1 files changed, 29 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 2d4b95974cc..c8f8dda0c2d 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -1944,6 +1944,35 @@ void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
   if (ExitIt == LoopChain.end())
     return;
 
+  // Rotating a loop exit to the bottom when there is a fallthrough to top
+  // trades the entry fallthrough for an exit fallthrough.
+  // If there is no bottom->top edge, but the chosen exit block does have
+  // a fallthrough, we break that fallthrough for nothing in return.
+
+  // Let's consider an example. We have a built chain of basic blocks
+  // B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block.
+  // By doing a rotation we get
+  // Bk+1, ..., Bn, B1, ..., Bk
+  // Break of fallthrough to B1 is compensated by a fallthrough from Bk.
+  // If we had a fallthrough Bk -> Bk+1 it is broken now.
+  // It might be compensated by fallthrough Bn -> B1.
+  // So we have a condition to avoid creation of extra branch by loop rotation.
+  // All below must be true to avoid loop rotation:
+  //   If there is a fallthrough to top (B1)
+  //   There was fallthrough from chosen exit block (Bk) to next one (Bk+1)
+  //   There is no fallthrough from bottom (Bn) to top (B1).
+  // Please note that there is no exit fallthrough from Bn because we checked it
+  // above.
+  if (ViableTopFallthrough) {
+    MachineBasicBlock *NextBlockInChain = *std::next(ExitIt);
+    MachineBasicBlock *Bottom = *std::prev(LoopChain.end());
+    if (NextBlockInChain->isPredecessor(ExitingBB))
+      if (!Top->isPredecessor(Bottom))
+        return;
+  }
+
+  DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB)
+               << " at bottom\n");
   std::rotate(LoopChain.begin(), std::next(ExitIt), LoopChain.end());
 }