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diff --git a/lib/Analysis/InstructionPrecedenceTracking.cpp b/lib/Analysis/InstructionPrecedenceTracking.cpp
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+++ b/lib/Analysis/InstructionPrecedenceTracking.cpp
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+//===-- InstructionPrecedenceTracking.cpp -----------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implements a class that is able to define some instructions as "special"
+// (e.g. as having implicit control flow, or writing memory, or having another
+// interesting property) and then efficiently answers queries of the types:
+// 1. Are there any special instructions in the block of interest?
+// 2. Return first of the special instructions in the given block;
+// 3. Check if the given instruction is preceeded by the first special
+//    instruction in the same block.
+// The class provides caching that allows to answer these queries quickly. The
+// user must make sure that the cached data is invalidated properly whenever
+// a content of some tracked block is changed.
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/InstructionPrecedenceTracking.h"
+#include "llvm/Analysis/ValueTracking.h"
+
+using namespace llvm;
+
+#ifndef NDEBUG
+static cl::opt<bool> ExpensiveAsserts(
+    "ipt-expensive-asserts",
+    cl::desc("Perform expensive assert validation on every query to Instruction"
+             " Precedence Tracking"),
+    cl::init(false), cl::Hidden);
+#endif
+
+const Instruction *InstructionPrecedenceTracking::getFirstSpecialInstruction(
+    const BasicBlock *BB) {
+#ifndef NDEBUG
+  // If there is a bug connected to invalid cache, turn on ExpensiveAsserts to
+  // catch this situation as early as possible.
+  if (ExpensiveAsserts)
+    validateAll();
+  else
+    validate(BB);
+#endif
+
+  if (FirstSpecialInsts.find(BB) == FirstSpecialInsts.end()) {
+    fill(BB);
+    assert(FirstSpecialInsts.find(BB) != FirstSpecialInsts.end() && "Must be!");
+  }
+  return FirstSpecialInsts[BB];
+}
+
+bool InstructionPrecedenceTracking::hasSpecialInstructions(
+    const BasicBlock *BB) {
+  return getFirstSpecialInstruction(BB) != nullptr;
+}
+
+bool InstructionPrecedenceTracking::isPreceededBySpecialInstruction(
+    const Instruction *Insn) {
+  const Instruction *MaybeFirstSpecial =
+      getFirstSpecialInstruction(Insn->getParent());
+  return MaybeFirstSpecial && OI.dominates(MaybeFirstSpecial, Insn);
+}
+
+void InstructionPrecedenceTracking::fill(const BasicBlock *BB) {
+  FirstSpecialInsts.erase(BB);
+  for (auto &I : *BB)
+    if (isSpecialInstruction(&I)) {
+      FirstSpecialInsts[BB] = &I;
+      return;
+    }
+
+  // Mark this block as having no special instructions.
+  FirstSpecialInsts[BB] = nullptr;
+}
+
+#ifndef NDEBUG
+void InstructionPrecedenceTracking::validate(const BasicBlock *BB) const {
+  auto It = FirstSpecialInsts.find(BB);
+  // Bail if we don't have anything cached for this block.
+  if (It == FirstSpecialInsts.end())
+    return;
+
+  for (const Instruction &Insn : *BB)
+    if (isSpecialInstruction(&Insn)) {
+      assert(It->second == &Insn &&
+             "Cached first special instruction is wrong!");
+      return;
+    }
+
+  assert(It->second == nullptr &&
+         "Block is marked as having special instructions but in fact it  has "
+         "none!");
+}
+
+void InstructionPrecedenceTracking::validateAll() const {
+  // Check that for every known block the cached value is correct.
+  for (auto &It : FirstSpecialInsts)
+    validate(It.first);
+}
+#endif
+
+void InstructionPrecedenceTracking::insertInstructionTo(const Instruction *Inst,
+                                                        const BasicBlock *BB) {
+  if (isSpecialInstruction(Inst))
+    FirstSpecialInsts.erase(BB);
+  OI.invalidateBlock(BB);
+}
+
+void InstructionPrecedenceTracking::removeInstruction(const Instruction *Inst) {
+  if (isSpecialInstruction(Inst))
+    FirstSpecialInsts.erase(Inst->getParent());
+  OI.invalidateBlock(Inst->getParent());
+}
+
+void InstructionPrecedenceTracking::clear() {
+  for (auto It : FirstSpecialInsts)
+    OI.invalidateBlock(It.first);
+  FirstSpecialInsts.clear();
+#ifndef NDEBUG
+  // The map should be valid after clearing (at least empty).
+  validateAll();
+#endif
+}
+
+bool ImplicitControlFlowTracking::isSpecialInstruction(
+    const Instruction *Insn) const {
+  // If a block's instruction doesn't always pass the control to its successor
+  // instruction, mark the block as having implicit control flow. We use them
+  // to avoid wrong assumptions of sort "if A is executed and B post-dominates
+  // A, then B is also executed". This is not true is there is an implicit
+  // control flow instruction (e.g. a guard) between them.
+  //
+  // TODO: Currently, isGuaranteedToTransferExecutionToSuccessor returns false
+  // for volatile stores and loads because they can trap. The discussion on
+  // whether or not it is correct is still ongoing. We might want to get rid
+  // of this logic in the future. Anyways, trapping instructions shouldn't
+  // introduce implicit control flow, so we explicitly allow them here. This
+  // must be removed once isGuaranteedToTransferExecutionToSuccessor is fixed.
+  if (isGuaranteedToTransferExecutionToSuccessor(Insn))
+    return false;
+  if (isa<LoadInst>(Insn)) {
+    assert(cast<LoadInst>(Insn)->isVolatile() &&
+           "Non-volatile load should transfer execution to successor!");
+    return false;
+  }
+  if (isa<StoreInst>(Insn)) {
+    assert(cast<StoreInst>(Insn)->isVolatile() &&
+           "Non-volatile store should transfer execution to successor!");
+    return false;
+  }
+  return true;
+}
+
+bool MemoryWriteTracking::isSpecialInstruction(
+    const Instruction *Insn) const {
+  return Insn->mayWriteToMemory();
+}