Vishwanatha-HD commented on code in PR #48180:
URL: https://github.com/apache/arrow/pull/48180#discussion_r2591499936
##########
cpp/src/arrow/compute/util.cc:
##########
@@ -299,7 +313,17 @@ void bytes_to_bits(int64_t hardware_flags, const int
num_bits, const uint8_t* by
}
int tail = num_bits % unroll;
if (tail) {
- uint64_t bytes_next = SafeLoadUpTo8Bytes(bytes + num_bits - tail, tail);
+ uint64_t bytes_next;
+#if ARROW_LITTLE_ENDIAN
+ bytes_next = SafeLoadUpTo8Bytes(bytes + num_bits - tail, tail);
+#else
+ // On Big-endian systems, for bytes_to_bits, load all tail bytes in
little-endian
+ // order to ensure compatibility with subsequent bit operations
+ bytes_next = 0;
+ for (int i = 0; i < tail; ++i) {
+ bytes_next |= static_cast<uint64_t>((bytes + num_bits - tail)[i]) << (8
* i);
+ }
+#endif
Review Comment:
@kou.. The SafeLoadUpTo8Bytes() function remains unchanged..
```
inline uint64_t SafeLoadUpTo8Bytes(const uint8_t* bytes, int num_bytes) {
ARROW_DCHECK(num_bytes >= 0 && num_bytes <= 8);
if (num_bytes == 8) {
return util::SafeLoad(reinterpret_cast<const uint64_t*>(bytes));
} else {
uint64_t word = 0;
for (int i = 0; i < num_bytes; ++i) {
word |= static_cast<uint64_t>(bytes[i]) << (8 * i);
}
return word;
}
}
```
The bytes_to_bits() function is handling the endianness fix independently..
If I do the endianness conversion inside SafeLoadUpTo8Bytes() function, rather
than here, then the testcase is not working..
```
if (tail) {
uint64_t bytes_next;
#if ARROW_LITTLE_ENDIAN
bytes_next = SafeLoadUpTo8Bytes(bytes + num_bits - tail, tail);
#else
// On Big-endian systems, for bytes_to_bits, load all tail bytes in
little-endian
// order to ensure compatibility with subsequent bit operations
bytes_next = 0;
for (int i = 0; i < tail; ++i) {
bytes_next |= static_cast<uint64_t>((bytes + num_bits - tail)[i]) <<
(8 * i);
}
#endif
bytes_next &= 0x0101010101010101ULL;
bytes_next |= (bytes_next >> 7); // Pairs of adjacent output bits in
individual bytes
bytes_next |= (bytes_next >> 14); // 4 adjacent output bits in
individual bytes
bytes_next |= (bytes_next >> 28); // All 8 output bits in the lowest
byte
bits[num_bits / 8] = static_cast<uint8_t>(bytes_next & 0xff);
}
```
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