Remove leveldb::port::kLittleEndian.

Clang 10 includes the optimizations described in
https://bugs.llvm.org/show_bug.cgi?id=41761. This means that the
platform-independent implementations of {Decode,Encode}Fixed{32,64}()
compile to one instruction on the most recent Clang and GCC.

PiperOrigin-RevId: 306330166

Author: pwnall

CMakeLists.txt

@@ -34,9 +34,6 @@ option(LEVELDB_BUILD_TESTS "Build LevelDB's unit tests" ON)
 option(LEVELDB_BUILD_BENCHMARKS "Build LevelDB's benchmarks" ON)
 option(LEVELDB_INSTALL "Install LevelDB's header and library" ON)
 
-include(TestBigEndian)
-test_big_endian(LEVELDB_IS_BIG_ENDIAN)
-
 include(CheckIncludeFile)
 check_include_file("unistd.h" HAVE_UNISTD_H)
 

port/port_config.h.in

@@ -30,10 +30,4 @@
 #cmakedefine01 HAVE_SNAPPY
 #endif  // !defined(HAVE_SNAPPY)
 
-// Define to 1 if your processor stores words with the most significant byte
-// first (like Motorola and SPARC, unlike Intel and VAX).
-#if !defined(LEVELDB_IS_BIG_ENDIAN)
-#cmakedefine01 LEVELDB_IS_BIG_ENDIAN
-#endif  // !defined(LEVELDB_IS_BIG_ENDIAN)
-
 #endif  // STORAGE_LEVELDB_PORT_PORT_CONFIG_H_

port/port_example.h

@@ -18,10 +18,6 @@ namespace port {
 // TODO(jorlow): Many of these belong more in the environment class rather than
 //               here. We should try moving them and see if it affects perf.
 
-// The following boolean constant must be true on a little-endian machine
-// and false otherwise.
-static const bool kLittleEndian = true /* or some other expression */;
-
 // ------------------ Threading -------------------
 
 // A Mutex represents an exclusive lock.

port/port_stdcxx.h

@@ -41,8 +41,6 @@
 namespace leveldb {
 namespace port {
 
-static const bool kLittleEndian = !LEVELDB_IS_BIG_ENDIAN;
-
 class CondVar;
 
 // Thinly wraps std::mutex.

util/coding.h

@@ -48,29 +48,13 @@ int VarintLength(uint64_t v);
 char* EncodeVarint32(char* dst, uint32_t value);
 char* EncodeVarint64(char* dst, uint64_t value);
 
-// TODO(costan): Remove port::kLittleEndian and the fast paths based on
-//               std::memcpy when clang learns to optimize the generic code, as
-//               described in https://bugs.llvm.org/show_bug.cgi?id=41761
-//
-// The platform-independent code in DecodeFixed{32,64}() gets optimized to mov
-// on x86 and ldr on ARM64, by both clang and gcc. However, only gcc optimizes
-// the platform-independent code in EncodeFixed{32,64}() to mov / str.
-
 // Lower-level versions of Put... that write directly into a character buffer
 // REQUIRES: dst has enough space for the value being written
 
 inline void EncodeFixed32(char* dst, uint32_t value) {
   uint8_t* const buffer = reinterpret_cast<uint8_t*>(dst);
 
-  if (port::kLittleEndian) {
-    // Fast path for little-endian CPUs. All major compilers optimize this to a
-    // single mov (x86_64) / str (ARM) instruction.
-    std::memcpy(buffer, &value, sizeof(uint32_t));
-    return;
-  }
-
-  // Platform-independent code.
-  // Currently, only gcc optimizes this to a single mov / str instruction.
+  // Recent clang and gcc optimize this to a single mov / str instruction.
   buffer[0] = static_cast<uint8_t>(value);
   buffer[1] = static_cast<uint8_t>(value >> 8);
   buffer[2] = static_cast<uint8_t>(value >> 16);
@@ -80,15 +64,7 @@ inline void EncodeFixed32(char* dst, uint32_t value) {
 inline void EncodeFixed64(char* dst, uint64_t value) {
   uint8_t* const buffer = reinterpret_cast<uint8_t*>(dst);
 
-  if (port::kLittleEndian) {
-    // Fast path for little-endian CPUs. All major compilers optimize this to a
-    // single mov (x86_64) / str (ARM) instruction.
-    std::memcpy(buffer, &value, sizeof(uint64_t));
-    return;
-  }
-
-  // Platform-independent code.
-  // Currently, only gcc optimizes this to a single mov / str instruction.
+  // Recent clang and gcc optimize this to a single mov / str instruction.
   buffer[0] = static_cast<uint8_t>(value);
   buffer[1] = static_cast<uint8_t>(value >> 8);
   buffer[2] = static_cast<uint8_t>(value >> 16);
@@ -105,16 +81,7 @@ inline void EncodeFixed64(char* dst, uint64_t value) {
 inline uint32_t DecodeFixed32(const char* ptr) {
   const uint8_t* const buffer = reinterpret_cast<const uint8_t*>(ptr);
 
-  if (port::kLittleEndian) {
-    // Fast path for little-endian CPUs. All major compilers optimize this to a
-    // single mov (x86_64) / ldr (ARM) instruction.
-    uint32_t result;
-    std::memcpy(&result, buffer, sizeof(uint32_t));
-    return result;
-  }
-
-  // Platform-independent code.
-  // Clang and gcc optimize this to a single mov / ldr instruction.
+  // Recent clang and gcc optimize this to a single mov / ldr instruction.
   return (static_cast<uint32_t>(buffer[0])) |
          (static_cast<uint32_t>(buffer[1]) << 8) |
          (static_cast<uint32_t>(buffer[2]) << 16) |
@@ -124,16 +91,7 @@ inline uint32_t DecodeFixed32(const char* ptr) {
 inline uint64_t DecodeFixed64(const char* ptr) {
   const uint8_t* const buffer = reinterpret_cast<const uint8_t*>(ptr);
 
-  if (port::kLittleEndian) {
-    // Fast path for little-endian CPUs. All major compilers optimize this to a
-    // single mov (x86_64) / ldr (ARM) instruction.
-    uint64_t result;
-    std::memcpy(&result, buffer, sizeof(uint64_t));
-    return result;
-  }
-
-  // Platform-independent code.
-  // Clang and gcc optimize this to a single mov / ldr instruction.
+  // Recent clang and gcc optimize this to a single mov / ldr instruction.
   return (static_cast<uint64_t>(buffer[0])) |
          (static_cast<uint64_t>(buffer[1]) << 8) |
          (static_cast<uint64_t>(buffer[2]) << 16) |