diff options
author | Victor Costan <costan@google.com> | 2020-04-13 23:18:12 +0000 |
---|---|---|
committer | Victor Costan <costan@google.com> | 2020-04-14 01:10:05 +0000 |
commit | 201f52201f5dd9701e7a8ceaa0ec4d344e69e022 (patch) | |
tree | b3b2fa7f9f9e3c6a3f744e72b003597e1d956ce1 /util | |
parent | ba369ddbaffcfe635dd620d1aa68473b56267065 (diff) | |
download | leveldb-201f52201f5dd9701e7a8ceaa0ec4d344e69e022.tar.gz |
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
Diffstat (limited to 'util')
-rw-r--r-- | util/coding.h | 50 |
1 files changed, 4 insertions, 46 deletions
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<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) | |