diff --git a/libc/src/__support/endian.h b/libc/src/__support/endian.h
new file mode 100644
--- /dev/null
+++ b/libc/src/__support/endian.h
@@ -0,0 +1,67 @@
+//===-- Endianness support ------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_SUPPORT_ENDIAN_H
+#define LLVM_LIBC_SRC_SUPPORT_ENDIAN_H
+
+#include <cstdint>
+
+namespace __llvm_libc {
+
+#if defined(LLVM_LIBC_IS_BIG_ENDIAN)
+#error "LLVM_LIBC_IS_BIG_ENDIAN cannot be defined directly."
+#endif
+#if defined(LLVM_LIBC_IS_LITTLE_ENDIAN)
+#error "LLVM_LIBC_IS_LITTLE_ENDIAN cannot be defined directly."
+#endif
+
+// We rely on compiler preprocessor defines to allow for cross compilation.
+#if !defined(__BYTE_ORDER__) || !defined(__ORDER_LITTLE_ENDIAN__) ||           \
+    !defined(__ORDER_BIG_ENDIAN__)
+#error "Missing preprocessor definitions for endianness detection."
+#endif
+
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+#define LLVM_LIBC_IS_LITTLE_ENDIAN 1
+#else
+#define LLVM_LIBC_IS_LITTLE_ENDIAN 0
+#endif
+
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define LLVM_LIBC_IS_BIG_ENDIAN 1
+#else
+#define LLVM_LIBC_IS_BIG_ENDIAN 0
+#endif
+
+// Converts the value to big endian representation. This is a no-op on big
+// endian architecture. We use template specialization to prevent accidental
+// integer promotion.
+template <typename T> static inline T ToBigEndian(T value);
+
+#if LLVM_LIBC_IS_LITTLE_ENDIAN
+template <> inline uint8_t ToBigEndian(uint8_t value) { return value; }
+template <> inline uint16_t ToBigEndian(uint16_t value) {
+  return __builtin_bswap16(value);
+}
+template <> inline uint32_t ToBigEndian(uint32_t value) {
+  return __builtin_bswap32(value);
+}
+template <> inline uint64_t ToBigEndian(uint64_t value) {
+  return __builtin_bswap64(value);
+}
+#endif
+#if LLVM_LIBC_IS_BIG_ENDIAN
+template <> inline uint8_t ToBigEndian(uint8_t value) { return value; }
+template <> inline uint16_t ToBigEndian(uint16_t value) { return value; }
+template <> inline uint32_t ToBigEndian(uint32_t value) { return value; }
+template <> inline uint64_t ToBigEndian(uint64_t value) { return value; }
+#endif
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_SUPPORT_ENDIAN_H