Move all sources from the llvm project into contrib/llvm-project.

This uses the new layout of the upstream repository, which was recently
migrated to GitHub, and converted into a "monorepo".  That is, most of
the earlier separate sub-projects with their own branches and tags were
consolidated into one top-level directory, and are now branched and
tagged together.

Updating the vendor area to match this layout is next.

1 files changed, 824 insertions, 0 deletions

diff --git a/contrib/llvm-project/llvm/lib/CodeGen/EarlyIfConversion.cpp b/contrib/llvm-project/llvm/lib/CodeGen/EarlyIfConversion.cpp
new file mode 100644
index 000000000000..0a83760befaa
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/CodeGen/EarlyIfConversion.cpp
@@ -0,0 +1,824 @@
+//===-- EarlyIfConversion.cpp - If-conversion on SSA form machine code ----===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Early if-conversion is for out-of-order CPUs that don't have a lot of
+// predicable instructions. The goal is to eliminate conditional branches that
+// may mispredict.
+//
+// Instructions from both sides of the branch are executed specutatively, and a
+// cmov instruction selects the result.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SparseSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineLoopInfo.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/MachineTraceMetrics.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/CodeGen/TargetRegisterInfo.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "early-ifcvt"
+
+// Absolute maximum number of instructions allowed per speculated block.
+// This bypasses all other heuristics, so it should be set fairly high.
+static cl::opt<unsigned>
+BlockInstrLimit("early-ifcvt-limit", cl::init(30), cl::Hidden,
+  cl::desc("Maximum number of instructions per speculated block."));
+
+// Stress testing mode - disable heuristics.
+static cl::opt<bool> Stress("stress-early-ifcvt", cl::Hidden,
+  cl::desc("Turn all knobs to 11"));
+
+STATISTIC(NumDiamondsSeen,  "Number of diamonds");
+STATISTIC(NumDiamondsConv,  "Number of diamonds converted");
+STATISTIC(NumTrianglesSeen, "Number of triangles");
+STATISTIC(NumTrianglesConv, "Number of triangles converted");
+
+//===----------------------------------------------------------------------===//
+//                                 SSAIfConv
+//===----------------------------------------------------------------------===//
+//
+// The SSAIfConv class performs if-conversion on SSA form machine code after
+// determining if it is possible. The class contains no heuristics; external
+// code should be used to determine when if-conversion is a good idea.
+//
+// SSAIfConv can convert both triangles and diamonds:
+//
+//   Triangle: Head              Diamond: Head
+//              | \                       /  \_
+//              |  \                     /    |
+//              |  [TF]BB              FBB    TBB
+//              |  /                     \    /
+//              | /                       \  /
+//             Tail                       Tail
+//
+// Instructions in the conditional blocks TBB and/or FBB are spliced into the
+// Head block, and phis in the Tail block are converted to select instructions.
+//
+namespace {
+class SSAIfConv {
+  const TargetInstrInfo *TII;
+  const TargetRegisterInfo *TRI;
+  MachineRegisterInfo *MRI;
+
+public:
+  /// The block containing the conditional branch.
+  MachineBasicBlock *Head;
+
+  /// The block containing phis after the if-then-else.
+  MachineBasicBlock *Tail;
+
+  /// The 'true' conditional block as determined by AnalyzeBranch.
+  MachineBasicBlock *TBB;
+
+  /// The 'false' conditional block as determined by AnalyzeBranch.
+  MachineBasicBlock *FBB;
+
+  /// isTriangle - When there is no 'else' block, either TBB or FBB will be
+  /// equal to Tail.
+  bool isTriangle() const { return TBB == Tail || FBB == Tail; }
+
+  /// Returns the Tail predecessor for the True side.
+  MachineBasicBlock *getTPred() const { return TBB == Tail ? Head : TBB; }
+
+  /// Returns the Tail predecessor for the  False side.
+  MachineBasicBlock *getFPred() const { return FBB == Tail ? Head : FBB; }
+
+  /// Information about each phi in the Tail block.
+  struct PHIInfo {
+    MachineInstr *PHI;
+    unsigned TReg, FReg;
+    // Latencies from Cond+Branch, TReg, and FReg to DstReg.
+    int CondCycles, TCycles, FCycles;
+
+    PHIInfo(MachineInstr *phi)
+      : PHI(phi), TReg(0), FReg(0), CondCycles(0), TCycles(0), FCycles(0) {}
+  };
+
+  SmallVector<PHIInfo, 8> PHIs;
+
+private:
+  /// The branch condition determined by AnalyzeBranch.
