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diff --git a/compiler-rt/lib/xray/xray_fdr_log_writer.h b/compiler-rt/lib/xray/xray_fdr_log_writer.h
new file mode 100644
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+++ b/compiler-rt/lib/xray/xray_fdr_log_writer.h
@@ -0,0 +1,231 @@
+//===-- xray_fdr_log_writer.h ---------------------------------------------===//
+//
+// 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 is a part of XRay, a function call tracing system.
+//
+//===----------------------------------------------------------------------===//
+#ifndef COMPILER_RT_LIB_XRAY_XRAY_FDR_LOG_WRITER_H_
+#define COMPILER_RT_LIB_XRAY_XRAY_FDR_LOG_WRITER_H_
+
+#include "xray_buffer_queue.h"
+#include "xray_fdr_log_records.h"
+#include <functional>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+namespace __xray {
+
+template <size_t Index> struct SerializerImpl {
+  template <class Tuple,
+            typename std::enable_if<
+                Index<std::tuple_size<
+                          typename std::remove_reference<Tuple>::type>::value,
+                      int>::type = 0> static void serializeTo(char *Buffer,
+                                                              Tuple &&T) {
+    auto P = reinterpret_cast<const char *>(&std::get<Index>(T));
+    constexpr auto Size = sizeof(std::get<Index>(T));
+    internal_memcpy(Buffer, P, Size);
+    SerializerImpl<Index + 1>::serializeTo(Buffer + Size,
+                                           std::forward<Tuple>(T));
+  }
+
+  template <class Tuple,
+            typename std::enable_if<
+                Index >= std::tuple_size<typename std::remove_reference<
+                             Tuple>::type>::value,
+                int>::type = 0>
+  static void serializeTo(char *, Tuple &&) {}
+};
+
+using Serializer = SerializerImpl<0>;
+
+template <class Tuple, size_t Index> struct AggregateSizesImpl {
+  static constexpr size_t value =
+      sizeof(typename std::tuple_element<Index, Tuple>::type) +
+      AggregateSizesImpl<Tuple, Index - 1>::value;
+};
+
+template <class Tuple> struct AggregateSizesImpl<Tuple, 0> {
+  static constexpr size_t value =
+      sizeof(typename std::tuple_element<0, Tuple>::type);
+};
+
+template <class Tuple> struct AggregateSizes {
+  static constexpr size_t value =
+      AggregateSizesImpl<Tuple, std::tuple_size<Tuple>::value - 1>::value;
+};
+
+template <MetadataRecord::RecordKinds Kind, class... DataTypes>
+MetadataRecord createMetadataRecord(DataTypes &&... Ds) {
+  static_assert(AggregateSizes<std::tuple<DataTypes...>>::value <=
+                    sizeof(MetadataRecord) - 1,
+                "Metadata payload longer than metadata buffer!");
+  MetadataRecord R;
+  R.Type = 1;
+  R.RecordKind = static_cast<uint8_t>(Kind);
+  Serializer::serializeTo(R.Data,
+                          std::make_tuple(std::forward<DataTypes>(Ds)...));
+  return R;
+}
+
+class FDRLogWriter {
+  BufferQueue::Buffer &Buffer;
+  char *NextRecord = nullptr;
+
+  template <class T> void writeRecord(const T &R) {
+    internal_memcpy(NextRecord, reinterpret_cast<const char *>(&R), sizeof(T));
+    NextRecord += sizeof(T);
+    // We need this atomic fence here to ensure that other threads attempting to
+    // read the bytes in the buffer will see the writes committed before the
+    // extents are updated.
+    atomic_thread_fence(memory_order_release);
+    atomic_fetch_add(Buffer.Extents, sizeof(T), memory_order_acq_rel);
+  }
+
+public:
+  explicit FDRLogWriter(BufferQueue::Buffer &B, char *P)
+      : Buffer(B), NextRecord(P) {
+    DCHECK_NE(Buffer.Data, nullptr);
+    DCHECK_NE(NextRecord, nullptr);
+  }
+
+  explicit FDRLogWriter(BufferQueue::Buffer &B)
+      : FDRLogWriter(B, static_cast<char *>(B.Data)) {}
+
+  template <MetadataRecord::RecordKinds Kind, class... Data>
+  bool writeMetadata(Data &&... Ds) {
+    // TODO: Check boundary conditions:
+    // 1) Buffer is full, and cannot handle one metadata record.
+    // 2) Buffer queue is finalising.
+    writeRecord(createMetadataRecord<Kind>(std::forward<Data>(Ds)...));
+    return true;
+  }
+
+  template <size_t N> size_t writeMetadataRecords(MetadataRecord (&Recs)[N]) {
+    constexpr auto Size = sizeof(MetadataRecord) * N;
+    internal_memcpy(NextRecord, reinterpret_cast<const char *>(Recs), Size);
+    NextRecord += Size;
+    // We need this atomic fence here to ensure that other threads attempting to
+    // read the bytes in the buffer will see the writes committed before the
+    // extents are updated.
