JIT: Refine post-dominance check in strength reduction (#104687)

When strength reduction has to find an insertion point for the new
primary IV update it needs to find a block that post-dominates all the
users of the IV. This was using `optReachable` before, but that is
conservative since it finds paths that may exit the loop. This
implements a more precise check.

src/coreclr/jit/compiler.h

@@ -2232,6 +2232,8 @@ class FlowGraphNaturalLoop
 
     bool MayExecuteBlockMultipleTimesPerIteration(BasicBlock* block);
 
+    bool IsPostDominatedOnLoopIteration(BasicBlock* block, BasicBlock* postDominator);
+
 #ifdef DEBUG
     static void Dump(FlowGraphNaturalLoop* loop);
 #endif // DEBUG

src/coreclr/jit/flowgraph.cpp

@@ -5801,6 +5801,76 @@ bool FlowGraphNaturalLoop::MayExecuteBlockMultipleTimesPerIteration(BasicBlock*
     return false;
 }
 
+//------------------------------------------------------------------------
+// IsPostDominatedOnLoopIteration:
+//   Check whether control will always flow through "postDominator" if starting
+//   at "block" and a backedge is taken.
+//
+// Parameters:
+//   block         - The basic block
+//   postDominator - Block to query postdominance of
+//
+// Returns:
+//   True if so.
+//
+bool FlowGraphNaturalLoop::IsPostDominatedOnLoopIteration(BasicBlock* block, BasicBlock* postDominator)
+{
+    assert(ContainsBlock(block) && ContainsBlock(postDominator));
+
+    unsigned index;
+    bool     gotIndex = TryGetLoopBlockBitVecIndex(block, &index);
+    assert(gotIndex);
+
+    Compiler*               comp = m_dfsTree->GetCompiler();
+    ArrayStack<BasicBlock*> stack(comp->getAllocator(CMK_Loops));
+
+    BitVecTraits traits = LoopBlockTraits();
+    BitVec       visited(BitVecOps::MakeEmpty(&traits));
+
+    stack.Push(block);
+    BitVecOps::AddElemD(&traits, visited, index);
+
+    auto queueSuccs = [=, &stack, &traits, &visited](BasicBlock* succ) {
+        if (succ == m_header)
+        {
+            // We managed to reach the header without going through "postDominator".
+            return BasicBlockVisit::Abort;
+        }
+
+        unsigned index;
+        if (!TryGetLoopBlockBitVecIndex(succ, &index) || !BitVecOps::IsMember(&traits, m_blocks, index))
+        {
+            // Block is not inside loop
+            return BasicBlockVisit::Continue;
+        }
+
+        if (!BitVecOps::TryAddElemD(&traits, visited, index))
+        {
+            // Block already visited
+            return BasicBlockVisit::Continue;
+        }
+
+        stack.Push(succ);
+        return BasicBlockVisit::Continue;
+    };
+
+    while (stack.Height() > 0)
+    {
+        BasicBlock* block = stack.Pop();
+        if (block == postDominator)
+        {
+            continue;
+        }
+
+        if (block->VisitAllSuccs(comp, queueSuccs) == BasicBlockVisit::Abort)
+        {
+            return false;
+        }
+    }
+
+    return true;
+}
+
 //------------------------------------------------------------------------
 // IterConst: Get the constant with which the iterator is modified
 //

src/coreclr/jit/inductionvariableopts.cpp

@@ -1853,16 +1853,8 @@ BasicBlock* StrengthReductionContext::FindUpdateInsertionPoint(ArrayStack<Cursor
         }
         else
         {
-            if (m_comp->optReachable(cursor.Block, m_loop->GetHeader(), insertionPoint))
+            if (!m_loop->IsPostDominatedOnLoopIteration(cursor.Block, insertionPoint))
             {
-                // Header is reachable without going through the insertion
-                // point, meaning that the insertion point does not
-                // post-dominate the use of an IV we want to replace.
-                //
-                // TODO-CQ: We only need to check whether the header is
-                // reachable from inside the loop, which is both cheaper and
-                // less conservative to check.
-                //
                 return nullptr;
             }
         }

src/coreclr/jit/redundantbranchopts.cpp

@@ -2224,13 +2224,11 @@ bool Compiler::optReachable(BasicBlock* const fromBlock, BasicBlock* const toBlo
                 return BasicBlockVisit::Abort;
             }
 
-            if (BitVecOps::IsMember(optReachableBitVecTraits, optReachableBitVec, succ->bbNum))
+            if (!BitVecOps::TryAddElemD(optReachableBitVecTraits, optReachableBitVec, succ->bbNum))
             {
                 return BasicBlockVisit::Continue;
             }
 
-            BitVecOps::AddElemD(optReachableBitVecTraits, optReachableBitVec, succ->bbNum);
-
             stack.Push(succ);
             return BasicBlockVisit::Continue;
         });