+  SmallVector<MachineOperand, 4> Cond;
+
+  /// Instructions in Head that define values used by the conditional blocks.
+  /// The hoisted instructions must be inserted after these instructions.
+  SmallPtrSet<MachineInstr*, 8> InsertAfter;
+
+  /// Register units clobbered by the conditional blocks.
+  BitVector ClobberedRegUnits;
+
+  // Scratch pad for findInsertionPoint.
+  SparseSet<unsigned> LiveRegUnits;
+
+  /// Insertion point in Head for speculatively executed instructions form TBB
+  /// and FBB.
+  MachineBasicBlock::iterator InsertionPoint;
+
+  /// Return true if all non-terminator instructions in MBB can be safely
+  /// speculated.
+  bool canSpeculateInstrs(MachineBasicBlock *MBB);
+
+  /// Find a valid insertion point in Head.
+  bool findInsertionPoint();
+
+  /// Replace PHI instructions in Tail with selects.
+  void replacePHIInstrs();
+
+  /// Insert selects and rewrite PHI operands to use them.
+  void rewritePHIOperands();
+
+public:
+  /// runOnMachineFunction - Initialize per-function data structures.
+  void runOnMachineFunction(MachineFunction &MF) {
+    TII = MF.getSubtarget().getInstrInfo();
+    TRI = MF.getSubtarget().getRegisterInfo();
+    MRI = &MF.getRegInfo();
+    LiveRegUnits.clear();
+    LiveRegUnits.setUniverse(TRI->getNumRegUnits());
+    ClobberedRegUnits.clear();
+    ClobberedRegUnits.resize(TRI->getNumRegUnits());
+  }
+
+  /// canConvertIf - If the sub-CFG headed by MBB can be if-converted,
+  /// initialize the internal state, and return true.
+  bool canConvertIf(MachineBasicBlock *MBB);
+
+  /// convertIf - If-convert the last block passed to canConvertIf(), assuming
+  /// it is possible. Add any erased blocks to RemovedBlocks.
+  void convertIf(SmallVectorImpl<MachineBasicBlock*> &RemovedBlocks);
+};
+} // end anonymous namespace
+
+
+/// canSpeculateInstrs - Returns true if all the instructions in MBB can safely
+/// be speculated. The terminators are not considered.
+///
+/// If instructions use any values that are defined in the head basic block,
+/// the defining instructions are added to InsertAfter.
+///
+/// Any clobbered regunits are added to ClobberedRegUnits.
+///
+bool SSAIfConv::canSpeculateInstrs(MachineBasicBlock *MBB) {
+  // Reject any live-in physregs. It's probably CPSR/EFLAGS, and very hard to
+  // get right.
+  if (!MBB->livein_empty()) {
+    LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " has live-ins.\n");
+    return false;
+  }
+
+  unsigned InstrCount = 0;
+
+  // Check all instructions, except the terminators. It is assumed that
+  // terminators never have side effects or define any used register values.
+  for (MachineBasicBlock::iterator I = MBB->begin(),
+       E = MBB->getFirstTerminator(); I != E; ++I) {
+    if (I->isDebugInstr())
+      continue;
+
+    if (++InstrCount > BlockInstrLimit && !Stress) {
+      LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " has more than "
+                        << BlockInstrLimit << " instructions.\n");
+      return false;
+    }
+
+    // There shouldn't normally be any phis in a single-predecessor block.
+    if (I->isPHI()) {
+      LLVM_DEBUG(dbgs() << "Can't hoist: " << *I);
+      return false;
+    }
+
+    // Don't speculate loads. Note that it may be possible and desirable to
+    // speculate GOT or constant pool loads that are guaranteed not to trap,
+    // but we don't support that for now.
+    if (I->mayLoad()) {
+      LLVM_DEBUG(dbgs() << "Won't speculate load: " << *I);
+      return false;
+    }
+
+    // We never speculate stores, so an AA pointer isn't necessary.
+    bool DontMoveAcrossStore = true;
+    if (!I->isSafeToMove(nullptr, DontMoveAcrossStore)) {
+      LLVM_DEBUG(dbgs() << "Can't speculate: " << *I);
+      return false;
+    }
+
+    // Check for any dependencies on Head instructions.
