AArch64: add branch fixup pass.

This is essentially a stripped-down version of the ConstandIslands pass (which
always had these two functions), providing just the features necessary for
correctness.

In particular there needs to be a way to resolve the situation where a
conditional branch's destination block ends up out of range.

This issue crops up when self-hosting for AArch64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175269 91177308-0d34-0410-b5e6-96231b3b80d8

6 files changed, 652 insertions, 0 deletions

diff --git a/lib/Target/AArch64/AArch64.h b/lib/Target/AArch64/AArch64.h
index a97aae7..4de4faa 100644
--- a/lib/Target/AArch64/AArch64.h
+++ b/lib/Target/AArch64/AArch64.h
@@ -31,6 +31,8 @@ FunctionPass *createAArch64ISelDAG(AArch64TargetMachine &TM,
 
 FunctionPass *createAArch64CleanupLocalDynamicTLSPass();
 
+FunctionPass *createAArch64BranchFixupPass();
+
 void LowerAArch64MachineInstrToMCInst(const MachineInstr *MI, MCInst &OutMI,
                                       AArch64AsmPrinter &AP);
 
diff --git a/lib/Target/AArch64/AArch64BranchFixupPass.cpp b/lib/Target/AArch64/AArch64BranchFixupPass.cpp
new file mode 100644
index 0000000..71233ba
--- /dev/null
+++ b/lib/Target/AArch64/AArch64BranchFixupPass.cpp
@@ -0,0 +1,600 @@
+//===-- AArch64BranchFixupPass.cpp - AArch64 branch fixup -----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains a pass that fixes AArch64 branches which have ended up out
+// of range for their immediate operands.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "aarch64-branch-fixup"
+#include "AArch64.h"
+#include "AArch64InstrInfo.h"
+#include "Utils/AArch64BaseInfo.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/Format.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/Statistic.h"
+using namespace llvm;
+
+STATISTIC(NumSplit,      "Number of uncond branches inserted");
+STATISTIC(NumCBrFixed,   "Number of cond branches fixed");
+
+/// Return the worst case padding that could result from unknown offset bits.
+/// This does not include alignment padding caused by known offset bits.
+///
+/// @param LogAlign log2(alignment)
+/// @param KnownBits Number of known low offset bits.
+static inline unsigned UnknownPadding(unsigned LogAlign, unsigned KnownBits) {
+  if (KnownBits < LogAlign)
+    return (1u << LogAlign) - (1u << KnownBits);
+  return 0;
+}
+
+namespace {
+  /// Due to limited PC-relative displacements, conditional branches to distant
+  /// blocks may need converting into an unconditional equivalent. For example:
+  ///     tbz w1, #0, far_away
+  /// becomes
+  ///     tbnz w1, #0, skip
+  ///     b far_away
+  ///   skip:
+  class AArch64BranchFixup : public MachineFunctionPass {
+    /// Information about the offset and size of a single basic block.
+    struct BasicBlockInfo {
+      /// Distance from the beginning of the function to the beginning of this
+      /// basic block.
+      ///
+      /// Offsets are computed assuming worst case padding before an aligned
+      /// block. This means that subtracting basic block offsets always gives a
+      /// conservative estimate of the real distance which may be smaller.
+      ///
+      /// Because worst case padding is used, the computed offset of an aligned
+      /// block may not actually be aligned.
+      unsigned Offset;
+
+      /// Size of the basic block in bytes.  If the block contains inline
+      /// assembly, this is a worst case estimate.
+      ///
+      /// The size does not include any alignment padding whether from the
+      /// beginning of the block, or from an aligned jump table at the end.
+      unsigned Size;
+
+      /// The number of low bits in Offset that are known to be exact.  The
+      /// remaining bits of Offset are an upper bound.
+      uint8_t KnownBits;
+
+      /// When non-zero, the block contains instructions (inline asm) of unknown
+      /// size.  The real size may be smaller than Size bytes by a multiple of 1
+      /// << Unalign.
+      uint8_t Unalign;
+
+      BasicBlockInfo() : Offset(0), Size(0), KnownBits(0), Unalign(0) {}
+
+      /// Compute the number of known offset bits internally to this block.
+      /// This number should be used to predict worst case padding when
+      /// splitting the block.
+      unsigned internalKnownBits() const {
+        unsigned Bits = Unalign ? Unalign : KnownBits;
+        // If the block size isn't a multiple of the known bits, assume the
+        // worst case padding.
+        if (Size & ((1u << Bits) - 1))
+          Bits = CountTrailingZeros_32(Size);
+        return Bits;
+      }
+
+      /// Compute the offset immediately following this block.  If LogAlign is