+    atomic_thread_fence(memory_order_release);
+    atomic_fetch_add(Buffer.Extents, Size, memory_order_acq_rel);
+    return Size;
+  }
+
+  enum class FunctionRecordKind : uint8_t {
+    Enter = 0x00,
+    Exit = 0x01,
+    TailExit = 0x02,
+    EnterArg = 0x03,
+  };
+
+  bool writeFunction(FunctionRecordKind Kind, int32_t FuncId, int32_t Delta) {
+    FunctionRecord R;
+    R.Type = 0;
+    R.RecordKind = uint8_t(Kind);
+    R.FuncId = FuncId;
+    R.TSCDelta = Delta;
+    writeRecord(R);
+    return true;
+  }
+
+  bool writeFunctionWithArg(FunctionRecordKind Kind, int32_t FuncId,
+                            int32_t Delta, uint64_t Arg) {
+    // We need to write the function with arg into the buffer, and then
+    // atomically update the buffer extents. This ensures that any reads
+    // synchronised on the buffer extents record will always see the writes
+    // that happen before the atomic update.
+    FunctionRecord R;
+    R.Type = 0;
+    R.RecordKind = uint8_t(Kind);
+    R.FuncId = FuncId;
+    R.TSCDelta = Delta;
+    MetadataRecord A =
+        createMetadataRecord<MetadataRecord::RecordKinds::CallArgument>(Arg);
+    NextRecord = reinterpret_cast<char *>(internal_memcpy(
+                     NextRecord, reinterpret_cast<char *>(&R), sizeof(R))) +
+                 sizeof(R);
+    NextRecord = reinterpret_cast<char *>(internal_memcpy(
+                     NextRecord, reinterpret_cast<char *>(&A), sizeof(A))) +
+                 sizeof(A);
+    // We need this atomic fence here to ensure that other threads attempting to
+    // read the bytes in the buffer will see the writes committed before the
+    // extents are updated.
+    atomic_thread_fence(memory_order_release);
+    atomic_fetch_add(Buffer.Extents, sizeof(R) + sizeof(A),
+                     memory_order_acq_rel);
+    return true;
+  }
+
+  bool writeCustomEvent(int32_t Delta, const void *Event, int32_t EventSize) {
+    // We write the metadata record and the custom event data into the buffer
+    // first, before we atomically update the extents for the buffer. This
+    // allows us to ensure that any threads reading the extents of the buffer
+    // will only ever see the full metadata and custom event payload accounted
+    // (no partial writes accounted).
+    MetadataRecord R =
+        createMetadataRecord<MetadataRecord::RecordKinds::CustomEventMarker>(
+            EventSize, Delta);
+    NextRecord = reinterpret_cast<char *>(internal_memcpy(
+                     NextRecord, reinterpret_cast<char *>(&R), sizeof(R))) +
+                 sizeof(R);
+    NextRecord = reinterpret_cast<char *>(
+                     internal_memcpy(NextRecord, Event, EventSize)) +
+                 EventSize;
+
+    // We need this atomic fence here to ensure that other threads attempting to
+    // read the bytes in the buffer will see the writes committed before the
+    // extents are updated.
+    atomic_thread_fence(memory_order_release);
+    atomic_fetch_add(Buffer.Extents, sizeof(R) + EventSize,
+                     memory_order_acq_rel);
+    return true;
+  }
+
+  bool writeTypedEvent(int32_t Delta, uint16_t EventType, const void *Event,
+                       int32_t EventSize) {
+    // We do something similar when writing out typed events, see
+    // writeCustomEvent(...) above for details.
+    MetadataRecord R =
+        createMetadataRecord<MetadataRecord::RecordKinds::TypedEventMarker>(
+            EventSize, Delta, EventType);
+    NextRecord = reinterpret_cast<char *>(internal_memcpy(
+                     NextRecord, reinterpret_cast<char *>(&R), sizeof(R))) +
+                 sizeof(R);
+    NextRecord = reinterpret_cast<char *>(
+                     internal_memcpy(NextRecord, Event, EventSize)) +
+                 EventSize;
+
+    // We need this atomic fence here to ensure that other threads attempting to
+    // read the bytes in the buffer will see the writes committed before the
+    // extents are updated.
+    atomic_thread_fence(memory_order_release);
+    atomic_fetch_add(Buffer.Extents, EventSize, memory_order_acq_rel);
+    return true;
+  }
+
+  char *getNextRecord() const { return NextRecord; }
+
+  void resetRecord() {
+    NextRecord = reinterpret_cast<char *>(Buffer.Data);
+    atomic_store(Buffer.Extents, 0, memory_order_release);
+  }
+
+  void undoWrites(size_t B) {
+    DCHECK_GE(NextRecord - B, reinterpret_cast<char *>(Buffer.Data));
+    NextRecord -= B;
+    atomic_fetch_sub(Buffer.Extents, B, memory_order_acq_rel);
+  }
+
+}; // namespace __xray
+
+} // namespace __xray
+
+#endif // COMPILER-RT_LIB_XRAY_XRAY_FDR_LOG_WRITER_H_