[llvm] [CFG] Add shortcut if CycleInfo is available (PR #188928)

[Robert Imschweiler via llvm-commits]

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@@ -235,10 +235,21 @@ static bool isReachableImpl(SmallVectorImpl<BasicBlock *> &Worklist,
     const Cycle *OuterC = nullptr;
     if (CI) {
       OuterC = CI->getTopLevelParentCycle(BB);
-      if (CyclesWithHoles.count(OuterC))
-        OuterC = nullptr;
-      else if (StopCycles.contains(OuterC))
-        return true;
+      if (OuterC) {
+        if (CyclesWithHoles.count(OuterC))
+          OuterC = nullptr;
+        else if (StopCycles.contains(OuterC))
+          return true;
+      } else {
+        // If BB is not part of a cycle, then it can't reach any block that
+        // dominates it. An exception is if the block is unreachable, as all
+        // reachable blocks dominate an unreachable block.
+        if (DT && DT->isReachableFromEntry(BB) &&
+            llvm::all_of(StopSet, [&](const BasicBlock *StopBB) {
+              return DT->dominates(StopBB, BB);
+            }))
+          continue;
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ro-i wrote:

doesn't `continue` mess with the `Limit` counter?

https://github.com/llvm/llvm-project/pull/188928