+      /// specified, return the offset the successor block will get if it has
+      /// this alignment.
+      unsigned postOffset(unsigned LogAlign = 0) const {
+        unsigned PO = Offset + Size;
+        if (!LogAlign)
+          return PO;
+        // Add alignment padding from the terminator.
+        return PO + UnknownPadding(LogAlign, internalKnownBits());
+      }
+
+      /// Compute the number of known low bits of postOffset.  If this block
+      /// contains inline asm, the number of known bits drops to the
+      /// instruction alignment.  An aligned terminator may increase the number
+      /// of know bits.
+      /// If LogAlign is given, also consider the alignment of the next block.
+      unsigned postKnownBits(unsigned LogAlign = 0) const {
+        return std::max(LogAlign, internalKnownBits());
+      }
+    };
+
+    std::vector<BasicBlockInfo> BBInfo;
+
+    /// One per immediate branch, keeping the machine instruction pointer,
+    /// conditional or unconditional, the max displacement, and (if IsCond is
+    /// true) the corresponding inverted branch opcode.
+    struct ImmBranch {
+      MachineInstr *MI;
+      unsigned OffsetBits : 31;
+      bool IsCond : 1;
+      ImmBranch(MachineInstr *mi, unsigned offsetbits, bool cond)
+        : MI(mi), OffsetBits(offsetbits), IsCond(cond) {}
+    };
+
+    /// Keep track of all the immediate branch instructions.
+    ///
+    std::vector<ImmBranch> ImmBranches;
+
+    MachineFunction *MF;
+    const AArch64InstrInfo *TII;
+  public:
+    static char ID;
+    AArch64BranchFixup() : MachineFunctionPass(ID) {}
+
+    virtual bool runOnMachineFunction(MachineFunction &MF);
+
+    virtual const char *getPassName() const {
+      return "AArch64 branch fixup pass";
+    }
+
+  private:
+    void initializeFunctionInfo();
+    MachineBasicBlock *splitBlockBeforeInstr(MachineInstr *MI);
+    void adjustBBOffsetsAfter(MachineBasicBlock *BB);
+    bool isBBInRange(MachineInstr *MI, MachineBasicBlock *BB,
+                     unsigned OffsetBits);
+    bool fixupImmediateBr(ImmBranch &Br);
+    bool fixupConditionalBr(ImmBranch &Br);
+
+    void computeBlockSize(MachineBasicBlock *MBB);
+    unsigned getOffsetOf(MachineInstr *MI) const;
+    void dumpBBs();
+    void verify();
+  };
+  char AArch64BranchFixup::ID = 0;
+}
+
+/// check BBOffsets
+void AArch64BranchFixup::verify() {
+#ifndef NDEBUG
+  for (MachineFunction::iterator MBBI = MF->begin(), E = MF->end();
+       MBBI != E; ++MBBI) {
+    MachineBasicBlock *MBB = MBBI;
+    unsigned MBBId = MBB->getNumber();
+    assert(!MBBId || BBInfo[MBBId - 1].postOffset() <= BBInfo[MBBId].Offset);
+  }
+#endif
+}
+
+/// print block size and offset information - debugging
+void AArch64BranchFixup::dumpBBs() {
+  DEBUG({
+    for (unsigned J = 0, E = BBInfo.size(); J !=E; ++J) {
+      const BasicBlockInfo &BBI = BBInfo[J];
+      dbgs() << format("%08x BB#%u\t", BBI.Offset, J)
+             << " kb=" << unsigned(BBI.KnownBits)
+             << " ua=" << unsigned(BBI.Unalign)
+             << format(" size=%#x\n", BBInfo[J].Size);
+    }
+  });
+}
+
+/// Returns an instance of the branch fixup pass.
+FunctionPass *llvm::createAArch64BranchFixupPass() {
+  return new AArch64BranchFixup();
+}
+
+bool AArch64BranchFixup::runOnMachineFunction(MachineFunction &mf) {
+  MF = &mf;
+  DEBUG(dbgs() << "***** AArch64BranchFixup ******");
+  TII = (const AArch64InstrInfo*)MF->getTarget().getInstrInfo();
+
+  // This pass invalidates liveness information when it splits basic blocks.
+  MF->getRegInfo().invalidateLiveness();
+
+  // Renumber all of the machine basic blocks in the function, guaranteeing that
+  // the numbers agree with the position of the block in the function.
+  MF->RenumberBlocks();
+
+  // Do the initial scan of the function, building up information about the
+  // sizes of each block and location of each immediate branch.
+  initializeFunctionInfo();
+
+  // Iteratively fix up branches until there is no change.
+  unsigned NoBRIters = 0;
+  bool MadeChange = false;
+  while (true) {
+    DEBUG(dbgs() << "Beginning iteration #" << NoBRIters << '\n');
+    bool BRChange = false;
+    for (unsigned i = 0, e = ImmBranches.size(); i != e; ++i)
+      BRChange |= fixupImmediateBr(ImmBranches[i]);
+    if (BRChange && ++NoBRIters > 30)
+      report_fatal_error("Branch Fix Up pass failed to converge!");
+    DEBUG(dumpBBs());
+
+    if (!BRChange)
+      break;
+    MadeChange = true;
+  }
+
+  // After a while, this might be made debug-only, but it is not expensive.
+  verify();
+
+  DEBUG(dbgs() << '\n'; dumpBBs());
+
+  BBInfo.clear();
+  ImmBranches.clear();
+
+  return MadeChange;
+}
+
