From 519718b1069280761ae6596eb09008ce536f6c07 Mon Sep 17 00:00:00 2001
From: "Aman Khalid (from Dev Box)" <amankhalid@microsoft.com>
Date: Thu, 23 Jan 2025 17:17:34 -0500
Subject: [PATCH] Don't run optSetBlockWeights when we have PGO data

---
 src/coreclr/jit/optimizer.cpp | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/src/coreclr/jit/optimizer.cpp b/src/coreclr/jit/optimizer.cpp
index c0049564371ebf..f427e3fc068d07 100644
--- a/src/coreclr/jit/optimizer.cpp
+++ b/src/coreclr/jit/optimizer.cpp
@@ -40,9 +40,20 @@ DataFlow::DataFlow(Compiler* pCompiler)
 PhaseStatus Compiler::optSetBlockWeights()
 {
     noway_assert(opts.OptimizationEnabled());
+    assert(m_dfsTree != nullptr);
+    const bool usingProfileWeights = fgIsUsingProfileWeights();
+
+    // Rely on profile synthesis to propagate weights when we have PGO data.
+    // TODO: Replace optSetBlockWeights with profile synthesis entirely.
+    if (usingProfileWeights)
+    {
+        // Leave breadcrumb for loop alignment
+        fgHasLoops = m_dfsTree->HasCycle();
+        return PhaseStatus::MODIFIED_NOTHING;
+    }
+
     bool madeChanges = false;
 
-    assert(m_dfsTree != nullptr);
     if (m_domTree == nullptr)
     {
         m_domTree = FlowGraphDominatorTree::Build(m_dfsTree);
@@ -59,8 +70,7 @@ PhaseStatus Compiler::optSetBlockWeights()
         optFindAndScaleGeneralLoopBlocks();
     }
 
-    bool       firstBBDominatesAllReturns = true;
-    const bool usingProfileWeights        = fgIsUsingProfileWeights();
+    bool firstBBDominatesAllReturns = true;
 
     fgComputeReturnBlocks();