Reverts wrong modification to MachineBlockPlacement & BranchFolding; uses a new strat...

[oota-llvm.git] / lib / CodeGen / SelectionDAG / SelectionDAGISel.cpp

diff --git a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index c075da4738ad6e012b0b6ced51cb88385957d47f..1676e2dfe08fb477b6cb01df9bec642449a880d7 100644 (file)
--- a/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -645,6 +645,47 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
   // at this point.
   FuncInfo->clear();
 
+  // XXX-update: Right after instruction selection, check through the
+  // intentionally added fake conditional branches and mark them as unremovable.
+  for (auto& MBB : *MF) {
+    // Check whether MBB has two successors which only contains an unconditional
+    // branch to the same destination.
+    if (MBB.succ_size() != 2 ||
+        !MBB.getLastNonDebugInstr()->isUnconditionalBranch()) {
+      continue;
+    }
+    auto MBBSuccIter = MBB.succ_begin();
+    auto* Succ1 = *MBBSuccIter;
+    MBBSuccIter++;
+    auto* Succ2 = *MBBSuccIter;
+
+    MachineBasicBlock* Succ1Succ = nullptr;
+    MachineBasicBlock* Succ2Succ = nullptr;
+    if ((Succ1->size() == 1 && Succ1->begin()->isUnconditionalBranch()) ||
+        (Succ1->size() == 0)) {
+      Succ1Succ = *Succ1->succ_begin();
+    }
+    if ((Succ2->size() == 1 && Succ2->begin()->isUnconditionalBranch()) ||
+        (Succ2->size() == 0)) {
+      Succ2Succ = *Succ2->succ_begin();
+    }
+
+    bool HasCommonDest = Succ1Succ && Succ1Succ == Succ2Succ;
+    if (HasCommonDest) {
+      auto MBBIter = MBB.end();
+      std::advance(MBBIter, -2);
+      assert(MBBIter->isConditionalBranch());
+      MBBIter->disableCanEliminateMachineInstr();
+      MBB.disableCanEliminateMachineBB();
+      Succ1->disableCanEliminateMachineBB();
+      Succ2->disableCanEliminateMachineBB();
+      Succ1Succ->disableCanEliminateMachineBB();
+      DEBUG(dbgs() << "Mark as unremovable machine basic block: " << MBB
+                   << "\nMark as unremovable branch instruction: " << *MBBIter
+                   << "\n");
+    }
+  }
+
   DEBUG(dbgs() << "*** MachineFunction at end of ISel ***\n");
   DEBUG(MF->print(dbgs()));