diff --git a/bolt/include/bolt/Core/MCPlusAnnotation.h b/bolt/include/bolt/Core/MCPlusAnnotation.h
new file mode 100644
--- /dev/null
+++ b/bolt/include/bolt/Core/MCPlusAnnotation.h
@@ -0,0 +1,41 @@
+//===- bolt/Core/MCPlusAnnotation.h - Interface for MCPlus ------*- 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 contains the definitions of annotation encoding/decoding functions.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef BOLT_CORE_MCPLUSANNOTATION_H
+#define BOLT_CORE_MCPLUSANNOTATION_H
+
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/MathExtras.h"
+
+/// We encode Index and Value into a 64-bit immediate operand value.
+inline int64_t encodeAnnotationImm(uint8_t Index, int64_t Value) {
+  if (LLVM_UNLIKELY(Value !=
+                    llvm::SignExtend64<56>(Value & 0xff'ffff'ffff'ffff)))
+    llvm::report_fatal_error("annotation value out of range");
+
+  Value &= 0xff'ffff'ffff'ffff;
+  Value |= (int64_t)Index << 56;
+
+  return Value;
+}
+
+/// Extract annotation index from immediate operand value.
+inline uint8_t extractAnnotationIndex(int64_t ImmValue) {
+  return ImmValue >> 56;
+}
+
+/// Extract annotation value from immediate operand value.
+inline int64_t extractAnnotationValue(int64_t ImmValue) {
+  return llvm::SignExtend64<56>(ImmValue & 0xff'ffff'ffff'ffffULL);
+}
+
+#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
@@ -15,6 +15,7 @@
 #define BOLT_CORE_MCPLUSBUILDER_H
 
 #include "bolt/Core/MCPlus.h"
+#include "bolt/Core/MCPlusAnnotation.h"
 #include "bolt/Core/Relocation.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/BitVector.h"
@@ -73,28 +74,6 @@
   /// A variable that is used to generate unique ids for annotation allocators
   AllocatorIdTy MaxAllocatorId = 0;
 
-  /// We encode Index and Value into a 64-bit immediate operand value.
-  static int64_t encodeAnnotationImm(unsigned Index, int64_t Value) {
-    assert(Index < 256 && "annotation index max value exceeded");
-    assert((Value == (Value << 8) >> 8) && "annotation value out of range");
-
-    Value &= 0xff'ffff'ffff'ffff;
-    Value |= (int64_t)Index << 56;
-
-    return Value;
-  }
-
-  /// Extract annotation index from immediate operand value.
-  static unsigned extractAnnotationIndex(int64_t ImmValue) {
-    return ImmValue >> 56;
-  }
-
-  /// Extract annotation value from immediate operand value.
-  static int64_t extractAnnotationValue(int64_t ImmValue) {
-    ImmValue &= 0xff'ffff'ffff'ffff;
-    return (ImmValue << 8) >> 8;
-  }
-
   MCInst *getAnnotationInst(const MCInst &Inst) const {
     if (Inst.getNumOperands() == 0)
       return nullptr;
diff --git a/bolt/unittests/Core/CMakeLists.txt b/bolt/unittests/Core/CMakeLists.txt
--- a/bolt/unittests/Core/CMakeLists.txt
+++ b/bolt/unittests/Core/CMakeLists.txt
@@ -1,8 +1,10 @@
 set(LLVM_LINK_COMPONENTS
   BOLTCore
   BOLTRewrite
+  BOLTUtils
   DebugInfoDWARF
   Object
+  MC
   ${LLVM_TARGETS_TO_BUILD}
   )
 
diff --git a/bolt/unittests/Core/MCPlusBuilder.cpp b/bolt/unittests/Core/MCPlusBuilder.cpp
--- a/bolt/unittests/Core/MCPlusBuilder.cpp
+++ b/bolt/unittests/Core/MCPlusBuilder.cpp
@@ -6,6 +6,7 @@
 #include "X86Subtarget.h"
 #endif // X86_AVAILABLE
 
+#include "bolt/Core/MCPlusAnnotation.h"
 #include "bolt/Rewrite/RewriteInstance.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"
@@ -110,3 +111,18 @@
 }
 
 #endif // X86_AVAILABLE
+
+TEST_P(MCPlusBuilderTester, Annotation) {
+  // Round-trip encoding-decoding check for negative values
+  int64_t Value = INT32_MIN;
+  uint8_t Index = 123;
+  int64_t EncodedAnnotation = encodeAnnotationImm(Index, Value);
+  int64_t DecodedValue = extractAnnotationValue(EncodedAnnotation);
+  uint8_t DecodedIndex = extractAnnotationIndex(EncodedAnnotation);
+  ASSERT_EQ(Value, DecodedValue);
+  ASSERT_EQ(Index, DecodedIndex);
+
+  // Large int64 should trigger an out of range assertion
+  Value = 0x1FF'FFFF'FFFF'FFFFULL;
+  ASSERT_DEATH(encodeAnnotationImm(0, Value), "annotation value out of range");
+}