+/// Return true if the specified basic block can fallthrough into the block
+/// immediately after it.
+static bool BBHasFallthrough(MachineBasicBlock *MBB) {
+  // Get the next machine basic block in the function.
+  MachineFunction::iterator MBBI = MBB;
+  // Can't fall off end of function.
+  if (llvm::next(MBBI) == MBB->getParent()->end())
+    return false;
+
+  MachineBasicBlock *NextBB = llvm::next(MBBI);
+  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(),
+       E = MBB->succ_end(); I != E; ++I)
+    if (*I == NextBB)
+      return true;
+
+  return false;
+}
+
+/// Do the initial scan of the function, building up information about the sizes
+/// of each block, and each immediate branch.
+void AArch64BranchFixup::initializeFunctionInfo() {
+  BBInfo.clear();
+  BBInfo.resize(MF->getNumBlockIDs());
+
+  // First thing, compute the size of all basic blocks, and see if the function
+  // has any inline assembly in it. If so, we have to be conservative about
+  // alignment assumptions, as we don't know for sure the size of any
+  // instructions in the inline assembly.
+  for (MachineFunction::iterator I = MF->begin(), E = MF->end(); I != E; ++I)
+    computeBlockSize(I);
+
+  // The known bits of the entry block offset are determined by the function
+  // alignment.
+  BBInfo.front().KnownBits = MF->getAlignment();
+
+  // Compute block offsets and known bits.
+  adjustBBOffsetsAfter(MF->begin());
+
+  // Now go back through the instructions and build up our data structures.
+  for (MachineFunction::iterator MBBI = MF->begin(), E = MF->end();
+       MBBI != E; ++MBBI) {
+    MachineBasicBlock &MBB = *MBBI;
+
+    for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
+         I != E; ++I) {
+      if (I->isDebugValue())
+        continue;
+
+      int Opc = I->getOpcode();
+      if (I->isBranch()) {
+        bool IsCond = false;
+
+        // The offsets encoded in instructions here scale by the instruction
+        // size (4 bytes), effectively increasing their range by 2 bits.
+        unsigned Bits = 0;
+        switch (Opc) {
+        default:
+          continue;  // Ignore other JT branches
+        case AArch64::TBZxii:
+        case AArch64::TBZwii:
+        case AArch64::TBNZxii:
+        case AArch64::TBNZwii:
+          IsCond = true;
+          Bits = 14 + 2;
+          break;
+        case AArch64::Bcc:
+        case AArch64::CBZx:
+        case AArch64::CBZw:
+        case AArch64::CBNZx:
+        case AArch64::CBNZw:
+          IsCond = true;
+          Bits = 19 + 2;
+          break;
+        case AArch64::Bimm:
+          Bits = 26 + 2;
+          break;
+        }
+
+        // Record this immediate branch.
+        ImmBranches.push_back(ImmBranch(I, Bits, IsCond));
+      }
+    }
+  }
+}
+
+/// Compute the size and some alignment information for MBB.  This function
+/// updates BBInfo directly.
+void AArch64BranchFixup::computeBlockSize(MachineBasicBlock *MBB) {
+  BasicBlockInfo &BBI = BBInfo[MBB->getNumber()];
+  BBI.Size = 0;
+  BBI.Unalign = 0;
+
+  for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;
+       ++I) {
+    BBI.Size += TII->getInstSizeInBytes(*I);
+    // For inline asm, GetInstSizeInBytes returns a conservative estimate.
+    // The actual size may be smaller, but still a multiple of the instr size.
+    if (I->isInlineAsm())
+      BBI.Unalign = 2;
+  }
+}
+
+/// Return the current offset of the specified machine instruction from the
+/// start of the function.  This offset changes as stuff is moved around inside
+/// the function.
+unsigned AArch64BranchFixup::getOffsetOf(MachineInstr *MI) const {
+  MachineBasicBlock *MBB = MI->getParent();
+
+  // The offset is composed of two things: the sum of the sizes of all MBB's
+  // before this instruction's block, and the offset from the start of the block
+  // it is in.
+  unsigned Offset = BBInfo[MBB->getNumber()].Offset;
+
+  // Sum instructions before MI in MBB.
+  for (MachineBasicBlock::iterator I = MBB->begin(); &*I != MI; ++I) {
+    assert(I != MBB->end() && "Didn't find MI in its own basic block?");
+    Offset += TII->getInstSizeInBytes(*I);
+  }
+  return Offset;
+}
+
+/// Split the basic block containing MI into two blocks, which are joined by
+/// an unconditional branch.  Update data structures and renumber blocks to
+/// account for this change and returns the newly created block.
+MachineBasicBlock *
+AArch64BranchFixup::splitBlockBeforeInstr(MachineInstr *MI) {
+  MachineBasicBlock *OrigBB = MI->getParent();
+
+  // Create a new MBB for the code after the OrigBB.
+  MachineBasicBlock *NewBB =
+    MF->CreateMachineBasicBlock(OrigBB->getBasicBlock());
+  MachineFunction::iterator MBBI = OrigBB; ++MBBI;
