JIT: Limit 3-opt to 1000 swaps per run (dotnet#112259)

Fixes dotnet#111988.

Author: amanasifkhalid

src/coreclr/jit/compiler.h

@@ -6352,6 +6352,7 @@ class Compiler
     class ThreeOptLayout
     {
         static bool EdgeCmp(const FlowEdge* left, const FlowEdge* right);
+        static constexpr unsigned maxSwaps = 1000;
 
         Compiler* compiler;
         PriorityQueue<FlowEdge*, decltype(&ThreeOptLayout::EdgeCmp)> cutPoints;

src/coreclr/jit/fgopt.cpp

@@ -5343,7 +5343,8 @@ bool Compiler::ThreeOptLayout::RunGreedyThreeOptPass(unsigned startPos, unsigned
     // and before the destination block, and swap the partitions to create fallthrough.
     // If it is, do the swap, and for the blocks before/after each cut point that lost fallthrough,
     // consider adding their successors/predecessors to 'cutPoints'.
-    while (!cutPoints.Empty())
+    unsigned numSwaps = 0;
+    while (!cutPoints.Empty() && (numSwaps < maxSwaps))
     {
         FlowEdge* const candidateEdge = cutPoints.Pop();
         candidateEdge->markUnvisited();
@@ -5498,8 +5499,10 @@ bool Compiler::ThreeOptLayout::RunGreedyThreeOptPass(unsigned startPos, unsigned
         }
 
         modified = true;
+        numSwaps++;
     }
 
+    cutPoints.Clear();
     return modified;
 }
 

src/coreclr/jit/priorityqueue.h

@@ -58,6 +58,11 @@ class PriorityQueue
         return data.empty();
     }
 
+    void Clear()
+    {
+        data.clear();
+    }
+
     // Insert new element at the back of the vector.
     // Then, while the new element has a higher priority than its parent, move the element up.
     void Push(const T& value)