Reuse DFS tree from LSRA through layout

Author: amanasifkhalid

src/coreclr/jit/fgopt.cpp

@@ -4663,10 +4663,6 @@ void Compiler::fgDoReversePostOrderLayout()
     }
 #endif // DEBUG
 
-    // Compute DFS of all blocks in the method, using profile data to determine the order successors are visited in.
-    //
-    m_dfsTree = fgComputeDfs</* useProfile */ true>();
-
     // If LSRA didn't create any new blocks, we can reuse its loop-aware RPO traversal,
     // which is cached in Compiler::fgBBs.
     // If the cache isn't available, we need to recompute the loop-aware RPO.
@@ -4675,15 +4671,21 @@ void Compiler::fgDoReversePostOrderLayout()
 
     if (rpoSequence == nullptr)
     {
-        rpoSequence                        = new (this, CMK_BasicBlock) BasicBlock*[m_dfsTree->GetPostOrderCount()];
+        assert(m_dfsTree == nullptr);
+        m_dfsTree                          = fgComputeDfs</* useProfile */ true>();
         FlowGraphNaturalLoops* const loops = FlowGraphNaturalLoops::Find(m_dfsTree);
-        unsigned                     index = 0;
-        auto                         addToSequence = [rpoSequence, &index](BasicBlock* block) {
+        rpoSequence                        = new (this, CMK_BasicBlock) BasicBlock*[m_dfsTree->GetPostOrderCount()];
+        unsigned index                     = 0;
+        auto     addToSequence             = [rpoSequence, &index](BasicBlock* block) {
             rpoSequence[index++] = block;
         };
 
         fgVisitBlocksInLoopAwareRPO(m_dfsTree, loops, addToSequence);
     }
+    else
+    {
+        assert(m_dfsTree != nullptr);
+    }
 
     // Fast path: We don't have any EH regions, so just reorder the blocks
     //

src/coreclr/jit/lsra.cpp

@@ -947,13 +947,16 @@ void LinearScan::setBlockSequence()
     bbVisitedSet = BitVecOps::MakeEmpty(traits);
 
     assert((blockSequence == nullptr) && (bbSeqCount == 0));
-    FlowGraphDfsTree* const dfsTree = compiler->fgComputeDfs</* useProfile */ true>();
+
+    compiler->m_dfsTree             = compiler->fgComputeDfs</* useProfile */ true>();
+    FlowGraphDfsTree* const dfsTree = compiler->m_dfsTree;
     blockSequence                   = new (compiler, CMK_LSRA) BasicBlock*[compiler->fgBBcount];
 
     if (compiler->opts.OptimizationEnabled() && dfsTree->HasCycle())
     {
-        // Ensure loop bodies are compact in the visitation order
-        FlowGraphNaturalLoops* const loops = FlowGraphNaturalLoops::Find(dfsTree);
+        // Ensure loop bodies are compact in the visitation order.
+        compiler->m_loops                  = FlowGraphNaturalLoops::Find(dfsTree);
+        FlowGraphNaturalLoops* const loops = compiler->m_loops;
         unsigned                     index = 0;
 
         auto addToSequence = [this, &index](BasicBlock* block) {
@@ -1319,6 +1322,7 @@ PhaseStatus LinearScan::doLinearScan()
     {
         assert(compiler->fgBBcount > bbSeqCount);
         compiler->fgBBs = nullptr;
+        compiler->fgInvalidateDfsTree();
     }
 
     return PhaseStatus::MODIFIED_EVERYTHING;