+    for (const MachineOperand &MO : I->operands()) {
+      if (MO.isRegMask()) {
+        LLVM_DEBUG(dbgs() << "Won't speculate regmask: " << *I);
+        return false;
+      }
+      if (!MO.isReg())
+        continue;
+      unsigned Reg = MO.getReg();
+
+      // Remember clobbered regunits.
+      if (MO.isDef() && TargetRegisterInfo::isPhysicalRegister(Reg))
+        for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units)
+          ClobberedRegUnits.set(*Units);
+
+      if (!MO.readsReg() || !TargetRegisterInfo::isVirtualRegister(Reg))
+        continue;
+      MachineInstr *DefMI = MRI->getVRegDef(Reg);
+      if (!DefMI || DefMI->getParent() != Head)
+        continue;
+      if (InsertAfter.insert(DefMI).second)
+        LLVM_DEBUG(dbgs() << printMBBReference(*MBB) << " depends on "
+                          << *DefMI);
+      if (DefMI->isTerminator()) {
+        LLVM_DEBUG(dbgs() << "Can't insert instructions below terminator.\n");
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
+
+/// Find an insertion point in Head for the speculated instructions. The
+/// insertion point must be:
+///
+/// 1. Before any terminators.
+/// 2. After any instructions in InsertAfter.
+/// 3. Not have any clobbered regunits live.
+///
+/// This function sets InsertionPoint and returns true when successful, it
+/// returns false if no valid insertion point could be found.
+///
+bool SSAIfConv::findInsertionPoint() {
+  // Keep track of live regunits before the current position.
+  // Only track RegUnits that are also in ClobberedRegUnits.
+  LiveRegUnits.clear();
+  SmallVector<unsigned, 8> Reads;
+  MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
+  MachineBasicBlock::iterator I = Head->end();
+  MachineBasicBlock::iterator B = Head->begin();
+  while (I != B) {
+    --I;
+    // Some of the conditional code depends in I.
+    if (InsertAfter.count(&*I)) {
+      LLVM_DEBUG(dbgs() << "Can't insert code after " << *I);
+      return false;
+    }
+
+    // Update live regunits.
+    for (const MachineOperand &MO : I->operands()) {
+      // We're ignoring regmask operands. That is conservatively correct.
+      if (!MO.isReg())
+        continue;
+      unsigned Reg = MO.getReg();
+      if (!TargetRegisterInfo::isPhysicalRegister(Reg))
+        continue;
+      // I clobbers Reg, so it isn't live before I.
+      if (MO.isDef())
+        for (MCRegUnitIterator Units(Reg, TRI); Units.isValid(); ++Units)
+          LiveRegUnits.erase(*Units);
+      // Unless I reads Reg.
+      if (MO.readsReg())
+        Reads.push_back(Reg);
+    }
+    // Anything read by I is live before I.
+    while (!Reads.empty())
+      for (MCRegUnitIterator Units(Reads.pop_back_val(), TRI); Units.isValid();
+           ++Units)
+        if (ClobberedRegUnits.test(*Units))
+          LiveRegUnits.insert(*Units);
+
+    // We can't insert before a terminator.
+    if (I != FirstTerm && I->isTerminator())
+      continue;
+
+    // Some of the clobbered registers are live before I, not a valid insertion
+    // point.
+    if (!LiveRegUnits.empty()) {
+      LLVM_DEBUG({
+        dbgs() << "Would clobber";
+        for (SparseSet<unsigned>::const_iterator
+             i = LiveRegUnits.begin(), e = LiveRegUnits.end(); i != e; ++i)
+          dbgs() << ' ' << printRegUnit(*i, TRI);
+        dbgs() << " live before " << *I;
+      });
+      continue;
+    }
+
+    // This is a valid insertion point.
+    InsertionPoint = I;
+    LLVM_DEBUG(dbgs() << "Can insert before " << *I);
+    return true;
+  }
+  LLVM_DEBUG(dbgs() << "No legal insertion point found.\n");
+  return false;
+}
+
+
+
+/// canConvertIf - analyze the sub-cfg rooted in MBB, and return true if it is
+/// a potential candidate for if-conversion. Fill out the internal state.
+///
+bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) {
+  Head = MBB;
+  TBB = FBB = Tail = nullptr;
+
+  if (Head->succ_size() != 2)
+    return false;
+  MachineBasicBlock *Succ0 = Head->succ_begin()[0];
+  MachineBasicBlock *Succ1 = Head->succ_begin()[1];
+
+  // Canonicalize so Succ0 has MBB as its single predecessor.
