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Remove leveldb::port::kLittleEndian.

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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
This commit is contained in:
Victor Costan 2020-04-13 23:18:12 +00:00
parent ba369ddbaf
commit 201f52201f
5 changed files with 4 additions and 61 deletions
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3
CMakeLists.txt
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@ -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)

6
port/port_config.h.in
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@ -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_

4
port/port_example.h
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@ -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.

2
port/port_stdcxx.h
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@ -41,8 +41,6 @@
namespace leveldb {
namespace port {
static const bool kLittleEndian = !LEVELDB_IS_BIG_ENDIAN;
class CondVar;
// Thinly wraps std::mutex.

50
util/coding.h
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@ -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) |