pitrou commented on code in PR #45108:
URL: https://github.com/apache/arrow/pull/45108#discussion_r1904334656
##########
cpp/src/arrow/compute/key_map_internal_avx2.cc:
##########
@@ -392,16 +392,32 @@ int SwissTable::extract_group_ids_avx2(const int
num_keys, const uint32_t* hashe
} else {
for (int i = 0; i < num_keys / unroll; ++i) {
__m256i hash = _mm256_loadu_si256(reinterpret_cast<const
__m256i*>(hashes) + i);
+ // Extend hash and local_slot to 64-bit to compute 64-bit group id
offsets to
+ // gather.
+ // This is to prevent index overflow issues in GH-44513.
+ // NB: Use zero-extend conversion for unsigned hash.
+ __m256i hash_lo = _mm256_cvtepu32_epi64(_mm256_castsi256_si128(hash));
+ __m256i hash_hi = _mm256_cvtepu32_epi64(_mm256_extracti128_si256(hash,
1));
__m256i local_slot =
_mm256_set1_epi64x(reinterpret_cast<const
uint64_t*>(local_slots)[i]);
local_slot = _mm256_shuffle_epi8(
Review Comment:
Hmm... so this first expands `_mm256_shuffle_epi8` from 8-bit to 32-bit
lanes, and then `_mm256_cvtepi32_epi64` below expands it from 32-bit to 64-bit
lanes? Would it be quicker to shuffle directly from 8-bit to 64-bit (twice, I
suppose)
(interestingly, `_mm256_shuffle_epi8` is faster than `_mm256_cvtepi32_epi64`
according to
https://www.intel.com/content/www/us/en/docs/intrinsics-guide/index.html#text=_mm256_shuffle_epi8&ig_expand=1798,6006,1628,6006)
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