TailDuplication: Record blocks that received the duplicated block. NFC.

[iteratee]

Summary

This will allow tail duplication during layout to handle the cfg changes more
cleanly.

[davidxl]

lib/CodeGen/TailDuplicator.cpp

139

.. = std::move(TDBBs);

Or do inplace update:

SmallVector<...> *TDBBP = (DuplicatePreds: DuplicatePreds : &TDBBs);
If (!tailDuplicate(IsSimple, MBB, *TDBBP, Copies);

[iteratee]

use std::move

[davidxl]

lgtm

[iteratee]

Committed in r279858

include/llvm/CodeGen/TailDuplicator.h

public:
   void initMF(MachineFunction &MF,
               const MachineBranchProbabilityInfo *MBPI,
               unsigned TailDupSize = 0);
   bool tailDuplicateBlocks();
   static bool isSimpleBB(MachineBasicBlock *TailBB);
   bool shouldTailDuplicate(bool IsSimple, MachineBasicBlock &TailBB);
   /// Returns true if TailBB can successfully be duplicated into PredBB
   bool canTailDuplicate(MachineBasicBlock *TailBB, MachineBasicBlock *PredBB);
-  bool tailDuplicateAndUpdate(bool IsSimple, MachineBasicBlock *MBB);
+  /// Tail duplicate a single basic block into its predecessors, and then clean
+  /// up.
+  /// If \p DuplicatePreds is not null, it will be updated to contain the list
+  /// of predecessors that received a copy of \p MBB.
+  bool tailDuplicateAndUpdate(
+      bool IsSimple, MachineBasicBlock *MBB,
+      SmallVectorImpl<MachineBasicBlock*> *DuplicatedPreds = nullptr);
 
 private:
   typedef TargetInstrInfo::RegSubRegPair RegSubRegPair;
 
   void addSSAUpdateEntry(unsigned OrigReg, unsigned NewReg,
                          MachineBasicBlock *BB);
   void processPHI(MachineInstr *MI, MachineBasicBlock *TailBB,
                   MachineBasicBlock *PredBB,

lib/CodeGen/TailDuplicator.cpp

while (MI != MBB->end()) {
         }
       }
       ++MI;
     }
   }
 }
 
 /// Tail duplicate the block and cleanup.
-bool TailDuplicator::tailDuplicateAndUpdate(bool IsSimple,
-                                            MachineBasicBlock *MBB) {
+/// \p IsSimple - return value of isSimpleBB
+/// \p MBB - block to be duplicated
+/// \p DuplicatedPreds - if non-null, \p DuplicatedPreds will contain a list of
+///     all Preds that received a copy of \p MBB.
+bool TailDuplicator::tailDuplicateAndUpdate(
+    bool IsSimple, MachineBasicBlock *MBB,
+    SmallVectorImpl<MachineBasicBlock*> *DuplicatedPreds) {
   // Save the successors list.
   SmallSetVector<MachineBasicBlock *, 8> Succs(MBB->succ_begin(),
                                                MBB->succ_end());
 
   SmallVector<MachineBasicBlock *, 8> TDBBs;
   SmallVector<MachineInstr *, 16> Copies;
   if (!tailDuplicate(IsSimple, MBB, TDBBs, Copies))
     return false;
 
+  if (DuplicatedPreds)
+    *DuplicatedPreds = TDBBs;
+
   ++NumTails;
 
   SmallVector<MachineInstr *, 8> NewPHIs;
   MachineSSAUpdater SSAUpdate(*MF, &NewPHIs);
 
   // TailBB's immediate successors are now successors of those predecessors
   // which duplicated TailBB. Add the predecessors as sources to the PHI
   // instructions.