+  MF->insert(MBBI, NewBB);
+
+  // Splice the instructions starting with MI over to NewBB.
+  NewBB->splice(NewBB->end(), OrigBB, MI, OrigBB->end());
+
+  // Add an unconditional branch from OrigBB to NewBB.
+  // Note the new unconditional branch is not being recorded.
+  // There doesn't seem to be meaningful DebugInfo available; this doesn't
+  // correspond to anything in the source.
+  BuildMI(OrigBB, DebugLoc(), TII->get(AArch64::Bimm)).addMBB(NewBB);
+  ++NumSplit;
+
+  // Update the CFG.  All succs of OrigBB are now succs of NewBB.
+  NewBB->transferSuccessors(OrigBB);
+
+  // OrigBB branches to NewBB.
+  OrigBB->addSuccessor(NewBB);
+
+  // Update internal data structures to account for the newly inserted MBB.
+  MF->RenumberBlocks(NewBB);
+
+  // Insert an entry into BBInfo to align it properly with the (newly
+  // renumbered) block numbers.
+  BBInfo.insert(BBInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
+
+  // Figure out how large the OrigBB is.  As the first half of the original
+  // block, it cannot contain a tablejump.  The size includes
+  // the new jump we added.  (It should be possible to do this without
+  // recounting everything, but it's very confusing, and this is rarely
+  // executed.)
+  computeBlockSize(OrigBB);
+
+  // Figure out how large the NewMBB is.  As the second half of the original
+  // block, it may contain a tablejump.
+  computeBlockSize(NewBB);
+
+  // All BBOffsets following these blocks must be modified.
+  adjustBBOffsetsAfter(OrigBB);
+
+  return NewBB;
+}
+
+void AArch64BranchFixup::adjustBBOffsetsAfter(MachineBasicBlock *BB) {
+  unsigned BBNum = BB->getNumber();
+  for(unsigned i = BBNum + 1, e = MF->getNumBlockIDs(); i < e; ++i) {
+    // Get the offset and known bits at the end of the layout predecessor.
+    // Include the alignment of the current block.
+    unsigned LogAlign = MF->getBlockNumbered(i)->getAlignment();
+    unsigned Offset = BBInfo[i - 1].postOffset(LogAlign);
+    unsigned KnownBits = BBInfo[i - 1].postKnownBits(LogAlign);
+
+    // This is where block i begins.  Stop if the offset is already correct,
+    // and we have updated 2 blocks.  This is the maximum number of blocks
+    // changed before calling this function.
+    if (i > BBNum + 2 &&
+        BBInfo[i].Offset == Offset &&
+        BBInfo[i].KnownBits == KnownBits)
+      break;
+
+    BBInfo[i].Offset = Offset;
+    BBInfo[i].KnownBits = KnownBits;
+  }
+}
+
+/// Returns true if the distance between specific MI and specific BB can fit in
+/// MI's displacement field.
+bool AArch64BranchFixup::isBBInRange(MachineInstr *MI,
+                                     MachineBasicBlock *DestBB,
+                                     unsigned OffsetBits) {
+  int64_t BrOffset   = getOffsetOf(MI);
+  int64_t DestOffset = BBInfo[DestBB->getNumber()].Offset;
+
+  DEBUG(dbgs() << "Branch of destination BB#" << DestBB->getNumber()
+               << " from BB#" << MI->getParent()->getNumber()
+               << " bits available=" << OffsetBits
+               << " from " << getOffsetOf(MI) << " to " << DestOffset
+               << " offset " << int(DestOffset-BrOffset) << "\t" << *MI);
+
+  return isIntN(OffsetBits, DestOffset - BrOffset);
+}
+
+/// Fix up an immediate branch whose destination is too far away to fit in its
+/// displacement field.
+bool AArch64BranchFixup::fixupImmediateBr(ImmBranch &Br) {
+  MachineInstr *MI = Br.MI;
+  MachineBasicBlock *DestBB = 0;
+  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+    if (MI->getOperand(i).isMBB()) {
+      DestBB = MI->getOperand(i).getMBB();
+      break;
+    }
+  }
+  assert(DestBB && "Branch with no destination BB?");
+
+  // Check to see if the DestBB is already in-range.
+  if (isBBInRange(MI, DestBB, Br.OffsetBits))
+    return false;
+
+  assert(Br.IsCond && "Only conditional branches should need fixup");
+  return fixupConditionalBr(Br);
+}
+
+/// Fix up a conditional branch whose destination is too far away to fit in its
+/// displacement field. It is converted to an inverse conditional branch + an
+/// unconditional branch to the destination.
+bool
+AArch64BranchFixup::fixupConditionalBr(ImmBranch &Br) {
+  MachineInstr *MI = Br.MI;
+  MachineBasicBlock *MBB = MI->getParent();
+  unsigned CondBrMBBOperand = 0;
+
+  // The general idea is to add an unconditional branch to the destination and
+  // invert the conditional branch to jump over it. Complications occur around
+  // fallthrough and unreachable ends to the block.
+  //   b.lt L1
+  //   =>
+  //   b.ge L2
+  //   b   L1
+  // L2:
+
+  // First we invert the conditional branch, by creating a replacement if
+  // necessary. This if statement contains all the special handling of different
