diff options
Diffstat (limited to 'snappy.cc')
| -rw-r--r-- | snappy.cc | 41 |
1 files changed, 37 insertions, 4 deletions
@@ -272,16 +272,49 @@ uint16* WorkingMemory::GetHashTable(size_t input_size, int* table_size) { } } // end namespace internal -// For 0 <= offset <= 4, GetUint32AtOffset(UNALIGNED_LOAD64(p), offset) will +// For 0 <= offset <= 4, GetUint32AtOffset(GetEightBytesAt(p), offset) will // equal UNALIGNED_LOAD32(p + offset). Motivation: On x86-64 hardware we have // empirically found that overlapping loads such as // UNALIGNED_LOAD32(p) ... UNALIGNED_LOAD32(p+1) ... UNALIGNED_LOAD32(p+2) // are slower than UNALIGNED_LOAD64(p) followed by shifts and casts to uint32. +// +// We have different versions for 64- and 32-bit; ideally we would avoid the +// two functions and just inline the UNALIGNED_LOAD64 call into +// GetUint32AtOffset, but GCC (at least not as of 4.6) is seemingly not clever +// enough to avoid loading the value multiple times then. For 64-bit, the load +// is done when GetEightBytesAt() is called, whereas for 32-bit, the load is +// done at GetUint32AtOffset() time. + +#ifdef ARCH_K8 + +typedef uint64 EightBytesReference; + +static inline EightBytesReference GetEightBytesAt(const char* ptr) { + return UNALIGNED_LOAD64(ptr); +} + static inline uint32 GetUint32AtOffset(uint64 v, int offset) { - DCHECK(0 <= offset && offset <= 4) << offset; + DCHECK_GE(offset, 0); + DCHECK_LE(offset, 4); return v >> (LittleEndian::IsLittleEndian() ? 8 * offset : 32 - 8 * offset); } +#else + +typedef const char* EightBytesReference; + +static inline EightBytesReference GetEightBytesAt(const char* ptr) { + return ptr; +} + +static inline uint32 GetUint32AtOffset(const char* v, int offset) { + DCHECK_GE(offset, 0); + DCHECK_LE(offset, 4); + return UNALIGNED_LOAD32(v + offset); +} + +#endif + // Flat array compression that does not emit the "uncompressed length" // prefix. Compresses "input" string to the "*op" buffer. // @@ -378,7 +411,7 @@ char* CompressFragment(const char* input, // though we don't yet know how big the literal will be. We handle that // by proceeding to the next iteration of the main loop. We also can exit // this loop via goto if we get close to exhausting the input. - uint64 input_bytes = 0; + EightBytesReference input_bytes; uint32 candidate_bytes = 0; do { @@ -397,7 +430,7 @@ char* CompressFragment(const char* input, if (PREDICT_FALSE(ip >= ip_limit)) { goto emit_remainder; } - input_bytes = UNALIGNED_LOAD64(insert_tail); + input_bytes = GetEightBytesAt(insert_tail); uint32 prev_hash = HashBytes(GetUint32AtOffset(input_bytes, 0), shift); table[prev_hash] = ip - base_ip - 1; uint32 cur_hash = HashBytes(GetUint32AtOffset(input_bytes, 1), shift); |