From 6b039d0e62c563d2d217ece14acbd94c14a23eb3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20Petryka?=
 <35800402+MichalPetryka@users.noreply.github.com>
Date: Thu, 18 Jun 2026 21:58:03 +0200
Subject: [PATCH] Fix PGO consistency asserts with Then only QMarks with
 likelyhood in AO

Removed explicit Then handling in favour of canonicalizing to else first which fixes the issue
---
 src/coreclr/jit/morph.cpp | 55 +++++++++++++++------------------------
 1 file changed, 21 insertions(+), 34 deletions(-)

diff --git a/src/coreclr/jit/morph.cpp b/src/coreclr/jit/morph.cpp
index 8f32ee00f0d34c..752e259f4c1d4f 100644
--- a/src/coreclr/jit/morph.cpp
+++ b/src/coreclr/jit/morph.cpp
@@ -14807,17 +14807,31 @@ bool Compiler::fgExpandQmarkStmt(BasicBlock* block, Statement* stmt, bool onlyEa
     }
 #endif // DEBUG
 
+    GenTreeColon* colon        = qmark->gtGetOp2()->AsColon();
+    bool          hasTrueExpr  = !colon->ThenNode()->OperIs(GT_NOP);
+    bool          hasFalseExpr = !colon->ElseNode()->OperIs(GT_NOP);
+    assert(hasTrueExpr || hasFalseExpr); // We expect to have at least one arm of the qmark!
+
+    // canonicalize to else
+    if (hasTrueExpr && !hasFalseExpr)
+    {
+        qmark->gtOp1 = gtReverseCond(qmark->gtGetOp1());
+        qmark->SetThenNodeLikelihood(qmark->ElseNodeLikelihood());
+
+        GenTree* op2 = colon->gtOp2;
+        colon->gtOp2 = colon->gtOp1;
+        colon->gtOp1 = op2;
+        hasTrueExpr  = false;
+        hasFalseExpr = true;
+    }
+    assert(hasFalseExpr); // we should always have false now
+
     // Retrieve the operands.
+    GenTree* trueExpr  = colon->ThenNode();
+    GenTree* falseExpr = colon->ElseNode();
     GenTree* condExpr  = qmark->gtGetOp1();
-    GenTree* trueExpr  = qmark->gtGetOp2()->AsColon()->ThenNode();
-    GenTree* falseExpr = qmark->gtGetOp2()->AsColon()->ElseNode();
-
     assert(!varTypeIsFloating(condExpr->TypeGet()));
 
-    bool hasTrueExpr  = !trueExpr->OperIs(GT_NOP);
-    bool hasFalseExpr = !falseExpr->OperIs(GT_NOP);
-    assert(hasTrueExpr || hasFalseExpr); // We expect to have at least one arm of the qmark!
-
     // Create remainder, cond and "else" blocks. After this, the blocks are in this order:
     //     block ... condBlock ... elseBlock ... remainderBlock
     //
@@ -14895,33 +14909,6 @@ bool Compiler::fgExpandQmarkStmt(BasicBlock* block, Statement* stmt, bool onlyEa
         thenEdge->setLikelihood(thenLikelihood / 100.0);
         elseEdge->setLikelihood(elseLikelihood / 100.0);
     }
-    else if (hasTrueExpr)
-    {
-        //                 false
-        //     S0 -->-- ~C -->-- T -->-- S1
-        //              |                |
-        //              +-->-------------+
-        //              bbj_cond(true)
-        //
-        // TODO: Remove unnecessary condition reversal
-        qmark->gtOp1 = condExpr = gtReverseCond(condExpr);
-
-        const unsigned thenLikelihood = qmark->ThenNodeLikelihood();
-        const unsigned elseLikelihood = qmark->ElseNodeLikelihood();
-
-        assert(condBlock->TargetIs(elseBlock));
-        FlowEdge* const thenEdge = fgAddRefPred(remainderBlock, condBlock);
-        FlowEdge* const elseEdge = condBlock->GetTargetEdge();
-        condBlock->SetCond(thenEdge, elseEdge);
-
-        // Since we have no false expr, use the one we'd already created.
-        thenBlock = elseBlock;
-        elseBlock = nullptr;
-
-        thenBlock->inheritWeightPercentage(condBlock, thenLikelihood);
-        thenEdge->setLikelihood(thenLikelihood / 100.0);
-        elseEdge->setLikelihood(elseLikelihood / 100.0);
-    }
     else if (hasFalseExpr)
     {
         //                false