andygrove commented on PR #558:
URL: https://github.com/apache/datafusion-comet/pull/558#issuecomment-2161643541
Comparison of safe code vs using unsafe read/write aligned:
```rust
pub fn copy_i32_to_i16(src: &[u8], dst: &mut [u8], num: usize) {
debug_assert!(src.len() >= num * 4, "Source slice is too small");
debug_assert!(dst.len() >= num * 2, "Destination slice is too small");
for i in 0..num {
let i32_value =
i32::from_le_bytes([src[i * 4], src[i * 4 + 1], src[i * 4 + 2],
src[i * 4 + 3]]);
// Downcast to i16, potentially losing data
let i16_value = i32_value as i16;
let i16_bytes = i16_value.to_le_bytes();
dst[i * 2] = i16_bytes[0];
dst[i * 2 + 1] = i16_bytes[1];
}
}
pub fn copy_i32_to_i16_unsafe(src: &[u8], dst: &mut [u8], num: usize) {
debug_assert!(src.len() >= num * 4, "Source slice is too small");
debug_assert!(dst.len() >= num * 2, "Destination slice is too small");
let src_ptr = src.as_ptr() as *const i32;
let dst_ptr = dst.as_mut_ptr() as *mut i16;
unsafe {
for i in 0..num {
dst_ptr
.add(i)
.write_unaligned(src_ptr.add(i).read_unaligned() as i16);
}
}
}
```
```
parquet_decode/decode_i32_to_i16_safe
time: [839.29 ns 839.57 ns 839.93 ns]
Found 4 outliers among 100 measurements (4.00%)
1 (1.00%) high mild
3 (3.00%) high severe
parquet_decode/decode_i32_to_i16_unsafe
time: [68.269 ps 68.400 ps 68.593 ps]
Found 7 outliers among 100 measurements (7.00%)
2 (2.00%) low mild
2 (2.00%) high mild
3 (3.00%) high severe
```
I am going to reimplement with the unaligned approach and see if that has
any safety issues
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