diff --git a/bolt/CMakeLists.txt b/bolt/CMakeLists.txt
--- a/bolt/CMakeLists.txt
+++ b/bolt/CMakeLists.txt
@@ -76,6 +76,10 @@
 add_subdirectory(tools)
 
 if (BOLT_INCLUDE_TESTS)
+  if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/unittest/googletest/include/gtest/gtest.h)
+    add_subdirectory(unittests)
+    list(APPEND BOLT_TEST_DEPS BoltUnitTests)
+  endif()
   add_subdirectory(test)
 endif()
 
diff --git a/bolt/include/bolt/Core/MCPlusBuilder.h b/bolt/include/bolt/Core/MCPlusBuilder.h
--- a/bolt/include/bolt/Core/MCPlusBuilder.h
+++ b/bolt/include/bolt/Core/MCPlusBuilder.h
@@ -1886,6 +1886,14 @@
   }
 };
 
+MCPlusBuilder *createX86MCPlusBuilder(const MCInstrAnalysis *,
+                                      const MCInstrInfo *,
+                                      const MCRegisterInfo *);
+
+MCPlusBuilder *createAArch64MCPlusBuilder(const MCInstrAnalysis *,
+                                          const MCInstrInfo *,
+                                          const MCRegisterInfo *);
+
 } // namespace bolt
 } // namespace llvm
 
diff --git a/bolt/include/bolt/Rewrite/RewriteInstance.h b/bolt/include/bolt/Rewrite/RewriteInstance.h
--- a/bolt/include/bolt/Rewrite/RewriteInstance.h
+++ b/bolt/include/bolt/Rewrite/RewriteInstance.h
@@ -547,6 +547,11 @@
   friend class RewriteInstanceDiff;
 };
 
+MCPlusBuilder *createMCPlusBuilder(const Triple::ArchType Arch,
+                                   const MCInstrAnalysis *Analysis,
+                                   const MCInstrInfo *Info,
+                                   const MCRegisterInfo *RegInfo);
+
 } // namespace bolt
 } // namespace llvm
 
diff --git a/bolt/include/bolt/Utils/CommandLineOpts.h b/bolt/include/bolt/Utils/CommandLineOpts.h
--- a/bolt/include/bolt/Utils/CommandLineOpts.h
+++ b/bolt/include/bolt/Utils/CommandLineOpts.h
@@ -44,7 +44,6 @@
 extern llvm::cl::opt<bool> HotData;
 extern llvm::cl::opt<bool> HotFunctionsAtEnd;
 extern llvm::cl::opt<bool> HotText;
-extern llvm::cl::opt<std::string> InputFilename;
 extern llvm::cl::opt<bool> Instrument;
 extern llvm::cl::opt<std::string> OutputFilename;
 extern llvm::cl::opt<std::string> PerfData;
diff --git a/bolt/lib/Core/MCPlusBuilder.cpp b/bolt/lib/Core/MCPlusBuilder.cpp
--- a/bolt/lib/Core/MCPlusBuilder.cpp
+++ b/bolt/lib/Core/MCPlusBuilder.cpp
@@ -416,49 +416,46 @@
   // AliasMap caches a mapping of registers to the set of registers that
   // alias (are sub or superregs of itself, including itself).
   static std::vector<BitVector> AliasMap;
-  static std::vector<MCPhysReg> SuperReg;
+  static std::vector<BitVector> SmallerAliasMap;
 
