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diff --git a/llvm/lib/Analysis/OrderedBasicBlock.cpp b/llvm/lib/Analysis/OrderedBasicBlock.cpp
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+//===- OrderedBasicBlock.cpp --------------------------------- -*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the OrderedBasicBlock class. OrderedBasicBlock
+// maintains an interface where clients can query if one instruction comes
+// before another in a BasicBlock. Since BasicBlock currently lacks a reliable
+// way to query relative position between instructions one can use
+// OrderedBasicBlock to do such queries. OrderedBasicBlock is lazily built on a
+// source BasicBlock and maintains an internal Instruction -> Position map. A
+// OrderedBasicBlock instance should be discarded whenever the source
+// BasicBlock changes.
+//
+// It's currently used by the CaptureTracker in order to find relative
+// positions of a pair of instructions inside a BasicBlock.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/OrderedBasicBlock.h"
+#include "llvm/IR/Instruction.h"
+using namespace llvm;
+
+OrderedBasicBlock::OrderedBasicBlock(const BasicBlock *BasicB)
+    : NextInstPos(0), BB(BasicB) {
+  LastInstFound = BB->end();
+}
+
+/// Given no cached results, find if \p A comes before \p B in \p BB.
+/// Cache and number out instruction while walking \p BB.
+bool OrderedBasicBlock::comesBefore(const Instruction *A,
+                                    const Instruction *B) {
+  const Instruction *Inst = nullptr;
+  assert(!(LastInstFound == BB->end() && NextInstPos != 0) &&
+         "Instruction supposed to be in NumberedInsts");
+  assert(A->getParent() == BB && "Instruction supposed to be in the block!");
+  assert(B->getParent() == BB && "Instruction supposed to be in the block!");
+
+  // Start the search with the instruction found in the last lookup round.
+  auto II = BB->begin();
+  auto IE = BB->end();
+  if (LastInstFound != IE)
+    II = std::next(LastInstFound);
+
+  // Number all instructions up to the point where we find 'A' or 'B'.
+  for (; II != IE; ++II) {
+    Inst = cast<Instruction>(II);
+    NumberedInsts[Inst] = NextInstPos++;
+    if (Inst == A || Inst == B)
+      break;
+  }
+
+  assert(II != IE && "Instruction not found?");
+  assert((Inst == A || Inst == B) && "Should find A or B");
+  LastInstFound = II;
+  return Inst != B;
+}
+
+/// Find out whether \p A dominates \p B, meaning whether \p A
+/// comes before \p B in \p BB. This is a simplification that considers
+/// cached instruction positions and ignores other basic blocks, being
+/// only relevant to compare relative instructions positions inside \p BB.
+bool OrderedBasicBlock::dominates(const Instruction *A, const Instruction *B) {
+  assert(A->getParent() == B->getParent() &&
+         "Instructions must be in the same basic block!");
+  assert(A->getParent() == BB && "Instructions must be in the tracked block!");
+
+  // First we lookup the instructions. If they don't exist, lookup will give us
+  // back ::end(). If they both exist, we compare the numbers. Otherwise, if NA
+  // exists and NB doesn't, it means NA must come before NB because we would
+  // have numbered NB as well if it didn't. The same is true for NB. If it
+  // exists, but NA does not, NA must come after it. If neither exist, we need
+  // to number the block and cache the results (by calling comesBefore).
+  auto NAI = NumberedInsts.find(A);
+  auto NBI = NumberedInsts.find(B);
+  if (NAI != NumberedInsts.end() && NBI != NumberedInsts.end())
+    return NAI->second < NBI->second;
+  if (NAI != NumberedInsts.end())
+    return true;
+  if (NBI != NumberedInsts.end())
+    return false;
+
+  return comesBefore(A, B);
+}
+
+void OrderedBasicBlock::eraseInstruction(const Instruction *I) {
+  if (LastInstFound != BB->end() && I == &*LastInstFound) {
+    if (LastInstFound == BB->begin()) {
+      LastInstFound = BB->end();
+      NextInstPos = 0;
+    } else
+      LastInstFound--;
+  }
+
+  NumberedInsts.erase(I);
+}
+
+void OrderedBasicBlock::replaceInstruction(const Instruction *Old,
+                                           const Instruction *New) {
+  auto OI = NumberedInsts.find(Old);
+  if (OI == NumberedInsts.end())
+    return;
+
+  NumberedInsts.insert({New, OI->second});
+  if (LastInstFound != BB->end() && Old == &*LastInstFound)
+    LastInstFound = New->getIterator();
+  NumberedInsts.erase(Old);
+}