Add config option to disable tier 0 JIT (#22370)

Add config option to disable tier 0 JIT

Fixes https://github.com/dotnet/coreclr/issues/21856
- For methods that don't have pregenerated code, using tier 0 JIT can improve startup perf, and disabling tier 0 JIT can be useful to sacrifice some startup time to avoid issues of running tier 0 code for too long. In some cases, it may also be desirable to avoid tiering up much later.
- A fixed value for the call count indicates that tier 0 call counting is disabled. When disabled, the method starts at tier 1.
- Also modified call counting to start from a predetermined threshold and count down to zero, as it simplifies some things, allows for methods to have different thresholds, and likely is what we would want eventually anyway
- Took a small step towards eliminating knowledge of specific tier levels in code that should not care, though more is to be done there

1 files changed, 2 insertions, 4 deletions

diff --git a/src/vm/tieredcompilation.h b/src/vm/tieredcompilation.h
index 072f8e16c4..2c021b89ec 100644
--- a/src/vm/tieredcompilation.h
+++ b/src/vm/tieredcompilation.h
@@ -34,9 +34,8 @@ public:
 #ifdef FEATURE_TIERED_COMPILATION
 
 public:
-    static bool RequiresCallCounting(MethodDesc* pMethodDesc);
-    void OnMethodCalled(MethodDesc* pMethodDesc, DWORD currentCallCount, BOOL* shouldStopCountingCallsRef, BOOL* wasPromotedToTier1Ref);
-    void OnMethodCallCountingStoppedWithoutTier1Promotion(MethodDesc* pMethodDesc);
+    void OnTier0MethodCalled(MethodDesc* pMethodDesc, bool isFirstCall, int currentCallCountLimit, BOOL* shouldStopCountingCallsRef, BOOL* wasPromotedToNextTierRef);
+    void OnMethodCallCountingStoppedWithoutTierPromotion(MethodDesc* pMethodDesc);
     void AsyncPromoteMethodToTier1(MethodDesc* pMethodDesc);
     void Shutdown();
     static CORJIT_FLAGS GetJitFlags(NativeCodeVersion nativeCodeVersion);
@@ -69,7 +68,6 @@ private:
     ADID m_domainId;
     BOOL m_isAppDomainShuttingDown;
     DWORD m_countOptimizationThreadsRunning;
-    DWORD m_callCountOptimizationThreshhold;
     DWORD m_optimizationQuantumMs;
     SArray<MethodDesc*>* m_methodsPendingCountingForTier1;
     HANDLE m_tieringDelayTimerHandle;