+  // branch types.
+  if (MI->getOpcode() == AArch64::Bcc) {
+    // The basic block is operand number 1 for Bcc
+    CondBrMBBOperand = 1;
+
+    A64CC::CondCodes CC = (A64CC::CondCodes)MI->getOperand(0).getImm();
+    CC = A64InvertCondCode(CC);
+    MI->getOperand(0).setImm(CC);
+  } else {
+    MachineInstrBuilder InvertedMI;
+    int InvertedOpcode;
+    switch (MI->getOpcode()) {
+    default: llvm_unreachable("Unknown branch type");
+    case AArch64::TBZxii: InvertedOpcode = AArch64::TBNZxii; break;
+    case AArch64::TBZwii: InvertedOpcode = AArch64::TBNZwii; break;
+    case AArch64::TBNZxii: InvertedOpcode = AArch64::TBZxii; break;
+    case AArch64::TBNZwii: InvertedOpcode = AArch64::TBZwii; break;
+    case AArch64::CBZx: InvertedOpcode = AArch64::CBNZx; break;
+    case AArch64::CBZw: InvertedOpcode = AArch64::CBNZw; break;
+    case AArch64::CBNZx: InvertedOpcode = AArch64::CBZx; break;
+    case AArch64::CBNZw: InvertedOpcode = AArch64::CBZw; break;
+    }
+
+    InvertedMI = BuildMI(*MBB, MI, MI->getDebugLoc(), TII->get(InvertedOpcode));
+    for (unsigned i = 0, e= MI->getNumOperands(); i != e; ++i) {
+      InvertedMI.addOperand(MI->getOperand(i));
+      if (MI->getOperand(i).isMBB())
+        CondBrMBBOperand = i;
+    }
+
+    MI->eraseFromParent();
+    MI = Br.MI = InvertedMI;
+  }
+
+  // If the branch is at the end of its MBB and that has a fall-through block,
+  // direct the updated conditional branch to the fall-through
+  // block. Otherwise, split the MBB before the next instruction.
+  MachineInstr *BMI = &MBB->back();
+  bool NeedSplit = (BMI != MI) || !BBHasFallthrough(MBB);
+
+  ++NumCBrFixed;
+  if (BMI != MI) {
+    if (llvm::next(MachineBasicBlock::iterator(MI)) == prior(MBB->end()) &&
+        BMI->getOpcode() == AArch64::Bimm) {
+      // Last MI in the BB is an unconditional branch. We can swap destinations:
+      // b.eq L1 (temporarily b.ne L1 after first change)
+      // b   L2
+      // =>
+      // b.ne L2
+      // b   L1
+      MachineBasicBlock *NewDest = BMI->getOperand(0).getMBB();
+      if (isBBInRange(MI, NewDest, Br.OffsetBits)) {
+        DEBUG(dbgs() << "  Invert Bcc condition and swap its destination with "
+                     << *BMI);
+        MachineBasicBlock *DestBB = MI->getOperand(CondBrMBBOperand).getMBB();
+        BMI->getOperand(0).setMBB(DestBB);
+        MI->getOperand(CondBrMBBOperand).setMBB(NewDest);
+        return true;
+      }
+    }
+  }
+
+  if (NeedSplit) {
+    MachineBasicBlock::iterator MBBI = MI; ++MBBI;
+    splitBlockBeforeInstr(MBBI);
+    // No need for the branch to the next block. We're adding an unconditional
+    // branch to the destination.
+    int delta = TII->getInstSizeInBytes(MBB->back());
+    BBInfo[MBB->getNumber()].Size -= delta;
+    MBB->back().eraseFromParent();
+    // BBInfo[SplitBB].Offset is wrong temporarily, fixed below
+  }
+
+  // After splitting and removing the unconditional branch from the original BB,
+  // the structure is now:
+  // oldbb:
+  //   [things]
+  //   b.invertedCC L1
+  // splitbb/fallthroughbb:
+  //   [old b L2/real continuation]
+  //
+  // We now have to change the conditional branch to point to splitbb and add an
+  // unconditional branch after it to L1, giving the final structure:
+  // oldbb:
+  //   [things]
+  //   b.invertedCC splitbb
+  //   b L1
+  // splitbb/fallthroughbb:
+  //   [old b L2/real continuation]
+  MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
+
+  DEBUG(dbgs() << "  Insert B to BB#"
+               << MI->getOperand(CondBrMBBOperand).getMBB()->getNumber()
+               << " also invert condition and change dest. to BB#"
+               << NextBB->getNumber() << "\n");
+
+  // Insert a new unconditional branch and fixup the destination of the
+  // conditional one.  Also update the ImmBranch as well as adding a new entry
+  // for the new branch.
+  BuildMI(MBB, DebugLoc(), TII->get(AArch64::Bimm))
+    .addMBB(MI->getOperand(CondBrMBBOperand).getMBB());
+  MI->getOperand(CondBrMBBOperand).setMBB(NextBB);
+
+  BBInfo[MBB->getNumber()].Size += TII->getInstSizeInBytes(MBB->back());
+
+  // 26 bits written down in Bimm, specifying a multiple of 4.
+  unsigned OffsetBits = 26 + 2;
+  ImmBranches.push_back(ImmBranch(&MBB->back(), OffsetBits, false));
+
+  adjustBBOffsetsAfter(MBB);
+  return true;
+}
diff --git a/lib/Target/AArch64/AArch64InstrInfo.cpp b/lib/Target/AArch64/AArch64InstrInfo.cpp
index b4e9e8d..7b93463 100644
--- a/lib/Target/AArch64/AArch64InstrInfo.cpp
+++ b/lib/Target/AArch64/AArch64InstrInfo.cpp
@@ -558,6 +558,48 @@ void AArch64InstrInfo::getAddressConstraints(const MachineInstr &MI,
   }
 }
 
