diff --git a/clang-tools-extra/clangd/refactor/Rename.h b/clang-tools-extra/clangd/refactor/Rename.h
--- a/clang-tools-extra/clangd/refactor/Rename.h
+++ b/clang-tools-extra/clangd/refactor/Rename.h
@@ -47,6 +47,13 @@
 /// in another file (per the index).
 llvm::Expected<FileEdits> rename(const RenameInputs &RInputs);
 
+/// Generates rename edits that replaces all given occurrences with the
+/// NewName.
+/// Exposed for testing only.
+llvm::Expected<Edit> buildRenameEdit(llvm::StringRef InitialCode,
+                                     std::vector<Range> Occurrences,
+                                     llvm::StringRef NewName);
+
 } // namespace clangd
 } // namespace clang
 
diff --git a/clang-tools-extra/clangd/refactor/Rename.cpp b/clang-tools-extra/clangd/refactor/Rename.cpp
--- a/clang-tools-extra/clangd/refactor/Rename.cpp
+++ b/clang-tools-extra/clangd/refactor/Rename.cpp
@@ -20,6 +20,7 @@
 #include "clang/Tooling/Refactoring/Rename/USRFindingAction.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/FormatVariadic.h"
 
 namespace clang {
 namespace clangd {
@@ -297,34 +298,6 @@
   return AffectedFiles;
 }
 
-llvm::Expected<std::pair<size_t, size_t>> toRangeOffset(const clangd::Range &R,
-                                                        llvm::StringRef Code) {
-  auto StartOffset = positionToOffset(Code, R.start);
-  if (!StartOffset)
-    return StartOffset.takeError();
-  auto EndOffset = positionToOffset(Code, R.end);
-  if (!EndOffset)
-    return EndOffset.takeError();
-  return std::make_pair(*StartOffset, *EndOffset);
-};
-
-llvm::Expected<Edit> buildRenameEdit(llvm::StringRef InitialCode,
-                                     const std::vector<Range> &Occurrences,
-                                     llvm::StringRef NewName) {
-  tooling::Replacements RenameEdit;
-  for (const Range &Occurrence : Occurrences) {
-    // FIXME: !positionToOffset is O(N), optimize it.
-    auto RangeOffset = toRangeOffset(Occurrence, InitialCode);
-    if (!RangeOffset)
-      return RangeOffset.takeError();
-    auto ByteLength = RangeOffset->second - RangeOffset->first;
-    if (auto Err = RenameEdit.add(tooling::Replacement(
-            InitialCode, RangeOffset->first, ByteLength, NewName)))
-      return std::move(Err);
-  }
-  return Edit(InitialCode, std::move(RenameEdit));
-}
-
 // Index-based rename, it renames all occurrences outside of the main file.
 //
 // The cross-file rename is purely based on the index, as we don't want to
@@ -358,7 +331,7 @@
         llvm::inconvertibleErrorCode());
 
   FileEdits Results;
-  for (const auto &FileAndOccurrences : AffectedFiles) {
+  for (auto &FileAndOccurrences : AffectedFiles) {
     llvm::StringRef FilePath = FileAndOccurrences.first();
 
     auto AffectedFileCode = GetFileContent(FilePath);
@@ -366,11 +339,14 @@
       elog("Fail to read file content: {0}", AffectedFileCode.takeError());
       continue;
     }
-
-    auto RenameEdit = buildRenameEdit(*AffectedFileCode,
-                                      FileAndOccurrences.getValue(), NewName);
-    if (!RenameEdit)
-      return RenameEdit.takeError();
+    auto RenameEdit = buildRenameEdit(
+        *AffectedFileCode, std::move(FileAndOccurrences.second), NewName);
+    if (!RenameEdit) {
+      return llvm::make_error<llvm::StringError>(
+          llvm::formatv("fail to build rename edit for file {0}: {1}", FilePath,
+                        llvm::toString(RenameEdit.takeError())),
+          llvm::inconvertibleErrorCode());
+    }
     if (!RenameEdit->Replacements.empty())
       Results.insert({FilePath, std::move(*RenameEdit)});
   }
@@ -465,5 +441,51 @@
   return Results;
 }
 