   if (AliasMap.size() > 0) {
     if (OnlySmaller)
-      return AliasMap[Reg];
-    return AliasMap[SuperReg[Reg]];
+      return SmallerAliasMap[Reg];
+    return AliasMap[Reg];
   }
+
   // Build alias map
   for (MCPhysReg I = 0, E = RegInfo->getNumRegs(); I != E; ++I) {
     BitVector BV(RegInfo->getNumRegs(), false);
     BV.set(I);
-    AliasMap.emplace_back(std::move(BV));
-    SuperReg.emplace_back(I);
+    AliasMap.emplace_back(BV);
+    SmallerAliasMap.emplace_back(BV);
+  }
+
+  // Cache all aliases for each register
+  for (MCPhysReg I = 1, E = RegInfo->getNumRegs(); I != E; ++I) {
+    for (MCRegAliasIterator AI(I, RegInfo, true); AI.isValid(); ++AI)
+      AliasMap[I].set(*AI);
   }
+
+  // Propagate smaller alias info upwards. Skip reg 0 (mapped to NoRegister)
   std::queue<MCPhysReg> Worklist;
-  // Propagate alias info upwards. Skip reg 0 (mapped to NoRegister)
   for (MCPhysReg I = 1, E = RegInfo->getNumRegs(); I < E; ++I)
     Worklist.push(I);
   while (!Worklist.empty()) {
     MCPhysReg I = Worklist.front();
     Worklist.pop();
     for (MCSubRegIterator SI(I, RegInfo); SI.isValid(); ++SI)
-      AliasMap[I] |= AliasMap[*SI];
+      SmallerAliasMap[I] |= SmallerAliasMap[*SI];
     for (MCSuperRegIterator SI(I, RegInfo); SI.isValid(); ++SI)
       Worklist.push(*SI);
   }
-  // Propagate parent reg downwards
-  for (MCPhysReg I = 1, E = RegInfo->getNumRegs(); I < E; ++I)
-    Worklist.push(I);
-  while (!Worklist.empty()) {
-    MCPhysReg I = Worklist.front();
-    Worklist.pop();
-    for (MCSubRegIterator SI(I, RegInfo); SI.isValid(); ++SI) {
-      SuperReg[*SI] = SuperReg[I];
-      Worklist.push(*SI);
-    }
-  }
 
   LLVM_DEBUG({
     dbgs() << "Dumping reg alias table:\n";
     for (MCPhysReg I = 0, E = RegInfo->getNumRegs(); I != E; ++I) {
       dbgs() << "Reg " << I << ": ";
-      const BitVector &BV = AliasMap[SuperReg[I]];
+      const BitVector &BV = AliasMap[I];
       int Idx = BV.find_first();
       while (Idx != -1) {
         dbgs() << Idx << " ";
@@ -469,8 +466,8 @@
   });
 
   if (OnlySmaller)
-    return AliasMap[Reg];
-  return AliasMap[SuperReg[Reg]];
+    return SmallerAliasMap[Reg];
+  return AliasMap[Reg];
 }
 
 uint8_t MCPlusBuilder::getRegSize(MCPhysReg Reg) const {
diff --git a/bolt/lib/Rewrite/RewriteInstance.cpp b/bolt/lib/Rewrite/RewriteInstance.cpp
--- a/bolt/lib/Rewrite/RewriteInstance.cpp
+++ b/bolt/lib/Rewrite/RewriteInstance.cpp
@@ -321,27 +321,6 @@
 
 extern const char *BoltRevision;
 
-extern MCPlusBuilder *createX86MCPlusBuilder(const MCInstrAnalysis *,
-                                             const MCInstrInfo *,
-                                             const MCRegisterInfo *);
-extern MCPlusBuilder *createAArch64MCPlusBuilder(const MCInstrAnalysis *,
-                                                 const MCInstrInfo *,
-                                                 const MCRegisterInfo *);
-
-} // namespace bolt
-} // namespace llvm
-
-namespace {
-
-bool refersToReorderedSection(ErrorOr<BinarySection &> Section) {
-  auto Itr =
-      std::find_if(opts::ReorderData.begin(), opts::ReorderData.end(),
-                   [&](const std::string &SectionName) {
-                     return (Section && Section->getName() == SectionName);
-                   });
-  return Itr != opts::ReorderData.end();
-}
-
 MCPlusBuilder *createMCPlusBuilder(const Triple::ArchType Arch,
                                    const MCInstrAnalysis *Analysis,
                                    const MCInstrInfo *Info,
@@ -359,6 +338,20 @@
   llvm_unreachable("architecture unsupported by MCPlusBuilder");
 }
 
