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SIMD Vectorization Demo

This demo is made by Wang Lipeng and CHE Yulin from Hong Kong University of Science and Technology. Feel free to give this repo a star if you like it.

intel intrinsics guide: see https://software.intel.com/sites/landingpage/IntrinsicsGuide/

bechmarking CPU info: see src/playground, using single-thread

Tokenizing

/	type
input	form body
output	dubbo package

algorithms	elapsed time
serial transform time	380 ms
sse4 transform time	240 ms
avx2 transform time	90 ms

• workaround for __tzcnt_u32 (requiring bmi) in sse4 (handle 16bits currently, else modify zero if-sentence)

inlineint__tzcnt_u32_using_popcnt_cmpeq(unsignedint x) {
int ret_cnt = 0;
int half_bits = x & (0xff);
if (half_bits == 0) {
half_bits = (x >> 8) & (0xff);
ret_cnt = 8;
}
__m128i pivot_u = _mm_set1_epi16(half_bits);
__m128i inspected_ele = _mm_set_epi16(
0xff, 0x7f, 0x3f, 0x1f, 0xf, 0x7, 0x3, 0x1);
__m128i trunc_pivot_u = _mm_and_si128(pivot_u, inspected_ele);
__m128i pivot_new = _mm_set1_epi16(0);
__m128i cmp_res = _mm_cmpeq_epi16(trunc_pivot_u, pivot_new);
int mask = _mm_movemask_epi8(cmp_res); // 16 bits
ret_cnt += _mm_popcnt_u32(mask) >> 1;
return ret_cnt;
}

• idea: advance with simd cmp and tzcnt

// 1st: advance fo find first '='while (true) {
__m128i pivot_u = _mm_set1_epi8('=');
__m128i inspected_ele = _mm_loadu_si128((__m128i *) (str + i));
__m128i cmp_res = _mm_cmpeq_epi8(pivot_u, inspected_ele);
int mask = _mm_movemask_epi8(cmp_res); // 16 bitsint advance = (mask == 0 ?
16 : __tzcnt_u32_using_popcnt_cmpeq(mask));
i += advance;
if (advance < 16) { break; }
}
off[next] = i;
next++;

• details: see src/fast_tokenizer/tokenizer.cpp, src/playground/test_split.cpp

Base64 Decoding/Encoding (i.e, Serialization/Deserialization)

• base64 encoding includes 1) unpacking and 2) ascii translation (to)

• base64 decoding includes 1) ascii translation (from) and 2) packing

• the translation table

base64	ascii-dec	ascii-hex	offset (diff)
0-25	65-90	0x41-0x5a	65
26-51	97-122	0x61-0x7a	71
52-61	48-57	0x30-0x39	-4
62	43, i.e., '+'	0x2b	-19
63	47, i.e., '/'	0x2f	-16
others	invalid	invalid	invalid

Base64 Decoding Vectorization

ascii code to base64 translation vectorization (simplified)

// attention: each SIMD lane is 128-bitsconst __m256i lut_roll = _mm256_setr_epi8(
0, 16, 19, 4, -65, -65, -71, -71,
0, 0, 0, 0, 0, 0, 0, 0,
0, 16, 19, 4, -65, -65, -71, -71,
0, 0, 0, 0, 0, 0, 0, 0
);
const __m256i mask_2F = _mm256_set1_epi8(0x2f);
// 1st: shift right for 4 32bit-wise-elements in each SIMD lane// shift to get high 4bits
__m256i hi_nibbles = _mm256_srli_epi32(str, 4);
// 2nd: cmpeq to distinguish 0x2f ('/') from 0x2b ('+') // when the byte is 0x2f, the result byte in eq_2F is 0xff (-1) const __m256i eq_2F = _mm256_cmpeq_epi8(str, mask_2F);
// 3rd: add -1 when the byte is 0x2f, // lookup lut_roll for offsets to add, shuffling in each SIMD Lane// shuffling using the previous high 4-bits in the byte (see 1st step)const __m256i roll = _mm256_shuffle_epi8(lut_roll, _mm256_add_epi8(eq_2F, hi_nibbles));
// 4th: add offsets
str = _mm256_add_epi8(str, roll);

Benchmark

• implementations

benchmark main file: src/playground/test_serialization.cpp

algorithms	link
naive-comp	src/naive_base64/naive_base64.cpp
naive-lookup	src/naive_base64/naive_base64.cpp
chromium	src/fast_base64/chromiumbase64.c
fast-avx2	src/fast_base64/fastavxbase64.c

• measure: we report TPS (throughput including both serialization and deserialization time)

algorithms	idea
naive-comp	ascii-base64 translation using computations
naive-lookup	ascii-base64 translation using two look-up tables
chromium	use separate lookup tables for saving some computations
fast-avx2	apply avx2, and serial fallback using chromium

• message len 200

algorithms	TPS-total	TPS-serializtion	TPS-deserializtion
naive_comp	201.448 MB/s	467.150 MB/s	354.179 MB/s
naive_lookup	490.444 MB/s	1080.221 MB/s	898.286 MB/s
chromium	493.397 MB/s	1039.237 MB/s	939.390 MB/s
avx2_chromium	887.260 MB/s	1838.769 MB/s	1714.608 MB/s

• message len 80

algorithms	TPS-total	TPS-serializtion	TPS-deserializtion
naive_comp	149.235 MB/s	288.609 MB/s	309.029 MB/s
naive_lookup	394.224 MB/s	740.423 MB/s	843.135 MB/s
chromium	409.404 MB/s	725.954 MB/s	938.900 MB/s
avx2_chromium	556.984 MB/s	974.215 MB/s	1300.531 MB/s

• message len 50

algorithms	TPS-total	TPS-serializtion	TPS-deserializtion
naive_comp	115.787 MB/s	218.357 MB/s	246.493 MB/s
naive_lookup	330.488 MB/s	598.695 MB/s	737.719 MB/s
chromium	335.035 MB/s	595.675 MB/s	765.698 MB/s
avx2_chromium	385.963 MB/s	656.360 MB/s	936.887 MB/s