+llvm::Expected<Edit> buildRenameEdit(llvm::StringRef InitialCode,
+                                     std::vector<Range> Occurrences,
+                                     llvm::StringRef NewName) {
+  llvm::sort(Occurrences);
+  // These two always correspond to the same position.
+  Position LastPos{0, 0};
+  size_t LastOffset = 0;
+
+  auto Offset = [&](const Position &P) -> llvm::Expected<size_t> {
+    assert(LastPos <= P && "malformed input");
+    Position Shifted = {
+        P.line - LastPos.line,
+        P.line > LastPos.line ? P.character : P.character - LastPos.character};
+    auto ShiftedOffset =
+        positionToOffset(InitialCode.substr(LastOffset), Shifted);
+    if (!ShiftedOffset)
+      return llvm::make_error<llvm::StringError>(
+          llvm::formatv("fail to convert the position {0} to offset ({1})", P,
+                        llvm::toString(ShiftedOffset.takeError())),
+          llvm::inconvertibleErrorCode());
+    LastPos = P;
+    LastOffset += *ShiftedOffset;
+    return LastOffset;
+  };
+
+  std::vector<std::pair</*start*/ size_t, /*end*/ size_t>> OccurrencesOffsets;
+  for (const auto &R : Occurrences) {
+    auto StartOffset = Offset(R.start);
+    if (!StartOffset)
+      return StartOffset.takeError();
+    auto EndOffset = Offset(R.end);
+    if (!EndOffset)
+      return EndOffset.takeError();
+    OccurrencesOffsets.push_back({*StartOffset, *EndOffset});
+  }
+
+  tooling::Replacements RenameEdit;
+  for (const auto &R : OccurrencesOffsets) {
+    auto ByteLength = R.second - R.first;
+    if (auto Err = RenameEdit.add(
+            tooling::Replacement(InitialCode, R.first, ByteLength, NewName)))
+      return std::move(Err);
+  }
+  return Edit(InitialCode, std::move(RenameEdit));
+}
+
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/RenameTests.cpp b/clang-tools-extra/clangd/unittests/RenameTests.cpp
--- a/clang-tools-extra/clangd/unittests/RenameTests.cpp
+++ b/clang-tools-extra/clangd/unittests/RenameTests.cpp
@@ -638,6 +638,33 @@
       UnorderedElementsAre(Pair(Eq(Path), Eq(expectedResult(Code, NewName)))));
 }
 
+TEST(CrossFileRenameTests, BuildRenameEdits) {
+  Annotations Code("[[😂]]");
+  auto LSPRange = Code.range();
+  auto Edit = buildRenameEdit(Code.code(), {LSPRange}, "abc");
+  ASSERT_TRUE(bool(Edit)) << Edit.takeError();
+  ASSERT_EQ(1UL, Edit->Replacements.size());
+  EXPECT_EQ(4UL, Edit->Replacements.begin()->getLength());
+
+  // Test invalid range.
+  LSPRange.end = {-1, 0}; // out of range
+  Edit = buildRenameEdit(Code.code(), {LSPRange}, "abc");
+  EXPECT_FALSE(Edit);
+  EXPECT_THAT(llvm::toString(Edit.takeError()),
+              testing::HasSubstr("fail to convert"));
+
+  // Normal ascii characters.
+  Annotations T(R"cpp(
+    [[range]]
+              [[range]]
+      [[range]]
+  )cpp");
+  Edit = buildRenameEdit(T.code(), T.ranges(), "abc");
+  ASSERT_TRUE(bool(Edit)) << Edit.takeError();
+  EXPECT_EQ(applyEdits(FileEdits{{T.code(), std::move(*Edit)}}).front().second,
+            expectedResult(Code, expectedResult(T, "abc")));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang