JIT: Add FlowEdge destination block member (#98243)

Part of #93020. Per conversation in #98054, I'm going to try to replace BasicBlock's block successor pointers (bbTarget, bbFalseTarget, etc) with FlowEdge pointers to simplify access to successor edges. To do this, each edge is going to need access to its destination block, so that access to successor blocks is still simple. As a first step, add a destination block member to FlowEdge.

Author: amanasifkhalid

src/coreclr/jit/block.cpp

@@ -142,7 +142,7 @@ FlowEdge* Compiler::BlockPredsWithEH(BasicBlock* blk)
     {
         if (bbInExnFlowRegions(tryIndex, bb) && !bb->isBBCallFinallyPairTail())
         {
-            res = new (this, CMK_FlowEdge) FlowEdge(bb, res);
+            res = new (this, CMK_FlowEdge) FlowEdge(bb, blk, res);
 
 #if MEASURE_BLOCK_SIZE
             genFlowNodeCnt += 1;
@@ -164,7 +164,7 @@ FlowEdge* Compiler::BlockPredsWithEH(BasicBlock* blk)
                 for (BasicBlock* filterBlk = enclosingDsc->ebdFilter; filterBlk != enclosingDsc->ebdHndBeg;
                      filterBlk             = filterBlk->Next())
                 {
-                    res = new (this, CMK_FlowEdge) FlowEdge(filterBlk, res);
+                    res = new (this, CMK_FlowEdge) FlowEdge(filterBlk, blk, res);
 
                     assert(filterBlk->VisitEHEnclosedHandlerSecondPassSuccs(this, [blk](BasicBlock* succ) {
                         return succ == blk ? BasicBlockVisit::Abort : BasicBlockVisit::Continue;
@@ -239,7 +239,7 @@ FlowEdge* Compiler::BlockDominancePreds(BasicBlock* blk)
                 for (BasicBlock* filterBlk = enclosingDsc->ebdFilter; filterBlk != enclosingDsc->ebdHndBeg;
                      filterBlk             = filterBlk->Next())
                 {
-                    res = new (this, CMK_FlowEdge) FlowEdge(filterBlk, res);
+                    res = new (this, CMK_FlowEdge) FlowEdge(filterBlk, blk, res);
 
                     assert(filterBlk->VisitEHEnclosedHandlerSecondPassSuccs(this, [blk](BasicBlock* succ) {
                         return succ == blk ? BasicBlockVisit::Abort : BasicBlockVisit::Continue;

src/coreclr/jit/block.h

@@ -2059,6 +2059,9 @@ struct FlowEdge
     // The source of the control flow
     BasicBlock* m_sourceBlock;
 
+    // The destination of the control flow
+    BasicBlock* m_destBlock;
+
     // Edge weights
     weight_t m_edgeWeightMin;
     weight_t m_edgeWeightMax;
@@ -2074,9 +2077,10 @@ struct FlowEdge
     bool m_likelihoodSet;
 
 public:
-    FlowEdge(BasicBlock* block, FlowEdge* rest)
+    FlowEdge(BasicBlock* sourceBlock, BasicBlock* destBlock, FlowEdge* rest)
         : m_nextPredEdge(rest)
-        , m_sourceBlock(block)
+        , m_sourceBlock(sourceBlock)
+        , m_destBlock(destBlock)
         , m_edgeWeightMin(0)
         , m_edgeWeightMax(0)
         , m_likelihood(0)
@@ -2102,14 +2106,28 @@ struct FlowEdge
 
     BasicBlock* getSourceBlock() const
     {
+        assert(m_sourceBlock != nullptr);
         return m_sourceBlock;
     }
 
     void setSourceBlock(BasicBlock* newBlock)
     {
+        assert(newBlock != nullptr);
         m_sourceBlock = newBlock;
     }
 
+    BasicBlock* getDestinationBlock() const
+    {
+        assert(m_destBlock != nullptr);
+        return m_destBlock;
+    }
+
+    void setDestinationBlock(BasicBlock* newBlock)
+    {
+        assert(newBlock != nullptr);
+        m_destBlock = newBlock;
+    }
+
     weight_t edgeWeightMin() const
     {
         return m_edgeWeightMin;

src/coreclr/jit/fgdiagnostic.cpp

@@ -2683,6 +2683,10 @@ unsigned BBPredsChecker::CheckBBPreds(BasicBlock* block, unsigned curTraversalSt
         }
 
         assert(CheckJump(blockPred, block));
+
+        // Make sure the pred edge's destination block is correct
+        //
+        assert(pred->getDestinationBlock() == block);
     }
 
     // Make sure preds are in increasing BBnum order

src/coreclr/jit/fgflow.cpp

@@ -204,7 +204,7 @@ FlowEdge* Compiler::fgAddRefPred(BasicBlock* block, BasicBlock* blockPred, FlowE
 
         // Create new edge in the list in the correct ordered location.
         //
-        flow = new (this, CMK_FlowEdge) FlowEdge(blockPred, *listp);
+        flow = new (this, CMK_FlowEdge) FlowEdge(blockPred, block, *listp);
         flow->incrementDupCount();
         *listp = flow;
 

src/coreclr/jit/optimizer.cpp

@@ -196,7 +196,7 @@ void Compiler::optScaleLoopBlocks(BasicBlock* begBlk, BasicBlock* endBlk)
         // Is this a back edge?
         if (predBlock->bbNum >= begBlk->bbNum)
         {
-            backedgeList = new (this, CMK_FlowEdge) FlowEdge(predBlock, backedgeList);
+            backedgeList = new (this, CMK_FlowEdge) FlowEdge(predBlock, begBlk, backedgeList);
 
 #if MEASURE_BLOCK_SIZE
             genFlowNodeCnt += 1;