+  if (Succ0->pred_size() != 1)
+    std::swap(Succ0, Succ1);
+
+  if (Succ0->pred_size() != 1 || Succ0->succ_size() != 1)
+    return false;
+
+  Tail = Succ0->succ_begin()[0];
+
+  // This is not a triangle.
+  if (Tail != Succ1) {
+    // Check for a diamond. We won't deal with any critical edges.
+    if (Succ1->pred_size() != 1 || Succ1->succ_size() != 1 ||
+        Succ1->succ_begin()[0] != Tail)
+      return false;
+    LLVM_DEBUG(dbgs() << "\nDiamond: " << printMBBReference(*Head) << " -> "
+                      << printMBBReference(*Succ0) << "/"
+                      << printMBBReference(*Succ1) << " -> "
+                      << printMBBReference(*Tail) << '\n');
+
+    // Live-in physregs are tricky to get right when speculating code.
+    if (!Tail->livein_empty()) {
+      LLVM_DEBUG(dbgs() << "Tail has live-ins.\n");
+      return false;
+    }
+  } else {
+    LLVM_DEBUG(dbgs() << "\nTriangle: " << printMBBReference(*Head) << " -> "
+                      << printMBBReference(*Succ0) << " -> "
+                      << printMBBReference(*Tail) << '\n');
+  }
+
+  // This is a triangle or a diamond.
+  // If Tail doesn't have any phis, there must be side effects.
+  if (Tail->empty() || !Tail->front().isPHI()) {
+    LLVM_DEBUG(dbgs() << "No phis in tail.\n");
+    return false;
+  }
+
+  // The branch we're looking to eliminate must be analyzable.
+  Cond.clear();
+  if (TII->analyzeBranch(*Head, TBB, FBB, Cond)) {
+    LLVM_DEBUG(dbgs() << "Branch not analyzable.\n");
+    return false;
+  }
+
+  // This is weird, probably some sort of degenerate CFG.
+  if (!TBB) {
+    LLVM_DEBUG(dbgs() << "AnalyzeBranch didn't find conditional branch.\n");
+    return false;
+  }
+
+  // Make sure the analyzed branch is conditional; one of the successors
+  // could be a landing pad. (Empty landing pads can be generated on Windows.)
+  if (Cond.empty()) {
+    LLVM_DEBUG(dbgs() << "AnalyzeBranch found an unconditional branch.\n");
+    return false;
+  }
+
+  // AnalyzeBranch doesn't set FBB on a fall-through branch.
+  // Make sure it is always set.
+  FBB = TBB == Succ0 ? Succ1 : Succ0;
+
+  // Any phis in the tail block must be convertible to selects.
+  PHIs.clear();
+  MachineBasicBlock *TPred = getTPred();
+  MachineBasicBlock *FPred = getFPred();
+  for (MachineBasicBlock::iterator I = Tail->begin(), E = Tail->end();
+       I != E && I->isPHI(); ++I) {
+    PHIs.push_back(&*I);
+    PHIInfo &PI = PHIs.back();
+    // Find PHI operands corresponding to TPred and FPred.
+    for (unsigned i = 1; i != PI.PHI->getNumOperands(); i += 2) {
+      if (PI.PHI->getOperand(i+1).getMBB() == TPred)
+        PI.TReg = PI.PHI->getOperand(i).getReg();
+      if (PI.PHI->getOperand(i+1).getMBB() == FPred)
+        PI.FReg = PI.PHI->getOperand(i).getReg();
+    }
+    assert(TargetRegisterInfo::isVirtualRegister(PI.TReg) && "Bad PHI");
+    assert(TargetRegisterInfo::isVirtualRegister(PI.FReg) && "Bad PHI");
+
+    // Get target information.
+    if (!TII->canInsertSelect(*Head, Cond, PI.TReg, PI.FReg,
+                              PI.CondCycles, PI.TCycles, PI.FCycles)) {
+      LLVM_DEBUG(dbgs() << "Can't convert: " << *PI.PHI);
+      return false;
+    }
+  }
+
+  // Check that the conditional instructions can be speculated.
+  InsertAfter.clear();
+  ClobberedRegUnits.reset();
+  if (TBB != Tail && !canSpeculateInstrs(TBB))
+    return false;
+  if (FBB != Tail && !canSpeculateInstrs(FBB))
+    return false;
+
+  // Try to find a valid insertion point for the speculated instructions in the
+  // head basic block.
