diff options
Diffstat (limited to 'snappy.cc')
| -rw-r--r-- | snappy.cc | 35 |
1 files changed, 11 insertions, 24 deletions
@@ -675,14 +675,15 @@ static inline void Report(const char *algorithm, size_t compressed_size, // bool TryFastAppend(const char* ip, size_t available, size_t length); // }; -namespace internal { - -// Mapping from i in range [0,4] to a mask to extract the bottom 8*i bits -static const uint32 wordmask[] = { - 0u, 0xffu, 0xffffu, 0xffffffu, 0xffffffffu -}; - -} // end namespace internal +// Mapping from n in range [0,4] to a mask to extract the bottom 8*n bits. +static inline uint32 WordMask(int n) { + DCHECK_GE(n, 0); + DCHECK_LE(n, 4); + // This needs to be wider than uint32 otherwise `mask << 32` will be + // undefined. + uint64 mask = 0xffffffff; + return ~(mask << (8 * n)); +} // Helper class for decompression class SnappyDecompressor { @@ -770,20 +771,6 @@ class SnappyDecompressor { #endif const char* ip = ip_; - // For position-independent executables, accessing global arrays can be - // slow. Move wordmask array onto the stack to mitigate this. - uint32 wordmask[sizeof(internal::wordmask)/sizeof(uint32)]; - // Do not use memcpy to copy internal::wordmask to - // wordmask. LLVM converts stack arrays to global arrays if it detects - // const stack arrays and this hurts the performance of position - // independent code. This change is temporary and can be reverted when - // https://reviews.llvm.org/D30759 is approved. - wordmask[0] = internal::wordmask[0]; - wordmask[1] = internal::wordmask[1]; - wordmask[2] = internal::wordmask[2]; - wordmask[3] = internal::wordmask[3]; - wordmask[4] = internal::wordmask[4]; - // We could have put this refill fragment only at the beginning of the loop. // However, duplicating it at the end of each branch gives the compiler more // scope to optimize the <ip_limit_ - ip> expression based on the local @@ -825,7 +812,7 @@ class SnappyDecompressor { // Long literal. const size_t literal_length_length = literal_length - 60; literal_length = - (LittleEndian::Load32(ip) & wordmask[literal_length_length]) + 1; + (LittleEndian::Load32(ip) & WordMask(literal_length_length)) + 1; ip += literal_length_length; } @@ -848,7 +835,7 @@ class SnappyDecompressor { MAYBE_REFILL(); } else { const size_t entry = char_table[c]; - const size_t trailer = LittleEndian::Load32(ip) & wordmask[entry >> 11]; + const size_t trailer = LittleEndian::Load32(ip) & WordMask(entry >> 11); const size_t length = entry & 0xff; ip += entry >> 11; |