JIT: Run new block layout only in backend (#107634)

Author: amanasifkhalid

src/coreclr/jit/compiler.cpp

@@ -5235,8 +5235,7 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     m_pLowering->FinalizeOutgoingArgSpace();
 
     // We can not add any new tracked variables after this point.
-    lvaTrackedFixed                    = true;
-    const unsigned numBlocksBeforeLSRA = fgBBcount;
+    lvaTrackedFixed = true;
 
     // Now that lowering is completed we can proceed to perform register allocation
     //
@@ -5250,8 +5249,10 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 
     if (opts.OptimizationEnabled())
     {
-        // LSRA may introduce new blocks. If it does, rerun layout.
-        if ((fgBBcount != numBlocksBeforeLSRA) && JitConfig.JitDoReversePostOrderLayout())
+        // We won't introduce new blocks from here on out,
+        // so run the new block layout.
+        //
+        if (JitConfig.JitDoReversePostOrderLayout())
         {
             auto lateLayoutPhase = [this] {
                 fgDoReversePostOrderLayout();
@@ -5272,7 +5273,12 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
         DoPhase(this, PHASE_DETERMINE_FIRST_COLD_BLOCK, &Compiler::fgDetermineFirstColdBlock);
 
         // Now that the flowgraph is finalized, run post-layout optimizations.
+        //
         DoPhase(this, PHASE_OPTIMIZE_POST_LAYOUT, &Compiler::optOptimizePostLayout);
+
+        // Determine start of cold region if we are hot/cold splitting
+        //
+        DoPhase(this, PHASE_DETERMINE_FIRST_COLD_BLOCK, &Compiler::fgDetermineFirstColdBlock);
     }
 
 #if FEATURE_LOOP_ALIGN

src/coreclr/jit/fgopt.cpp

@@ -3341,22 +3341,12 @@ bool Compiler::fgReorderBlocks(bool useProfile)
 
     if (useProfile)
     {
+        // Don't run the new layout until we get to the backend,
+        // since LSRA can introduce new blocks, and lowering can churn the flowgraph.
+        //
         if (JitConfig.JitDoReversePostOrderLayout())
         {
-            fgDoReversePostOrderLayout();
-            fgMoveColdBlocks();
-
-            if (compHndBBtabCount != 0)
-            {
-                fgRebuildEHRegions();
-            }
-
-            // Renumber blocks to facilitate LSRA's order of block visitation
-            // TODO: Consider removing this, and using traversal order in lSRA
-            //
-            fgRenumberBlocks();
-
-            return true;
+            return (newRarelyRun || movedBlocks || optimizedSwitches);
         }
 
         // We will be reordering blocks, so ensure the false target of a BBJ_COND block is its next block