+  if (!findInsertionPoint())
+    return false;
+
+  if (isTriangle())
+    ++NumTrianglesSeen;
+  else
+    ++NumDiamondsSeen;
+  return true;
+}
+
+/// replacePHIInstrs - Completely replace PHI instructions with selects.
+/// This is possible when the only Tail predecessors are the if-converted
+/// blocks.
+void SSAIfConv::replacePHIInstrs() {
+  assert(Tail->pred_size() == 2 && "Cannot replace PHIs");
+  MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
+  assert(FirstTerm != Head->end() && "No terminators");
+  DebugLoc HeadDL = FirstTerm->getDebugLoc();
+
+  // Convert all PHIs to select instructions inserted before FirstTerm.
+  for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
+    PHIInfo &PI = PHIs[i];
+    LLVM_DEBUG(dbgs() << "If-converting " << *PI.PHI);
+    unsigned DstReg = PI.PHI->getOperand(0).getReg();
+    TII->insertSelect(*Head, FirstTerm, HeadDL, DstReg, Cond, PI.TReg, PI.FReg);
+    LLVM_DEBUG(dbgs() << "          --> " << *std::prev(FirstTerm));
+    PI.PHI->eraseFromParent();
+    PI.PHI = nullptr;
+  }
+}
+
+/// rewritePHIOperands - When there are additional Tail predecessors, insert
+/// select instructions in Head and rewrite PHI operands to use the selects.
+/// Keep the PHI instructions in Tail to handle the other predecessors.
+void SSAIfConv::rewritePHIOperands() {
+  MachineBasicBlock::iterator FirstTerm = Head->getFirstTerminator();
+  assert(FirstTerm != Head->end() && "No terminators");
+  DebugLoc HeadDL = FirstTerm->getDebugLoc();
+
+  // Convert all PHIs to select instructions inserted before FirstTerm.
+  for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
+    PHIInfo &PI = PHIs[i];
+    unsigned DstReg = 0;
+
+    LLVM_DEBUG(dbgs() << "If-converting " << *PI.PHI);
+    if (PI.TReg == PI.FReg) {
+      // We do not need the select instruction if both incoming values are
+      // equal.
+      DstReg = PI.TReg;
+    } else {
+      unsigned PHIDst = PI.PHI->getOperand(0).getReg();
+      DstReg = MRI->createVirtualRegister(MRI->getRegClass(PHIDst));
+      TII->insertSelect(*Head, FirstTerm, HeadDL,
+                         DstReg, Cond, PI.TReg, PI.FReg);
+      LLVM_DEBUG(dbgs() << "          --> " << *std::prev(FirstTerm));
+    }
+
+    // Rewrite PHI operands TPred -> (DstReg, Head), remove FPred.
+    for (unsigned i = PI.PHI->getNumOperands(); i != 1; i -= 2) {
+      MachineBasicBlock *MBB = PI.PHI->getOperand(i-1).getMBB();
+      if (MBB == getTPred()) {
+        PI.PHI->getOperand(i-1).setMBB(Head);
+        PI.PHI->getOperand(i-2).setReg(DstReg);
+      } else if (MBB == getFPred()) {
+        PI.PHI->RemoveOperand(i-1);
+        PI.PHI->RemoveOperand(i-2);
+      }
+    }
+    LLVM_DEBUG(dbgs() << "          --> " << *PI.PHI);
+  }
+}
+
+/// convertIf - Execute the if conversion after canConvertIf has determined the
+/// feasibility.
+///
+/// Any basic blocks erased will be added to RemovedBlocks.
+///
+void SSAIfConv::convertIf(SmallVectorImpl<MachineBasicBlock*> &RemovedBlocks) {
+  assert(Head && Tail && TBB && FBB && "Call canConvertIf first.");
+
+  // Update statistics.
+  if (isTriangle())
+    ++NumTrianglesConv;
+  else
+    ++NumDiamondsConv;
+
+  // Move all instructions into Head, except for the terminators.
+  if (TBB != Tail)
+    Head->splice(InsertionPoint, TBB, TBB->begin(), TBB->getFirstTerminator());
+  if (FBB != Tail)
+    Head->splice(InsertionPoint, FBB, FBB->begin(), FBB->getFirstTerminator());
+
+  // Are there extra Tail predecessors?
