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Move the extract masks variable out in zippy. I see a consistent 1.5-2% improvement for ARM. Probably because ARM has more relaxed address computation than x86 https://www.godbolt.org/z/bfM1ezx41. I don't think this is a compiler bug or it can do something about it

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PiperOrigin-RevId: 387569896
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Snappy Team
2021-07-29 13:26:45 +00:00
committed by Victor Costan
parent c8f7641646
commit f2db8f77ce
1 changed files with 18 additions and 9 deletions
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snappy.cc
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@@ -124,12 +124,8 @@ constexpr std::array<int16_t, 256> MakeTable(index_sequence<seq...>) {
return std::array<int16_t, 256>{LengthMinusOffset(seq)...};
}
// We maximally co-locate the two tables so that only one register needs to be
// reserved for the table address.
struct {
alignas(64) const std::array<int16_t, 256> length_minus_offset;
uint32_t extract_masks[4]; // Used for extracting offset based on tag type.
} table = {MakeTable(make_index_sequence<256>{}), {0, 0xFF, 0xFFFF, 0}};
alignas(64) const std::array<int16_t, 256> kLengthMinusOffset =
MakeTable(make_index_sequence<256>{});
// Any hash function will produce a valid compressed bitstream, but a good
// hash function reduces the number of collisions and thus yields better
@@ -1075,7 +1071,20 @@ size_t AdvanceToNextTagX86Optimized(const uint8_t** ip_p, size_t* tag) {
// Extract the offset for copy-1 and copy-2 returns 0 for literals or copy-4.
inline uint32_t ExtractOffset(uint32_t val, size_t tag_type) {
return val & table.extract_masks[tag_type];
// For x86 non-static storage works better. For ARM static storage is better.
// TODO: Once the array is recognized as a register, improve the
// readability for x86.
#if defined(__x86_64__)
constexpr uint64_t kExtractMasksCombined = 0x0000FFFF00FF0000ull;
uint16_t result;
memcpy(&result,
reinterpret_cast<const char*>(&kExtractMasksCombined) + 2 * tag_type,
sizeof(result));
return val & result;
#else
static constexpr uint32_t kExtractMasks[4] = {0, 0xFF, 0xFFFF, 0};
return val & kExtractMasks[tag_type];
#endif
};
// Core decompression loop, when there is enough data available.
@@ -1108,7 +1117,7 @@ std::pair<const uint8_t*, ptrdiff_t> DecompressBranchless(
assert(tag == ip[-1]);
// For literals tag_type = 0, hence we will always obtain 0 from
// ExtractLowBytes. For literals offset will thus be kLiteralOffset.
ptrdiff_t len_min_offset = table.length_minus_offset[tag];
ptrdiff_t len_min_offset = kLengthMinusOffset[tag];
#if defined(__aarch64__)
size_t tag_type = AdvanceToNextTagARMOptimized(&ip, &tag);
#else
@@ -1328,7 +1337,7 @@ class SnappyDecompressor {
if (!writer->AppendFromSelf(copy_offset, length, &op)) goto exit;
} else {
const ptrdiff_t entry = table.length_minus_offset[c];
const ptrdiff_t entry = kLengthMinusOffset[c];
preload = LittleEndian::Load32(ip);
const uint32_t trailer = ExtractLowBytes(preload, c & 3);
const uint32_t length = entry & 0xff;