Tweak the loop rotation logic to check whether the loop is naturally

laid out in a form with a fallthrough into the header and a fallthrough
out of the bottom. In that case, leave the loop alone because any
rotation will introduce unnecessary branches. If either side looks like
it will require an explicit branch, then the rotation won't add any, do
it to ensure the branch occurs outside of the loop (if possible) and
maximize the benefit of the fallthrough in the bottom.

llvm-svn: 154806

1 files changed, 51 insertions, 11 deletions

diff --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index 6226237..511a558 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -215,6 +215,8 @@ class MachineBlockPlacement : public MachineFunctionPass {
                                       MachineLoop &L,
                                       const BlockFilterSet &LoopBlockSet);
   void buildLoopChains(MachineFunction &F, MachineLoop &L);
+  void rotateLoop(BlockChain &LoopChain, MachineBasicBlock *ExitingBB,
+                  const BlockFilterSet &LoopBlockSet);
   void buildCFGChains(MachineFunction &F);
 
 public:
@@ -659,6 +661,54 @@ MachineBlockPlacement::findBestLoopExit(MachineFunction &F,
   return ExitingBB;
 }
 
+/// \brief Attempt to rotate an exiting block to the bottom of the loop.
+///
+/// Once we have built a chain, try to rotate it to line up the hot exit block
+/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
+/// branches. For example, if the loop has fallthrough into its header and out
+/// of its bottom already, don't rotate it.
+void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
+                                       MachineBasicBlock *ExitingBB,
+                                       const BlockFilterSet &LoopBlockSet) {
+  if (!ExitingBB)
+    return;
+
+  MachineBasicBlock *Top = *LoopChain.begin();
+  bool ViableTopFallthrough = false;
+  for (MachineBasicBlock::pred_iterator PI = Top->pred_begin(),
+                                        PE = Top->pred_end();
+       PI != PE; ++PI) {
+    BlockChain *PredChain = BlockToChain[*PI];
+    if (!LoopBlockSet.count(*PI) &&
+        (!PredChain || *PI == *llvm::prior(PredChain->end()))) {
+      ViableTopFallthrough = true;
+      break;
+    }
+  }
+
+  // If the header has viable fallthrough, check whether the current loop
+  // bottom is a viable exiting block. If so, bail out as rotating will
+  // introduce an unnecessary branch.
+  if (ViableTopFallthrough) {
+    MachineBasicBlock *Bottom = *llvm::prior(LoopChain.end());
+    for (MachineBasicBlock::succ_iterator SI = Bottom->succ_begin(),
+                                          SE = Bottom->succ_end();
+         SI != SE; ++SI) {
+      BlockChain *SuccChain = BlockToChain[*SI];
+      if (!LoopBlockSet.count(*SI) &&
+          (!SuccChain || *SI == *SuccChain->begin()))
+        return;
+    }
+  }
+
+  BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
+                                          ExitingBB);
+  if (ExitIt == LoopChain.end())
+    return;
+
+  std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
+}
+
 /// \brief Forms basic block chains from the natural loop structures.
 ///
 /// These chains are designed to preserve the existing *structure* of the code
@@ -709,17 +759,7 @@ void MachineBlockPlacement::buildLoopChains(MachineFunction &F,
   }
 
   buildChain(L.getHeader(), LoopChain, BlockWorkList, &LoopBlockSet);
-
-  // Once we have built a chain, try to rotate it to line up the hot exit block
-  // with fallthrough out of the loop (if we have a valid exit block for that).
-  if (ExitingBB) {
-    BlockChain::iterator ExitIt = std::find(LoopChain.begin(), LoopChain.end(),
-                                            ExitingBB);
-
-    if (ExitIt != LoopChain.end()) {
-      std::rotate(LoopChain.begin(), llvm::next(ExitIt), LoopChain.end());
-    }
-  }
+  rotateLoop(LoopChain, ExitingBB, LoopBlockSet);
 
   DEBUG({
     // Crash at the end so we get all of the debugging output first.