+  bool ExtraPreds = Tail->pred_size() != 2;
+  if (ExtraPreds)
+    rewritePHIOperands();
+  else
+    replacePHIInstrs();
+
+  // Fix up the CFG, temporarily leave Head without any successors.
+  Head->removeSuccessor(TBB);
+  Head->removeSuccessor(FBB, true);
+  if (TBB != Tail)
+    TBB->removeSuccessor(Tail, true);
+  if (FBB != Tail)
+    FBB->removeSuccessor(Tail, true);
+
+  // Fix up Head's terminators.
+  // It should become a single branch or a fallthrough.
+  DebugLoc HeadDL = Head->getFirstTerminator()->getDebugLoc();
+  TII->removeBranch(*Head);
+
+  // Erase the now empty conditional blocks. It is likely that Head can fall
+  // through to Tail, and we can join the two blocks.
+  if (TBB != Tail) {
+    RemovedBlocks.push_back(TBB);
+    TBB->eraseFromParent();
+  }
+  if (FBB != Tail) {
+    RemovedBlocks.push_back(FBB);
+    FBB->eraseFromParent();
+  }
+
+  assert(Head->succ_empty() && "Additional head successors?");
+  if (!ExtraPreds && Head->isLayoutSuccessor(Tail)) {
+    // Splice Tail onto the end of Head.
+    LLVM_DEBUG(dbgs() << "Joining tail " << printMBBReference(*Tail)
+                      << " into head " << printMBBReference(*Head) << '\n');
+    Head->splice(Head->end(), Tail,
+                     Tail->begin(), Tail->end());
+    Head->transferSuccessorsAndUpdatePHIs(Tail);
+    RemovedBlocks.push_back(Tail);
+    Tail->eraseFromParent();
+  } else {
+    // We need a branch to Tail, let code placement work it out later.
+    LLVM_DEBUG(dbgs() << "Converting to unconditional branch.\n");
+    SmallVector<MachineOperand, 0> EmptyCond;
+    TII->insertBranch(*Head, Tail, nullptr, EmptyCond, HeadDL);
+    Head->addSuccessor(Tail);
+  }
+  LLVM_DEBUG(dbgs() << *Head);
+}
+
+
+//===----------------------------------------------------------------------===//
+//                           EarlyIfConverter Pass
+//===----------------------------------------------------------------------===//
+
+namespace {
+class EarlyIfConverter : public MachineFunctionPass {
+  const TargetInstrInfo *TII;
+  const TargetRegisterInfo *TRI;
+  MCSchedModel SchedModel;
+  MachineRegisterInfo *MRI;
+  MachineDominatorTree *DomTree;
+  MachineLoopInfo *Loops;
+  MachineTraceMetrics *Traces;
+  MachineTraceMetrics::Ensemble *MinInstr;
+  SSAIfConv IfConv;
+
+public:
+  static char ID;
+  EarlyIfConverter() : MachineFunctionPass(ID) {}
+  void getAnalysisUsage(AnalysisUsage &AU) const override;
+  bool runOnMachineFunction(MachineFunction &MF) override;
+  StringRef getPassName() const override { return "Early If-Conversion"; }
+
+private:
+  bool tryConvertIf(MachineBasicBlock*);
+  void updateDomTree(ArrayRef<MachineBasicBlock*> Removed);
+  void updateLoops(ArrayRef<MachineBasicBlock*> Removed);
+  void invalidateTraces();
+  bool shouldConvertIf();
+};
+} // end anonymous namespace
+
+char EarlyIfConverter::ID = 0;
+char &llvm::EarlyIfConverterID = EarlyIfConverter::ID;
+
+INITIALIZE_PASS_BEGIN(EarlyIfConverter, DEBUG_TYPE,
+                      "Early If Converter", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachineTraceMetrics)
+INITIALIZE_PASS_END(EarlyIfConverter, DEBUG_TYPE,
+                    "Early If Converter", false, false)
+
+void EarlyIfConverter::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.addRequired<MachineBranchProbabilityInfo>();
+  AU.addRequired<MachineDominatorTree>();
+  AU.addPreserved<MachineDominatorTree>();
+  AU.addRequired<MachineLoopInfo>();
+  AU.addPreserved<MachineLoopInfo>();
+  AU.addRequired<MachineTraceMetrics>();
+  AU.addPreserved<MachineTraceMetrics>();
+  MachineFunctionPass::getAnalysisUsage(AU);
+}
+
+/// Update the dominator tree after if-conversion erased some blocks.