+} // namespace bolt
+} // namespace llvm
+
+namespace {
+
+bool refersToReorderedSection(ErrorOr<BinarySection &> Section) {
+  auto Itr =
+      std::find_if(opts::ReorderData.begin(), opts::ReorderData.end(),
+                   [&](const std::string &SectionName) {
+                     return (Section && Section->getName() == SectionName);
+                   });
+  return Itr != opts::ReorderData.end();
+}
+
 } // anonymous namespace
 
 RewriteInstance::RewriteInstance(ELFObjectFileBase *File, const int Argc,
diff --git a/bolt/lib/Utils/CommandLineOpts.cpp b/bolt/lib/Utils/CommandLineOpts.cpp
--- a/bolt/lib/Utils/CommandLineOpts.cpp
+++ b/bolt/lib/Utils/CommandLineOpts.cpp
@@ -131,14 +131,6 @@
         "will put hot code into 2M pages. This requires relocation."),
     cl::ZeroOrMore, cl::cat(BoltCategory));
 
-cl::opt<std::string>
-InputFilename(
-  cl::Positional,
-  cl::desc("<executable>"),
-  cl::Required,
-  cl::cat(BoltCategory),
-  cl::sub(*cl::AllSubCommands));
-
 cl::opt<bool>
     Instrument("instrument",
                cl::desc("instrument code to generate accurate profile data"),
diff --git a/bolt/test/CMakeLists.txt b/bolt/test/CMakeLists.txt
--- a/bolt/test/CMakeLists.txt
+++ b/bolt/test/CMakeLists.txt
@@ -8,6 +8,12 @@
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py
   )
+configure_lit_site_cfg(
+  ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.site.cfg.py.in
+  ${CMAKE_CURRENT_BINARY_DIR}/Unit/lit.site.cfg.py
+  MAIN_CONFIG
+  ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
+  )
 
 set(BOLT_TEST_PARAMS
   bolt_site_config=${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
diff --git a/bolt/test/Unit/CMakeLists.txt b/bolt/test/Unit/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/bolt/test/Unit/CMakeLists.txt
@@ -0,0 +1,17 @@
+add_custom_target(bolt-unit-test-deps)
+add_dependencies(bolt-unit-test-deps bolt-test-depends)
+
+add_lit_testsuites(BOLT-UNIT
+  ${CMAKE_CURRENT_SOURCE_DIR}
+  DEPENDS bolt-unit-test-deps)
+
+configure_lit_site_cfg(
+  ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.py.in
+  ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg.py
+  MAIN_CONFIG
+  ${CMAKE_CURRENT_SOURCE_DIR}/lit.cfg.py)
+
+add_lit_testsuite(check-golt-unit "Running bolt unit test suite"
+  ${CMAKE_CURRENT_BINARY_DIR}
+  EXCLUDE_FROM_CHECK_ALL
+  DEPENDS bolt-unit-test-deps)
diff --git a/bolt/test/Unit/lit.cfg.py b/bolt/test/Unit/lit.cfg.py
