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perf(codegen): further reduce memory allocations in generate_line_offset_tables #13056

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Aug 13, 2025
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perf(codegen): further reduce memory allocations in generate_line_offset_tables #13056

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11 changes: 6 additions & 5 deletions crates/oxc_codegen/src/sourcemap_builder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -312,6 +312,9 @@ impl<'a> SourcemapBuilder<'a> {
let mut lines = vec![];
let mut column_offsets = IndexVec::new();

// Used as a buffer to reduce memory reallocations
let mut columns = vec![];

// Process content line-by-line.
// For each line, start by assuming line will be entirely ASCII, and read byte-by-byte.
// If line is all ASCII, UTF-8 columns and UTF-16 columns are the same,
Expand All @@ -325,7 +328,6 @@ impl<'a> SourcemapBuilder<'a> {
column_offsets_id: None,
});

let mut columns = vec![]; // Used as a buffer to reduce memory reallocations.
let remaining = &content.as_bytes()[line_byte_offset as usize..];
for (byte_offset_from_line_start, b) in remaining.iter().enumerate() {
#[expect(clippy::cast_possible_truncation)]
Expand All @@ -351,8 +353,6 @@ impl<'a> SourcemapBuilder<'a> {
line.column_offsets_id =
Some(ColumnOffsetsId::from_usize(column_offsets.len()));

columns.clear();

// Loop through rest of line char-by-char.
// `chunk_byte_offset` in this loop is byte offset from start of this 1st
// Unicode char.
Expand Down Expand Up @@ -396,8 +396,9 @@ impl<'a> SourcemapBuilder<'a> {
// Record column offsets
column_offsets.push(ColumnOffsets {
byte_offset_to_first: byte_offset_from_line_start,
columns: columns.into_iter().collect(),
columns: columns.clone().into_boxed_slice(),
});
columns.clear();

// Revert back to outer loop for next line
continue 'lines;
Expand All @@ -410,7 +411,7 @@ impl<'a> SourcemapBuilder<'a> {
// Record column offsets
column_offsets.push(ColumnOffsets {
byte_offset_to_first: byte_offset_from_line_start,
columns: columns.into_iter().collect(),
columns: columns.into_boxed_slice(),
});

break 'lines;
Expand Down
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