zanmato1984 commented on code in PR #45108:
URL: https://github.com/apache/arrow/pull/45108#discussion_r1913028296


##########
cpp/src/arrow/compute/key_map_internal_avx2.cc:
##########
@@ -392,16 +387,30 @@ 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 from. 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(
-          local_slot, _mm256_setr_epi32(0x80808000, 0x80808001, 0x80808002, 
0x80808003,
-                                        0x80808004, 0x80808005, 0x80808006, 
0x80808007));
-      local_slot = _mm256_mullo_epi32(local_slot, 
_mm256_set1_epi32(byte_size));
-      __m256i pos = _mm256_srlv_epi32(hash, _mm256_set1_epi32(bits_hash_ - 
log_blocks_));
-      pos = _mm256_mullo_epi32(pos, _mm256_set1_epi32(byte_multiplier));
-      pos = _mm256_add_epi32(pos, local_slot);
-      __m256i group_id = _mm256_i32gather_epi32(elements, pos, 1);
+      __m256i local_slot_lo = _mm256_shuffle_epi8(
+          local_slot, _mm256_setr_epi32(0x80808000, 0x80808080, 0x80808001, 
0x80808080,
+                                        0x80808002, 0x80808080, 0x80808003, 
0x80808080));
+      __m256i local_slot_hi = _mm256_shuffle_epi8(
+          local_slot, _mm256_setr_epi32(0x80808004, 0x80808080, 0x80808005, 
0x80808080,
+                                        0x80808006, 0x80808080, 0x80808007, 
0x80808080));
+      local_slot_lo = _mm256_mul_epu32(local_slot_lo, 
_mm256_set1_epi32(byte_size));
+      local_slot_lo = _mm256_mul_epu32(local_slot_hi, 
_mm256_set1_epi32(byte_size));

Review Comment:
   Thank you for spotting this! Let me check how things go wrong (or not go 
wrong).



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