+void EarlyIfConverter::updateDomTree(ArrayRef<MachineBasicBlock*> Removed) {
+  // convertIf can remove TBB, FBB, and Tail can be merged into Head.
+  // TBB and FBB should not dominate any blocks.
+  // Tail children should be transferred to Head.
+  MachineDomTreeNode *HeadNode = DomTree->getNode(IfConv.Head);
+  for (unsigned i = 0, e = Removed.size(); i != e; ++i) {
+    MachineDomTreeNode *Node = DomTree->getNode(Removed[i]);
+    assert(Node != HeadNode && "Cannot erase the head node");
+    while (Node->getNumChildren()) {
+      assert(Node->getBlock() == IfConv.Tail && "Unexpected children");
+      DomTree->changeImmediateDominator(Node->getChildren().back(), HeadNode);
+    }
+    DomTree->eraseNode(Removed[i]);
+  }
+}
+
+/// Update LoopInfo after if-conversion.
+void EarlyIfConverter::updateLoops(ArrayRef<MachineBasicBlock*> Removed) {
+  if (!Loops)
+    return;
+  // If-conversion doesn't change loop structure, and it doesn't mess with back
+  // edges, so updating LoopInfo is simply removing the dead blocks.
+  for (unsigned i = 0, e = Removed.size(); i != e; ++i)
+    Loops->removeBlock(Removed[i]);
+}
+
+/// Invalidate MachineTraceMetrics before if-conversion.
+void EarlyIfConverter::invalidateTraces() {
+  Traces->verifyAnalysis();
+  Traces->invalidate(IfConv.Head);
+  Traces->invalidate(IfConv.Tail);
+  Traces->invalidate(IfConv.TBB);
+  Traces->invalidate(IfConv.FBB);
+  Traces->verifyAnalysis();
+}
+
+// Adjust cycles with downward saturation.
+static unsigned adjCycles(unsigned Cyc, int Delta) {
+  if (Delta < 0 && Cyc + Delta > Cyc)
+    return 0;
+  return Cyc + Delta;
+}
+
+/// Apply cost model and heuristics to the if-conversion in IfConv.
+/// Return true if the conversion is a good idea.
+///
+bool EarlyIfConverter::shouldConvertIf() {
+  // Stress testing mode disables all cost considerations.
+  if (Stress)
+    return true;
+
+  if (!MinInstr)
+    MinInstr = Traces->getEnsemble(MachineTraceMetrics::TS_MinInstrCount);
+
+  MachineTraceMetrics::Trace TBBTrace = MinInstr->getTrace(IfConv.getTPred());
+  MachineTraceMetrics::Trace FBBTrace = MinInstr->getTrace(IfConv.getFPred());
+  LLVM_DEBUG(dbgs() << "TBB: " << TBBTrace << "FBB: " << FBBTrace);
+  unsigned MinCrit = std::min(TBBTrace.getCriticalPath(),
+                              FBBTrace.getCriticalPath());
+
+  // Set a somewhat arbitrary limit on the critical path extension we accept.
+  unsigned CritLimit = SchedModel.MispredictPenalty/2;
+
+  // If-conversion only makes sense when there is unexploited ILP. Compute the
+  // maximum-ILP resource length of the trace after if-conversion. Compare it
+  // to the shortest critical path.
+  SmallVector<const MachineBasicBlock*, 1> ExtraBlocks;
+  if (IfConv.TBB != IfConv.Tail)
+    ExtraBlocks.push_back(IfConv.TBB);
+  unsigned ResLength = FBBTrace.getResourceLength(ExtraBlocks);
+  LLVM_DEBUG(dbgs() << "Resource length " << ResLength
+                    << ", minimal critical path " << MinCrit << '\n');
+  if (ResLength > MinCrit + CritLimit) {
+    LLVM_DEBUG(dbgs() << "Not enough available ILP.\n");
+    return false;
+  }
+
+  // Assume that the depth of the first head terminator will also be the depth
+  // of the select instruction inserted, as determined by the flag dependency.
+  // TBB / FBB data dependencies may delay the select even more.
+  MachineTraceMetrics::Trace HeadTrace = MinInstr->getTrace(IfConv.Head);
+  unsigned BranchDepth =
+      HeadTrace.getInstrCycles(*IfConv.Head->getFirstTerminator()).Depth;
+  LLVM_DEBUG(dbgs() << "Branch depth: " << BranchDepth << '\n');
+
+  // Look at all the tail phis, and compute the critical path extension caused
+  // by inserting select instructions.
