Revert "[BranchFolding] Restrict tail merging loop blocks after MBP"

This reverts commit r278463 because it hits the bot.

llvm-svn: 278484

1 files changed, 9 insertions, 23 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index a2d304bb078..1a783b61276 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1005,24 +1005,6 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
     MachineBasicBlock *IBB = &*I;
     MachineBasicBlock *PredBB = &*std::prev(I);
     MergePotentials.clear();
-    MachineLoop *ML;
-
-    // Bail if merging after placement and IBB is the loop header because
-    // -- If merging predecessors that belong to the same loop as IBB, the
-    // common tail of merged predecessors may become the loop top if block
-    // placement is called again and the predecessors may branch to this common
-    // tail and require more branches. This can be relaxed if
-    // MachineBlockPlacement::findBestLoopTop is more flexible.
-    // --If merging predecessors that do not belong to the same loop as IBB, the
-    // loop info of IBB's loop and the other loops may be affected. Calling the
-    // block placement again may make big change to the layout and eliminate the
-    // reason to do tail merging here.
-    if (AfterBlockPlacement && MLI) {
-      ML = MLI->getLoopFor(IBB);
-      if (ML && IBB == ML->getHeader())
-        continue;
-    }
-
     for (MachineBasicBlock *PBB : I->predecessors()) {
       if (MergePotentials.size() == TailMergeThreshold)
         break;
@@ -1042,12 +1024,16 @@ bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
       if (PBB->hasEHPadSuccessor())
         continue;
 
-      // After block placement, only consider predecessors that belong to the
-      // same loop as IBB.  The reason is the same as above when skipping loop
-      // header.
+      // Bail out if the loop header (IBB) is not the top of the loop chain
+      // after the block placement.  Otherwise, the common tail of IBB's
+      // predecessors may become the loop top if block placement is called again
+      // and the predecessors may branch to this common tail.
+      // FIXME: Relaxed this check if the algorithm of finding loop top is
+      // changed in MBP.
       if (AfterBlockPlacement && MLI)
-        if (ML != MLI->getLoopFor(PBB))
-          continue;
+        if (MachineLoop *ML = MLI->getLoopFor(IBB))
+          if (IBB == ML->getHeader() && ML == MLI->getLoopFor(PBB))
+            continue;
 
       MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
       SmallVector<MachineOperand, 4> Cond;