JIT: Fix edge likelihoods in fgOptimizeBranch (#113235)

Part of #113108. When converting an unconditional jump to a test block into a duplicate test block, fgOptimizeBranch flips the condition (unnecessarily -- I plan to fix this someday), which means we need to derive the new true edge's likelihood from the cloned false edge, and vice versa. Now that late profile repair is enabled, if we get this wrong, the flipped edge likelihoods will flatten loop weights, and inflate non-loop weights. I suspect fixing this will also revert some of the regressions from late profile repair.

Author: amanasifkhalid

src/coreclr/jit/fgopt.cpp

@@ -2768,15 +2768,18 @@ bool Compiler::fgOptimizeBranch(BasicBlock* bJump)
     FlowEdge* const destFalseEdge = bDest->GetFalseEdge();
     FlowEdge* const destTrueEdge  = bDest->GetTrueEdge();
 
-    // bJump now falls through into the next block
+    // bJump now falls through into the next block.
+    // Note that we're deriving the false edge's likelihood from 'destTrueEdge',
+    // because the comparison in 'bJump' is flipped.
+    // Similarly, we will derive the true edge's likelihood from 'destFalseEdge'.
     //
     BasicBlock* const bDestFalseTarget = bJump->Next();
-    FlowEdge* const   falseEdge        = fgAddRefPred(bDestFalseTarget, bJump, destFalseEdge);
+    FlowEdge* const   falseEdge        = fgAddRefPred(bDestFalseTarget, bJump, destTrueEdge);
 
     // bJump now jumps to bDest's normal jump target
     //
     fgRedirectTargetEdge(bJump, bDestNormalTarget);
-    bJump->GetTargetEdge()->setLikelihood(destTrueEdge->getLikelihood());
+    bJump->GetTargetEdge()->setLikelihood(destFalseEdge->getLikelihood());
 
     bJump->SetCond(bJump->GetTargetEdge(), falseEdge);
 
@@ -2787,18 +2790,16 @@ bool Compiler::fgOptimizeBranch(BasicBlock* bJump)
         // bJump no longer flows into bDest
         //
         bDest->decreaseBBProfileWeight(bJump->bbWeight);
-        bDestNormalTarget->decreaseBBProfileWeight(bJump->bbWeight * destFalseEdge->getLikelihood());
-        bDestFalseTarget->decreaseBBProfileWeight(bJump->bbWeight * destTrueEdge->getLikelihood());
 
         // Propagate bJump's weight into its new successors
         //
-        bDestNormalTarget->increaseBBProfileWeight(bJump->GetTrueEdge()->getLikelyWeight());
-        bDestFalseTarget->increaseBBProfileWeight(falseEdge->getLikelyWeight());
+        bDestNormalTarget->setBBProfileWeight(bDestNormalTarget->computeIncomingWeight());
+        bDestFalseTarget->setBBProfileWeight(bDestFalseTarget->computeIncomingWeight());
 
         if ((bDestNormalTarget->NumSucc() > 0) || (bDestFalseTarget->NumSucc() > 0))
         {
             JITDUMP("fgOptimizeBranch: New flow out of " FMT_BB " needs to be propagated. Data %s inconsistent.\n",
-                    fgPgoConsistent ? "is now" : "was already");
+                    bJump->bbNum, fgPgoConsistent ? "is now" : "was already");
             fgPgoConsistent = false;
         }
     }