JIT: Add 3-opt implementation for improving upon RPO-based block layo…

…ut (dotnet#103450)

src/coreclr/jit/block.h

@@ -593,6 +593,9 @@ struct FlowEdge
     // The count of duplicate "edges" (used for switch stmts or degenerate branches)
     unsigned m_dupCount;
 
+    // Convenience flag for phases that need to track edge visitation
+    bool m_visited;
+
     // True if likelihood has been set
     INDEBUG(bool m_likelihoodSet);
 
@@ -603,6 +606,7 @@ struct FlowEdge
         , m_destBlock(destBlock)
         , m_likelihood(0)
         , m_dupCount(0)
+        , m_visited(false)
 #ifdef DEBUG
         , m_likelihoodSet(false)
 #endif // DEBUG
@@ -687,6 +691,23 @@ struct FlowEdge
         assert(m_dupCount >= 1);
         m_dupCount--;
     }
+
+    bool visited() const
+    {
+        return m_visited;
+    }
+
+    void markVisited()
+    {
+        assert(!visited());
+        m_visited = true;
+    }
+
+    void markUnvisited()
+    {
+        assert(visited());
+        m_visited = false;
+    }
 };
 
 //------------------------------------------------------------------------

src/coreclr/jit/compiler.cpp

@@ -5163,6 +5163,7 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
             auto lateLayoutPhase = [this] {
                 fgDoReversePostOrderLayout();
                 fgMoveColdBlocks();
+                fgSearchImprovedLayout();
 
                 if (compHndBBtabCount != 0)
                 {

src/coreclr/jit/compiler.h

@@ -38,6 +38,7 @@ XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
 #include "cycletimer.h"
 #include "blockset.h"
 #include "arraystack.h"
+#include "priorityqueue.h"
 #include "hashbv.h"
 #include "jitexpandarray.h"
 #include "valuenum.h"
@@ -6213,6 +6214,29 @@ class Compiler
     bool fgReorderBlocks(bool useProfile);
     void fgDoReversePostOrderLayout();
     void fgMoveColdBlocks();
+    void fgSearchImprovedLayout();
+
+    class ThreeOptLayout
+    {
+        static bool EdgeCmp(const FlowEdge* left, const FlowEdge* right);
+
+        Compiler* compiler;
+        PriorityQueue<FlowEdge*, decltype(&ThreeOptLayout::EdgeCmp)> cutPoints;
+        unsigned* ordinals;
+        BasicBlock** blockOrder;
+        BasicBlock** tempOrder;
+        unsigned numCandidateBlocks;
+        unsigned currEHRegion;
+
+        void ConsiderEdge(FlowEdge* edge);
+        void AddNonFallthroughSuccs(unsigned blockPos);
+        void AddNonFallthroughPreds(unsigned blockPos);
+        bool RunThreeOptPass(BasicBlock* startBlock, BasicBlock* endBlock);
+
+    public:
+        ThreeOptLayout(Compiler* comp);
+        void Run();
+    };
 
     template <bool hasEH>
     void fgMoveHotJumps();