neilconway commented on code in PR #26039:
URL: https://github.com/apache/datafusion/pull/26039#discussion_r4186481784
##########
datafusion/functions/src/strings.rs:
##########
@@ -1217,6 +1219,73 @@ pub(crate) fn append_view(
views_buffer.push(sub_view);
}
+/// Values of at most this many bytes are stored inline in their view.
+const MAX_INLINE_LEN: usize = MAX_INLINE_VIEW_LEN as usize;
+
+/// Returns the view for a substring of the value an existing view refers to.
+///
+/// # Arguments
+/// - view: The original view value
+/// - source: The bytes the original view refers to
+/// - range: The byte range within `source` of the substring to return a view
for
+///
+/// Substrings longer than 12 bytes point into the same data buffer as `view`;
+/// shorter ones are stored inline in the new view.
+///
+/// This uses shifts and masks rather than [`make_view`], which picks copy code
+/// based on the substring's length; the CPU often mispredicts that choice when
+/// lengths vary from row to row.
+#[inline]
+pub(crate) fn sub_view(view: u128, source: &[u8], range: Range<usize>) -> u128
{
+ debug_assert!(range.start <= range.end && range.end <= source.len());
+
+ // The substring's first bytes, in the low-order bits. Any bits past the
+ // end of the substring are masked off by `inline_view`.
+ let leading_bytes = if source.len() <= MAX_INLINE_LEN {
+ // `source` is stored in `view` itself, after its 4-byte length.
+ (view >> 32) >> (8 * range.start)
+ } else {
+ // `source` has more than 12 bytes, so read the 12 bytes starting at
+ // `range.start`, or the last 12 bytes if that would run past the end,
+ // and skip any that come before `range.start`.
+ let window_start = range.start.min(source.len() - MAX_INLINE_LEN);
+ let window = source[window_start..window_start + MAX_INLINE_LEN]
+ .try_into()
+ .unwrap();
+ read_12_bytes(window) >> (8 * (range.start - window_start))
+ };
+
+ let len = range.len();
+ if len <= MAX_INLINE_LEN {
+ inline_view(leading_bytes, len)
+ } else {
+ let original = ByteView::from(view);
+ ByteView {
+ length: len as u32,
+ prefix: leading_bytes as u32,
+ offset: original.offset + range.start as u32,
+ ..original
+ }
+ .as_u128()
+ }
+}
Review Comment:
Yep, makes sense!
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