new file mode 100644
--- /dev/null
+++ b/bolt/test/Unit/lit.cfg.py
@@ -0,0 +1,22 @@
+# -*- Python -*-
+
+# Configuration file for the 'lit' test runner.
+
+import os
+import subprocess
+
+import lit.formats
+
+# name: The name of this test suite.
+config.name = 'BOLT-Unit'
+
+# suffixes: A list of file extensions to treat as test files.
+config.suffixes = []
+
+# test_source_root: The root path where tests are located.
+# test_exec_root: The root path where tests should be run.
+config.test_exec_root = os.path.join(config.bolt_obj_root, 'unittests')
+config.test_source_root = config.test_exec_root
+
+# testFormat: The test format to use to interpret tests.
+config.test_format = lit.formats.GoogleTest(config.llvm_build_mode, 'Tests')
diff --git a/bolt/test/Unit/lit.site.cfg.py.in b/bolt/test/Unit/lit.site.cfg.py.in
new file mode 100644
--- /dev/null
+++ b/bolt/test/Unit/lit.site.cfg.py.in
@@ -0,0 +1,28 @@
+@LIT_SITE_CFG_IN_HEADER@
+
+config.llvm_src_root = "@LLVM_SOURCE_DIR@"
+config.llvm_obj_root = "@LLVM_BINARY_DIR@"
+config.llvm_tools_dir = "@LLVM_TOOLS_DIR@"
+config.llvm_libs_dir = "@LLVM_LIBS_DIR@"
+config.llvm_build_mode = "@LLVM_BUILD_MODE@"
+config.lit_tools_dir = "@LLVM_LIT_TOOLS_DIR@"
+config.bolt_obj_root = "@BOLT_BINARY_DIR@"
+config.bolt_src_root = "@BOLT_SOURCE_DIR@"
+config.target_triple = "@TARGET_TRIPLE@"
+config.python_executable = "@Python3_EXECUTABLE@"
+
+# Support substitution of the tools and libs dirs with user parameters. This is
+# used when we can't determine the tool dir at configuration time.
+try:
+    config.llvm_tools_dir = config.llvm_tools_dir % lit_config.params
+    config.llvm_libs_dir = config.llvm_libs_dir % lit_config.params
+    config.llvm_build_mode = config.llvm_build_mode % lit_config.params
+except KeyError as e:
+    key, = e.args
+    lit_config.fatal("unable to find %r parameter, use '--param=%s=VALUE'" % (key,key))
+
+import lit.llvm
+lit.llvm.initialize(lit_config, config)
+
+# Let the main config do the real work.
+lit_config.load_config(config, "@BOLT_SOURCE_DIR@/test/Unit/lit.cfg.py")
diff --git a/bolt/tools/driver/llvm-bolt.cpp b/bolt/tools/driver/llvm-bolt.cpp
--- a/bolt/tools/driver/llvm-bolt.cpp
+++ b/bolt/tools/driver/llvm-bolt.cpp
@@ -45,6 +45,11 @@
 static cl::OptionCategory *Perf2BoltCategories[] = {&AggregatorCategory,
                                                     &BoltOutputCategory};
 
