify the runtime about the current value of the counters
  * (invocation counters and backedge counters). The frequency of these notifications is

src/hotspot/share/oops/methodData.hpp

@@ -38,7 +38,9 @@
 class BytecodeStream;
 
 // The MethodData object collects counts and other profile information
-// during zeroth-tier (interpretive) and first-tier execution.
+// during zeroth-tier (interpreter) and third-tier (C1 with full profiling)
+// execution.
+//
 // The profile is used later by compilation heuristics.  Some heuristics
 // enable use of aggressive (or "heroic") optimizations.  An aggressive
 // optimization often has a down-side, a corner case that it handles