diff --git a/src/coreclr/jit/runtimelookup.cpp b/src/coreclr/jit/runtimelookup.cpp
index 325ca9956a2171..e2209b54e1d4dd 100644
--- a/src/coreclr/jit/runtimelookup.cpp
+++ b/src/coreclr/jit/runtimelookup.cpp
@@ -224,7 +224,6 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
                 }
 
                 GenTree* ctxTree = call->gtArgs.GetArgByIndex(0)->GetNode();
-                GenTree* sigNode = call->gtArgs.GetArgByIndex(1)->GetNode();
 
                 // Prepare slotPtr tree (TODO: consider sharing this part with impRuntimeLookup)
                 GenTree* slotPtrTree   = gtCloneExpr(ctxTree);
@@ -275,11 +274,11 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
                 //     if (*fastPathValue == null)
                 //         goto fallbackBb;
                 //
-                // fastPathBb(BBJ_ALWAYS):              [weight: 0.8]
+                // fastPathBb(BBJ_ALWAYS):              [weight: 1.0]
                 //     rtLookupLcl = *fastPathValue;
                 //     goto block;
                 //
-                // fallbackBb(BBJ_NONE):                [weight: 0.2]
+                // fallbackBb(BBJ_NONE):                [weight: 0.0]
                 //     rtLookupLcl = HelperCall();
                 //
                 // block(...):                          [weight: 1.0]
@@ -320,15 +319,15 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
                     //         goto fallbackBb;
                     //     ...
                     //
-                    // nullcheckBb(BBJ_COND):               [weight: 0.8]
+                    // nullcheckBb(BBJ_COND):               [weight: 1.0]
                     //     if (*fastPathValue == null)
                     //         goto fallbackBb;
                     //
-                    // fastPathBb(BBJ_ALWAYS):              [weight: 0.64]
+                    // fastPathBb(BBJ_ALWAYS):              [weight: 1.0]
                     //     rtLookupLcl = *fastPathValue;
                     //     goto block;
                     //
-                    // fallbackBb(BBJ_NONE):                [weight: 0.36]
+                    // fallbackBb(BBJ_NONE):                [weight: 0.0]
                     //     rtLookupLcl = HelperCall();
                     //
                     // block(...):                          [weight: 1.0]
@@ -387,27 +386,15 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
 
                 //
                 // Re-distribute weights (see '[weight: X]' on the diagrams above)
-                // TODO: consider marking fallbackBb as rarely-taken
                 //
                 block->inheritWeight(prevBb);
                 if (needsSizeCheck)
                 {
                     sizeCheckBb->inheritWeight(prevBb);
-                    // 80% chance we pass nullcheck
-                    nullcheckBb->inheritWeightPercentage(sizeCheckBb, 80);
-                    // 64% (0.8 * 0.8) chance we pass both nullcheck and sizecheck
-                    fastPathBb->inheritWeightPercentage(nullcheckBb, 80);
-                    // 100-64=36% chance we fail either nullcheck or sizecheck
-                    fallbackBb->inheritWeightPercentage(sizeCheckBb, 36);
-                }
-                else
-                {
-                    nullcheckBb->inheritWeight(prevBb);
-                    // 80% chance we pass nullcheck
-                    fastPathBb->inheritWeightPercentage(nullcheckBb, 80);
-                    // 20% chance we fail nullcheck (TODO: Consider making it cold (0%))
-                    fallbackBb->inheritWeightPercentage(nullcheckBb, 20);
                 }
+                nullcheckBb->inheritWeight(prevBb);
+                fastPathBb->inheritWeight(prevBb);
+                fallbackBb->bbSetRunRarely();
 
                 //
                 // Update loop info if loop table is known to be valid
@@ -437,6 +424,12 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
                     assert(BasicBlock::sameEHRegion(prevBb, sizeCheckBb));
                 }
 
+                // Merge prevBb with nullcheckBb (or sizeBb) if possible to simplify layout
+                if (fgCanCompactBlocks(prevBb, prevBb->GetUniqueSucc()))
+                {
+                    fgCompactBlocks(prevBb, prevBb->GetUniqueSucc());
+                }
+
                 // Scan current block again, the current call will be ignored because of ClearExpRuntimeLookup.
                 // We don't try to re-use expansions for the same lookups in the current block here - CSE is responsible
                 // for that
@@ -447,14 +440,5 @@ PhaseStatus Compiler::fgExpandRuntimeLookups()
             }
         }
     }
-
-    if (result == PhaseStatus::MODIFIED_EVERYTHING)
-    {
-        if (opts.OptimizationEnabled())
-        {
-            fgReorderBlocks(/* useProfileData */ false);
-            fgUpdateChangedFlowGraph(FlowGraphUpdates::COMPUTE_BASICS);
-        }
-    }
     return result;
 }