Remove folly::Endian::little from JSIndexedRAMBundle (#57109)

Summary:
Pull Request resolved: #57109

changelog: [internal]

The RAM bundle format stores offsets and lengths in little-endian, which previously required pulling in folly's byte-swapping library. This removes the folly dependency by implementing a simple little-endian decoder inline, cutting the build graph and making the code more self-contained.

Reviewed By: javache

Differential Revision: D107870286

fbshipit-source-id: c6f8399785189fad743123ecc51434c0feff9706

Author: sammy-SC

packages/react-native/ReactCommon/cxxreact/JSIndexedRAMBundle.cpp

@@ -9,16 +9,26 @@
 
 #ifndef RCT_REMOVE_LEGACY_ARCH
 
-#include <glog/logging.h>
+#include <bit>
+#include <cstdint>
 #include <fstream>
 #include <memory>
 #include <sstream>
 
-#include <folly/lang/Bits.h>
 #include <glog/logging.h>
 
 namespace facebook::react {
 
+namespace {
+uint32_t littleEndianToHost(uint32_t value) {
+  if constexpr (std::endian::native == std::endian::big) {
+    return (value << 24) | ((value & 0x0000FF00U) << 8) |
+        ((value & 0x00FF0000U) >> 8) | (value >> 24);
+  }
+  return value;
+}
+} // namespace
+
 std::function<std::unique_ptr<JSModulesUnbundle>(std::string)>
 JSIndexedRAMBundle::buildFactory() {
   return [](const std::string& bundlePath) {
@@ -60,8 +70,8 @@ void JSIndexedRAMBundle::init() {
       "header size must exactly match the input file format");
 
   readBundle(reinterpret_cast<char*>(header), sizeof(header));
-  size_t numTableEntries = folly::Endian::little(header[1]);
-  std::streamsize startupCodeSize = folly::Endian::little(header[2]);
+  size_t numTableEntries = littleEndianToHost(header[1]);
+  std::streamsize startupCodeSize = littleEndianToHost(header[2]);
 
   // allocate memory for meta data and lookup table.
   m_table = ModuleTable(numTableEntries);
@@ -95,7 +105,7 @@ std::string JSIndexedRAMBundle::getModuleCode(const uint32_t id) const {
 
   // entries without associated code have offset = 0 and length = 0
   const uint32_t length =
-      moduleData != nullptr ? folly::Endian::little(moduleData->length) : 0;
+      moduleData != nullptr ? littleEndianToHost(moduleData->length) : 0;
   if (length == 0) {
     throw std::ios_base::failure(
         "Error loading module" + std::to_string(id) + "from RAM Bundle");
@@ -105,7 +115,7 @@ std::string JSIndexedRAMBundle::getModuleCode(const uint32_t id) const {
   readBundle(
       &ret.front(),
       length - 1,
-      m_baseOffset + folly::Endian::little(moduleData->offset));
+      m_baseOffset + littleEndianToHost(moduleData->offset));
   return ret;
 }
 

packages/react-native/ReactCommon/cxxreact/tests/JSIndexedRAMBundleTest.cpp

@@ -0,0 +1,76 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <cstdint>
+#include <memory>
+#include <string>
+
+#include <cxxreact/JSBigString.h>
+#include <cxxreact/JSIndexedRAMBundle.h>
+#include <gtest/gtest.h>
+
+#ifndef RCT_REMOVE_LEGACY_ARCH
+
+using namespace facebook::react;
+
+namespace {
+constexpr uint32_t kRAMBundleMagic = 0xFB0BD1E5;
+
+void appendLittleEndian32(std::string& out, uint32_t value) {
+  out.push_back(static_cast<char>(value & 0xFF));
+  out.push_back(static_cast<char>((value >> 8) & 0xFF));
+  out.push_back(static_cast<char>((value >> 16) & 0xFF));
+  out.push_back(static_cast<char>((value >> 24) & 0xFF));
+}
+
+void appendNullTerminatedSection(std::string& out, const std::string& section) {
+  out += section;
+  out.push_back('\0');
+}
+} // namespace
+
+TEST(JSIndexedRAMBundleTest, ReadsLittleEndianStartupAndModules) {
+  const std::string startup = "var startup = true;";
+  const std::string moduleZero = "module zero code";
+  const std::string moduleOne = "m1";
+
+  const auto startupCodeSize = static_cast<uint32_t>(startup.size() + 1);
+  const auto lengthZero = static_cast<uint32_t>(moduleZero.size() + 1);
+  const auto lengthOne = static_cast<uint32_t>(moduleOne.size() + 1);
+  const uint32_t offsetZero = startupCodeSize;
+  const uint32_t offsetOne = offsetZero + lengthZero;
+
+  std::string bundle;
+  appendLittleEndian32(bundle, kRAMBundleMagic);
+  appendLittleEndian32(bundle, 2);
+  appendLittleEndian32(bundle, startupCodeSize);
+  appendLittleEndian32(bundle, offsetZero);
+  appendLittleEndian32(bundle, lengthZero);
+  appendLittleEndian32(bundle, offsetOne);
+  appendLittleEndian32(bundle, lengthOne);
+  appendNullTerminatedSection(bundle, startup);
+  appendNullTerminatedSection(bundle, moduleZero);
+  appendNullTerminatedSection(bundle, moduleOne);
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+  JSIndexedRAMBundle ramBundle(std::make_unique<JSBigStdString>(bundle));
+
+  const auto startupCode = ramBundle.getStartupCode();
+  EXPECT_EQ(std::string(startupCode->c_str(), startupCode->size()), startup);
+
+  const auto firstModule = ramBundle.getModule(0);
+  EXPECT_EQ(firstModule.name, "0.js");
+  EXPECT_EQ(firstModule.code, moduleZero);
+
+  const auto secondModule = ramBundle.getModule(1);
+  EXPECT_EQ(secondModule.name, "1.js");
+  EXPECT_EQ(secondModule.code, moduleOne);
+#pragma GCC diagnostic pop
+}
+
+#endif // RCT_REMOVE_LEGACY_ARCH