From 201f52201f5dd9701e7a8ceaa0ec4d344e69e022 Mon Sep 17 00:00:00 2001 From: Victor Costan Date: Mon, 13 Apr 2020 23:18:12 +0000 Subject: [PATCH] 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 --- CMakeLists.txt | 3 --- port/port_config.h.in | 6 ------ port/port_example.h | 4 ---- port/port_stdcxx.h | 2 -- util/coding.h | 50 ++++---------------------------------------------- 5 files changed, 4 insertions(+), 61 deletions(-) diff --git a/CMakeLists.txt b/CMakeLists.txt index be41ba4..ae9b0f7 100644 --- a/CMakeLists.txt +++ b/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) diff --git a/port/port_config.h.in b/port/port_config.h.in index 2127315..272671d 100644 --- a/port/port_config.h.in +++ b/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_ \ No newline at end of file diff --git a/port/port_example.h b/port/port_example.h index 1a8fca2..a665910 100644 --- a/port/port_example.h +++ b/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. diff --git a/port/port_stdcxx.h b/port/port_stdcxx.h index e9cb0e5..2bda48d 100644 --- a/port/port_stdcxx.h +++ b/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. diff --git a/util/coding.h b/util/coding.h index 1983ae7..f0bb57b 100644 --- a/util/coding.h +++ b/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(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(value); buffer[1] = static_cast(value >> 8); buffer[2] = static_cast(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(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(value); buffer[1] = static_cast(value >> 8); buffer[2] = static_cast(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(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(buffer[0])) | (static_cast(buffer[1]) << 8) | (static_cast(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(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(buffer[0])) | (static_cast(buffer[1]) << 8) | (static_cast(buffer[2]) << 16) |