+static cl::opt<std::string> InputFilename(cl::Positional,
+                                          cl::desc("<executable>"),
+                                          cl::Required, cl::cat(BoltCategory),
+                                          cl::sub(*cl::AllSubCommands));
+
 static cl::opt<std::string>
 InputDataFilename("data",
   cl::desc("<data file>"),
diff --git a/bolt/unittests/CMakeLists.txt b/bolt/unittests/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/bolt/unittests/CMakeLists.txt
@@ -0,0 +1,8 @@
+add_custom_target(BoltUnitTests)
+set_target_properties(BoltUnitTests PROPERTIES FOLDER "Bolt tests")
+
+function(add_bolt_unittest test_dirname)
+  add_unittest(BoltUnitTests ${test_dirname} ${ARGN})
+endfunction()
+
+add_subdirectory(Core)
diff --git a/bolt/unittests/Core/CMakeLists.txt b/bolt/unittests/Core/CMakeLists.txt
new file mode 100644
--- /dev/null
+++ b/bolt/unittests/Core/CMakeLists.txt
@@ -0,0 +1,17 @@
+include_directories(
+  ${CMAKE_SOURCE_DIR}/lib/Target/AArch64
+  ${CMAKE_BINARY_DIR}/lib/Target/AArch64
+  )
+
+set(LLVM_LINK_COMPONENTS
+  BOLTRewrite
+  )
+
+add_bolt_unittest(CoreTests
+  MCPlusBuilder.cpp
+  )
+
+string(FIND "${LLVM_TARGETS_TO_BUILD}" "AArch64" POSITION)
+if (NOT ${POSITION} EQUAL -1)
+  target_compile_definitions(CoreTests PRIVATE AARCH64_AVAILABLE)
+endif()
diff --git a/bolt/unittests/Core/MCPlusBuilder.cpp b/bolt/unittests/Core/MCPlusBuilder.cpp
new file mode 100644
--- /dev/null
+++ b/bolt/unittests/Core/MCPlusBuilder.cpp
@@ -0,0 +1,82 @@
+#include "AArch64Subtarget.h"
+#include "bolt/Rewrite/RewriteInstance.h"
+#include "llvm/BinaryFormat/ELF.h"
+#include "llvm/DebugInfo/DWARF/DWARFContext.h"
+#include "llvm/Object/ELFObjectFile.h"
+#include "llvm/Support/TargetSelect.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace llvm::object;
+using namespace llvm::ELF;
+using namespace bolt;
+
+namespace {
+struct MCPlusBuilderTester : public testing::TestWithParam<Triple::ArchType> {
+  void SetUp() override {
+    initalizeLLVM();
+    prepareElf();
+    initializeBolt();
+  }
+
+protected:
+  void initalizeLLVM() {
+    llvm::InitializeAllTargetInfos();
+    llvm::InitializeAllTargetMCs();
+    llvm::InitializeAllAsmParsers();
+    llvm::InitializeAllDisassemblers();
+    llvm::InitializeAllTargets();
+    llvm::InitializeAllAsmPrinters();
+  }
+
+  void prepareElf() {
+    memcpy(ElfBuf, "\177ELF", 4);
+    ELF64LE::Ehdr *EHdr = reinterpret_cast<typename ELF64LE::Ehdr *>(ElfBuf);
+    EHdr->e_ident[llvm::ELF::EI_CLASS] = llvm::ELF::ELFCLASS64;
+    EHdr->e_ident[llvm::ELF::EI_DATA] = llvm::ELF::ELFDATA2LSB;
+    EHdr->e_machine = GetParam() == Triple::aarch64 ? EM_AARCH64 : EM_X86_64;
+    MemoryBufferRef Source(StringRef(ElfBuf, sizeof(ElfBuf)), "ELF");
+    ObjFile = cantFail(ObjectFile::createObjectFile(Source));
+  }
+
+  void initializeBolt() {
+    BC = BinaryContext::createBinaryContext(
+        ObjFile.get(), true, DWARFContext::create(*ObjFile.get()));
+    ASSERT_FALSE(!BC);
+    BC->initializeTarget(std::unique_ptr<MCPlusBuilder>(createMCPlusBuilder(
+        GetParam(), BC->MIA.get(), BC->MII.get(), BC->MRI.get())));
+  }
+
+  char ElfBuf[sizeof(typename ELF64LE::Ehdr)] = {};
+  std::unique_ptr<ObjectFile> ObjFile;
+  std::unique_ptr<BinaryContext> BC;
+};
+} // namespace
+
+#ifdef AARCH64_AVAILABLE
+
+INSTANTIATE_TEST_SUITE_P(AArch64, MCPlusBuilderTester,
+                         ::testing::Values(Triple::aarch64));
+
+TEST_P(MCPlusBuilderTester, AliasX0) {
+  uint64_t AliasesX0[] = {AArch64::W0, AArch64::X0, AArch64::W0_W1,
+                          AArch64::X0_X1, AArch64::X0_X1_X2_X3_X4_X5_X6_X7};
+  size_t AliasesX0Count = sizeof(AliasesX0) / sizeof(*AliasesX0);
+  const BitVector &BV = BC->MIB->getAliases(AArch64::X0);
+  ASSERT_EQ(BV.count(), AliasesX0Count);
+  for (size_t I = 0; I < AliasesX0Count; ++I) {
+    ASSERT_TRUE(BV[AliasesX0[I]]);
+  }
+}
+
+TEST_P(MCPlusBuilderTester, AliasSmallerX0) {
+  uint64_t AliasesX0[] = {AArch64::W0, AArch64::X0};
+  size_t AliasesX0Count = sizeof(AliasesX0) / sizeof(*AliasesX0);
+  const BitVector &BV = BC->MIB->getAliases(AArch64::X0, /*OnlySmaller*/ true);
+  ASSERT_EQ(BV.count(), AliasesX0Count);
+  for (size_t I = 0; I < AliasesX0Count; ++I) {
+    ASSERT_TRUE(BV[AliasesX0[I]]);
+  }
+}
+
+#endif // AARCH64_AVAILABLE