From b35bf301dd182e7a733f6932113bc59c2ca91788 Mon Sep 17 00:00:00 2001
From: Boshen <1430279+Boshen@users.noreply.github.com>
Date: Thu, 11 Sep 2025 05:28:51 +0000
Subject: [PATCH] perf(codegen): optimize sourcemap builder to reduce
 allocations (#13670)
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## Summary

This PR optimizes the sourcemap builder to reduce memory allocations and improve performance. Profiling showed that `update_generated_line_and_column` and `add_source_mapping` were performance bottlenecks.

### Changes Made

**Optimization 4: Optimize Line Offset Table Generation**
- Pre-allocate `columns` vector with capacity 256 to avoid frequent reallocations
- Replace `columns.clone().into_boxed_slice()` with `std::mem::take(&mut columns).into_boxed_slice()` to avoid unnecessary cloning
- Reserve capacity after taking the vector to maintain performance for subsequent lines

### Performance Impact

These changes reduce memory allocations when generating sourcemaps, especially for files with Unicode characters. The `clone()` operation was creating unnecessary copies of potentially large vectors on every Unicode line, which is now eliminated.

### Future Optimizations

Additional optimizations from the analysis that could be implemented in follow-up PRs:
1. Use SIMD-accelerated line break detection with `memchr`
2. Optimize UTF-16 column calculation to avoid iterator allocation
3. Add fast path for sequential token processing
5. Inline hot functions with `#[inline(always)]`

## Test Plan

- [x] All existing tests pass
- [x] No functional changes, only performance optimizations
- [x] Verified that sourcemap generation still works correctly

🤖 Generated with [Claude Code](https://claude.ai/code)
---
 crates/oxc_codegen/src/sourcemap_builder.rs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/crates/oxc_codegen/src/sourcemap_builder.rs b/crates/oxc_codegen/src/sourcemap_builder.rs
index 671671b630d8d..5df20385ce336 100644
--- a/crates/oxc_codegen/src/sourcemap_builder.rs
+++ b/crates/oxc_codegen/src/sourcemap_builder.rs
@@ -308,7 +308,8 @@ impl<'a> SourcemapBuilder<'a> {
         let mut column_offsets = IndexVec::new();
 
         // Used as a buffer to reduce memory reallocations
-        let mut columns = vec![];
+        // Pre-allocate with reasonable capacity to avoid frequent reallocations
+        let mut columns = Vec::with_capacity(256);
 
         // Process content line-by-line.
         // For each line, start by assuming line will be entirely ASCII, and read byte-by-byte.
@@ -389,11 +390,13 @@ impl<'a> SourcemapBuilder<'a> {
                             line_byte_offset += chunk_byte_offset;
 
                             // Record column offsets
+                            // Use mem::take to avoid clone - moves columns out and replaces with empty Vec
                             column_offsets.push(ColumnOffsets {
                                 byte_offset_to_first: byte_offset_from_line_start,
-                                columns: columns.clone().into_boxed_slice(),
+                                columns: std::mem::take(&mut columns).into_boxed_slice(),
                             });
-                            columns.clear();
+                            // Reserve capacity for next line to avoid reallocation
+                            columns.reserve(256);
 
                             // Revert back to outer loop for next line
                             continue 'lines;