+unsigned AArch64InstrInfo::getInstSizeInBytes(const MachineInstr &MI) const {
+  const MCInstrDesc &MCID = MI.getDesc();
+  const MachineBasicBlock &MBB = *MI.getParent();
+  const MachineFunction &MF = *MBB.getParent();
+  const MCAsmInfo &MAI = *MF.getTarget().getMCAsmInfo();
+
+  if (MCID.getSize())
+    return MCID.getSize();
+
+  if (MI.getOpcode() == AArch64::INLINEASM)
+    return getInlineAsmLength(MI.getOperand(0).getSymbolName(), MAI);
+
+  if (MI.isLabel())
+    return 0;
+
+  switch (MI.getOpcode()) {
+  case TargetOpcode::BUNDLE:
+    return getInstBundleLength(MI);
+  case TargetOpcode::IMPLICIT_DEF:
+  case TargetOpcode::KILL:
+  case TargetOpcode::PROLOG_LABEL:
+  case TargetOpcode::EH_LABEL:
+  case TargetOpcode::DBG_VALUE:
+    return 0;
+  case AArch64::TLSDESCCALL:
+    return 0;
+  default:
+    llvm_unreachable("Unknown instruction class");
+  }
+}
+
+unsigned AArch64InstrInfo::getInstBundleLength(const MachineInstr &MI) const {
+  unsigned Size = 0;
+  MachineBasicBlock::const_instr_iterator I = MI;
+  MachineBasicBlock::const_instr_iterator E = MI.getParent()->instr_end();
+  while (++I != E && I->isInsideBundle()) {
+    assert(!I->isBundle() && "No nested bundle!");
+    Size += getInstSizeInBytes(*I);
+  }
+  return Size;
+}
+
 bool llvm::rewriteA64FrameIndex(MachineInstr &MI, unsigned FrameRegIdx,
                                 unsigned FrameReg, int &Offset,
                                 const AArch64InstrInfo &TII) {
diff --git a/lib/Target/AArch64/AArch64InstrInfo.h b/lib/Target/AArch64/AArch64InstrInfo.h
index 0feb5a1..22a2ab4 100644
--- a/lib/Target/AArch64/AArch64InstrInfo.h
+++ b/lib/Target/AArch64/AArch64InstrInfo.h
@@ -83,6 +83,12 @@ public:
   ///    + imm % OffsetScale == 0
   void getAddressConstraints(const MachineInstr &MI, int &AccessScale,
                              int &MinOffset, int &MaxOffset) const;
+
+
+  unsigned getInstSizeInBytes(const MachineInstr &MI) const;
+
+  unsigned getInstBundleLength(const MachineInstr &MI) const;
+
 };
 
