[MERGE #2940 @pleath] Modify redeferral heuristic and add a cap.

Merge pull request #2940 from pleath:redefercap

Make the required inactive threshold grow exponentially (not linearly) with the number of previous compiles. Add a cap on the number of compiles (3 compiles, or 2 redeferrals, by default).

Author: pleath

lib/Common/ConfigFlagsList.h

@@ -422,7 +422,7 @@ PHASE(All)
 #define DEFAULT_CONFIG_ExtendedErrorStackForTestHost (false)
 #define DEFAULT_CONFIG_ForceSplitScope      (false)
 #define DEFAULT_CONFIG_DelayFullJITSmallFunc (0)
-
+#define DEFAULT_CONFIG_RedeferralCap         (3)
 
 //Following determines inline thresholds
 #define DEFAULT_CONFIG_InlineThreshold      (35)            //Default start
@@ -1157,6 +1157,7 @@ FLAGNR(Number,  ConstantArgumentInlineThreshold, "Maximum size in bytecodes of a
 FLAGNR(Number,  RecursiveInlineThreshold, "Maximum size in bytecodes of an inline candidate to inline recursively", DEFAULT_CONFIG_RecursiveInlineThreshold)
 FLAGNR(Number,  RecursiveInlineDepthMax, "Maximum depth of a recursive inline call", DEFAULT_CONFIG_RecursiveInlineDepthMax)
 FLAGNR(Number,  RecursiveInlineDepthMin, "Maximum depth of a recursive inline call", DEFAULT_CONFIG_RecursiveInlineDepthMin)
+FLAGNR(Number,  RedeferralCap,           "Number of compilations beyond which we stop redeferring a function", DEFAULT_CONFIG_RedeferralCap)
 FLAGNR(Number,  Loop                  , "Number of times to execute the script (useful for profiling short benchmarks and finding leaks)", DEFAULT_CONFIG_Loop)
 FLAGRA(Number,  LoopInterpretCount    , lic, "Number of times loop has to be interpreted before JIT Loop body", DEFAULT_CONFIG_LoopInterpretCount)
 FLAGNR(Number,  LoopProfileIterations , "Number of iterations of a loop that must be profiled before jitting the loop body", DEFAULT_CONFIG_LoopProfileIterations)

lib/Runtime/Base/FunctionBody.cpp

@@ -853,10 +853,23 @@ namespace Js
 
         if (!PHASE_FORCE(Js::RedeferralPhase, this) && !PHASE_STRESS(Js::RedeferralPhase, this))
         {
-            uint tmpThreshold;
-            auto fn = [&](){ tmpThreshold = 0xFFFFFFFF; };
-            tmpThreshold = UInt32Math::Mul(inactiveThreshold, this->GetCompileCount(), fn);
-            if (this->GetInactiveCount() < tmpThreshold)
+            uint compileCount = this->GetCompileCount();
+            if (compileCount >= (uint)CONFIG_FLAG(RedeferralCap))
+            {
+                return false;
+            }
+            // Redeferral threshold is k^x, where x is the number of previous compiles.
+            bool overflow = false;
+            uint currentThreshold = inactiveThreshold;
+            if (compileCount > 1)
+            {
+                currentThreshold = JavascriptNumber::DirectPowIntInt(&overflow, inactiveThreshold, compileCount);
+            }
+            if (overflow)
+            {
+                currentThreshold = 0xFFFFFFFF;
+            }
+            if (this->GetInactiveCount() < currentThreshold)
             {
                 return false;
             }