JIT: Improve weight distribution in internal blocks produced by casts

src/coreclr/jit/morph.cpp

@@ -17681,6 +17681,9 @@ void Compiler::fgExpandQmarkForCastInstOf(BasicBlock* block, Statement* stmt)
 
     assert(qmark->gtFlags & GTF_QMARK_CAST_INSTOF);
 
+    float currBbWeight = block->bbWeight;
+    float nextBbWeight = (block->bbNext != nullptr) ? block->bbNext->bbWeight : currBbWeight;
+
     // Get cond, true, false exprs for the qmark.
     GenTree* condExpr  = qmark->gtGetOp1();
     GenTree* trueExpr  = qmark->gtGetOp2()->AsColon()->ThenNode();
@@ -17766,11 +17769,18 @@ void Compiler::fgExpandQmarkForCastInstOf(BasicBlock* block, Statement* stmt)
     cond1Block->bbJumpDest = remainderBlock;
     cond2Block->bbJumpDest = remainderBlock;
 
-    // Set the weights; some are guesses.
+    // Currently, we don't instrument internal blocks, so the only way we can set weights to these blocks
+    // is to analyze successors and take a guess.
+    float cond2BlockWeight = 0;
+    if (currBbWeight > 0)
+    {
+        cond2BlockWeight = min(max(50.0f * nextBbWeight / currBbWeight + 50.0f, 50.0f), 100.0f);
+    }
+
     asgBlock->inheritWeight(block);
     cond1Block->inheritWeight(block);
-    cond2Block->inheritWeightPercentage(cond1Block, 50);
-    helperBlock->inheritWeightPercentage(cond2Block, 50);
+    cond2Block->inheritWeightPercentage(block, (UINT32)cond2BlockWeight);
+    helperBlock->inheritWeightPercentage(block, 100 - (UINT32)cond2BlockWeight);
 
     // Append cond1 as JTRUE to cond1Block
     GenTree*   jmpTree = gtNewOperNode(GT_JTRUE, TYP_VOID, condExpr);