 bool rewriteA64FrameIndex(MachineInstr &MI, unsigned FrameRegIdx,
diff --git a/lib/Target/AArch64/AArch64TargetMachine.cpp b/lib/Target/AArch64/AArch64TargetMachine.cpp
index 33739cf..df599d5 100644
--- a/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ b/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -66,6 +66,7 @@ TargetPassConfig *AArch64TargetMachine::createPassConfig(PassManagerBase &PM) {
 
 bool AArch64PassConfig::addPreEmitPass() {
   addPass(&UnpackMachineBundlesID);
+  addPass(createAArch64BranchFixupPass());
   return true;
 }
 
diff --git a/lib/Target/AArch64/CMakeLists.txt b/lib/Target/AArch64/CMakeLists.txt
index 06720a8..8164d6f 100644
--- a/lib/Target/AArch64/CMakeLists.txt
+++ b/lib/Target/AArch64/CMakeLists.txt
@@ -14,6 +14,7 @@ add_public_tablegen_target(AArch64CommonTableGen)
 
 add_llvm_target(AArch64CodeGen
   AArch64AsmPrinter.cpp
+  AArch64BranchFixupPass.cpp
   AArch64FrameLowering.cpp
   AArch64ISelDAGToDAG.cpp
   AArch64ISelLowering.cpp