+  MachineTraceMetrics::Trace TailTrace = MinInstr->getTrace(IfConv.Tail);
+  for (unsigned i = 0, e = IfConv.PHIs.size(); i != e; ++i) {
+    SSAIfConv::PHIInfo &PI = IfConv.PHIs[i];
+    unsigned Slack = TailTrace.getInstrSlack(*PI.PHI);
+    unsigned MaxDepth = Slack + TailTrace.getInstrCycles(*PI.PHI).Depth;
+    LLVM_DEBUG(dbgs() << "Slack " << Slack << ":\t" << *PI.PHI);
+
+    // The condition is pulled into the critical path.
+    unsigned CondDepth = adjCycles(BranchDepth, PI.CondCycles);
+    if (CondDepth > MaxDepth) {
+      unsigned Extra = CondDepth - MaxDepth;
+      LLVM_DEBUG(dbgs() << "Condition adds " << Extra << " cycles.\n");
+      if (Extra > CritLimit) {
+        LLVM_DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
+        return false;
+      }
+    }
+
+    // The TBB value is pulled into the critical path.
+    unsigned TDepth = adjCycles(TBBTrace.getPHIDepth(*PI.PHI), PI.TCycles);
+    if (TDepth > MaxDepth) {
+      unsigned Extra = TDepth - MaxDepth;
+      LLVM_DEBUG(dbgs() << "TBB data adds " << Extra << " cycles.\n");
+      if (Extra > CritLimit) {
+        LLVM_DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
+        return false;
+      }
+    }
+
+    // The FBB value is pulled into the critical path.
+    unsigned FDepth = adjCycles(FBBTrace.getPHIDepth(*PI.PHI), PI.FCycles);
+    if (FDepth > MaxDepth) {
+      unsigned Extra = FDepth - MaxDepth;
+      LLVM_DEBUG(dbgs() << "FBB data adds " << Extra << " cycles.\n");
+      if (Extra > CritLimit) {
+        LLVM_DEBUG(dbgs() << "Exceeds limit of " << CritLimit << '\n');
+        return false;
+      }
+    }
+  }
+  return true;
+}
+
+/// Attempt repeated if-conversion on MBB, return true if successful.
+///
+bool EarlyIfConverter::tryConvertIf(MachineBasicBlock *MBB) {
+  bool Changed = false;
+  while (IfConv.canConvertIf(MBB) && shouldConvertIf()) {
+    // If-convert MBB and update analyses.
+    invalidateTraces();
+    SmallVector<MachineBasicBlock*, 4> RemovedBlocks;
+    IfConv.convertIf(RemovedBlocks);
+    Changed = true;
+    updateDomTree(RemovedBlocks);
+    updateLoops(RemovedBlocks);
+  }
+  return Changed;
+}
+
+bool EarlyIfConverter::runOnMachineFunction(MachineFunction &MF) {
+  LLVM_DEBUG(dbgs() << "********** EARLY IF-CONVERSION **********\n"
+                    << "********** Function: " << MF.getName() << '\n');
+  if (skipFunction(MF.getFunction()))
+    return false;
+
+  // Only run if conversion if the target wants it.
+  const TargetSubtargetInfo &STI = MF.getSubtarget();
+  if (!STI.enableEarlyIfConversion())
+    return false;
+
+  TII = STI.getInstrInfo();
+  TRI = STI.getRegisterInfo();
+  SchedModel = STI.getSchedModel();
+  MRI = &MF.getRegInfo();
+  DomTree = &getAnalysis<MachineDominatorTree>();
+  Loops = getAnalysisIfAvailable<MachineLoopInfo>();
+  Traces = &getAnalysis<MachineTraceMetrics>();
+  MinInstr = nullptr;
+
+  bool Changed = false;
+  IfConv.runOnMachineFunction(MF);
+
+  // Visit blocks in dominator tree post-order. The post-order enables nested
+  // if-conversion in a single pass. The tryConvertIf() function may erase
+  // blocks, but only blocks dominated by the head block. This makes it safe to
+  // update the dominator tree while the post-order iterator is still active.
+  for (auto DomNode : post_order(DomTree))
+    if (tryConvertIf(DomNode->getBlock()))
+      Changed = true;
+
+  return Changed;
+}