src: convert all endian checks to constexpr

This is finally possible in C++20 without having to rely on
compiler-defined macros, assuming none of our supported platforms are
mixed-endian.

Refs: https://github.com/nodejs/node/pull/44411
PR-URL: https://github.com/nodejs/node/pull/52974
Reviewed-By: Yagiz Nizipli <yagiz.nizipli@sentry.io>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_buffer.cc

@@ -666,7 +666,7 @@ void Fill(const FunctionCallbackInfo<Value>& args) {
   } else if (enc == UCS2) {
     str_length = str_obj->Length() * sizeof(uint16_t);
     node::TwoByteValue str(env->isolate(), args[1]);
-    if (IsBigEndian())
+    if constexpr (IsBigEndian())
       SwapBytes16(reinterpret_cast<char*>(&str[0]), str_length);
 
     memcpy(ts_obj_data + start, *str, std::min(str_length, fill_length));
@@ -960,7 +960,7 @@ void IndexOfString(const FunctionCallbackInfo<Value>& args) {
       return args.GetReturnValue().Set(-1);
     }
 
-    if (IsBigEndian()) {
+    if constexpr (IsBigEndian()) {
       StringBytes::InlineDecoder decoder;
       if (decoder.Decode(env, needle, enc).IsNothing()) return;
       const uint16_t* decoded_string =

src/node_file.cc

@@ -2262,12 +2262,14 @@ static void WriteString(const FunctionCallbackInfo<Value>& args) {
       auto ext = string->GetExternalOneByteStringResource();
       buf = const_cast<char*>(ext->data());
       len = ext->length();
-    } else if (enc == UCS2 && IsLittleEndian() && string->IsExternalTwoByte()) {
+    } else if (enc == UCS2 && string->IsExternalTwoByte()) {
+      if constexpr (IsLittleEndian()) {
         auto ext = string->GetExternalStringResource();
         buf = reinterpret_cast<char*>(const_cast<uint16_t*>(ext->data()));
         len = ext->length() * sizeof(*ext->data());
       }
     }
+  }
 
   if (is_async) {  // write(fd, string, pos, enc, req)
     CHECK_NOT_NULL(req_wrap_async);

src/node_i18n.cc

@@ -111,7 +111,7 @@ MaybeLocal<Object> ToBufferEndian(Environment* env, MaybeStackBuffer<T>* buf) {
 
   static_assert(sizeof(T) == 1 || sizeof(T) == 2,
                 "Currently only one- or two-byte buffers are supported");
-  if (sizeof(T) > 1 && IsBigEndian()) {
+  if constexpr (sizeof(T) > 1 && IsBigEndian()) {
     SPREAD_BUFFER_ARG(ret.ToLocalChecked(), retbuf);
     SwapBytes16(retbuf_data, retbuf_length);
   }
@@ -128,7 +128,7 @@ void CopySourceBuffer(MaybeStackBuffer<UChar>* dest,
   dest->AllocateSufficientStorage(length_in_chars);
   char* dst = reinterpret_cast<char*>(**dest);
   memcpy(dst, data, length);
-  if (IsBigEndian()) {
+  if constexpr (IsBigEndian()) {
     SwapBytes16(dst, length);
   }
 }
@@ -527,7 +527,7 @@ void ConverterObject::Decode(const FunctionCallbackInfo<Value>& args) {
 
     char* value = reinterpret_cast<char*>(output) + beginning;
 
-    if (IsBigEndian()) {
+    if constexpr (IsBigEndian()) {
       SwapBytes16(value, length);
     }
 

src/string_bytes.cc

@@ -339,8 +339,7 @@ size_t StringBytes::Write(Isolate* isolate,
       // the Buffer, so we need to reorder on BE platforms.  See
       // https://nodejs.org/api/buffer.html regarding Node's "ucs2"
       // encoding specification
-      if (IsBigEndian())
-        SwapBytes16(buf, nbytes);
+      if constexpr (IsBigEndian()) SwapBytes16(buf, nbytes);
 
       break;
     }
@@ -756,7 +755,7 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
 
     case UCS2: {
       size_t str_len = buflen / 2;
-      if (IsBigEndian()) {
+      if constexpr (IsBigEndian()) {
         uint16_t* dst = node::UncheckedMalloc<uint16_t>(str_len);
         if (str_len != 0 && dst == nullptr) {
           *error = node::ERR_MEMORY_ALLOCATION_FAILED(isolate);
@@ -803,7 +802,7 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
   // Buffer, so we need to reorder on BE platforms.  See
   // https://nodejs.org/api/buffer.html regarding Node's "ucs2"
   // encoding specification
-  if (IsBigEndian()) {
+  if constexpr (IsBigEndian()) {
     uint16_t* dst = node::UncheckedMalloc<uint16_t>(buflen);
     if (dst == nullptr) {
       *error = node::ERR_MEMORY_ALLOCATION_FAILED(isolate);

src/util.h

@@ -37,6 +37,7 @@
 #include <cstring>
 
 #include <array>
+#include <bit>
 #include <limits>
 #include <memory>
 #include <set>
@@ -778,24 +779,16 @@ inline v8::MaybeLocal<v8::Value> ToV8Value(v8::Local<v8::Context> context,
         .Check();                                                              \
   } while (0)
 
-enum class Endianness { LITTLE, BIG };
-
-inline Endianness GetEndianness() {
-  // Constant-folded by the compiler.
-  const union {
-    uint8_t u8[2];
-    uint16_t u16;
-  } u = {{1, 0}};
-  return u.u16 == 1 ? Endianness::LITTLE : Endianness::BIG;
+constexpr inline bool IsLittleEndian() {
+  return std::endian::native == std::endian::little;
 }
 
-inline bool IsLittleEndian() {
-  return GetEndianness() == Endianness::LITTLE;
+constexpr inline bool IsBigEndian() {
+  return std::endian::native == std::endian::big;
 }
 
-inline bool IsBigEndian() {
-  return GetEndianness() == Endianness::BIG;
-}
+static_assert(IsLittleEndian() || IsBigEndian(),
+              "Node.js does not support mixed-endian systems");
 
 // Round up a to the next highest multiple of b.
 template <typename T>