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codegen: Optimize multiline comments handling with SIMD processing #13593

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fe0d9f4
Initial plan
Copilot Sep 8, 2025
77df5e6
Implement SIMD optimization for multiline comments handling
Copilot Sep 8, 2025
d88b2c0
[autofix.ci] apply automated fixes
autofix-ci[bot] Sep 8, 2025
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1 change: 1 addition & 0 deletions .gitignore
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Original file line number Diff line number Diff line change
Expand Up @@ -52,3 +52,4 @@ tasks/prettier_conformance/prettier/
# See also
# * https://stackoverflow.com/a/7335487
# * https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files#configuring-ignored-files-for-all-repositories-on-your-computer
tmp/
181 changes: 141 additions & 40 deletions crates/oxc_codegen/src/comment.rs
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Expand Up @@ -22,12 +22,12 @@ pub type CommentsMap = FxHashMap</* attached_to */ u32, Vec<Comment>>;
/// Standard split would turn `"line1\r\nline2"` into `["line1", "", "line2"]` because
/// it treats `\r` and `\n` as separate terminators. This iterator correctly produces
/// `["line1", "line2"]` by treating `\r\n` as a single terminator.
struct LineTerminatorSplitter<'a> {
pub(crate) struct LineTerminatorSplitter<'a> {
text: &'a str,
}

impl<'a> LineTerminatorSplitter<'a> {
fn new(text: &'a str) -> Self {
pub(crate) fn new(text: &'a str) -> Self {
Self { text }
}
}
Expand All @@ -40,54 +40,155 @@ impl<'a> Iterator for LineTerminatorSplitter<'a> {
return None;
}

for (index, &byte) in self.text.as_bytes().iter().enumerate() {
match byte {
b'\n' => {
// SAFETY: Byte at `index` is `\n`, so `index` and `index + 1` are both UTF-8 char boundaries.
// Therefore, slices up to `index` and from `index + 1` are both valid `&str`s.
unsafe {
let line = self.text.get_unchecked(..index);
self.text = self.text.get_unchecked(index + 1..);
return Some(line);
// Line terminators will be very rare in most text. So we try to make the search as quick as possible by:
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@overlookmotel overlookmotel Sep 8, 2025

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I'm not sure if this assumption is correct. Line breaks are fairly common in block comments.

// 1. Searching for line terminator bytes (`\r`, `\n`, `0xE2`) first, and only checking details once found.
// 2. Searching longer strings in chunks of 16 bytes using SIMD, and only doing the
// more expensive byte-by-byte search once a line terminator is found.

let bytes = self.text.as_bytes();
let mut consumed = 0;

// Search range of bytes for line terminators, byte by byte.
//
// Bytes between `ptr` and `last_ptr` (inclusive) are searched for `\r`, `\n`, or `0xE2`.
// If found, process the line terminator and return the line.
//
// SAFETY:
// * `ptr` and `last_ptr` must be within bounds of `bytes`.
// * `last_ptr` must be greater or equal to `ptr`.
// * For `0xE2` (LS/PS), `last_ptr` must be no later than 3 bytes before end of string.
// i.e. safe to read 3 bytes at `last_ptr`.
let mut search_bytes = |mut ptr: *const u8, last_ptr| -> Option<&'a str> {
loop {
// SAFETY: `ptr` is always less than or equal to `last_ptr`.
// `last_ptr` is within bounds of `bytes`, so safe to read a byte at `ptr`.
let byte = unsafe { *ptr };
match byte {
b'\n' => {
// SAFETY: `consumed` is initially 0, and only updated to valid UTF-8 boundaries.
// `index` is on `\n`, so `index` and `index + 1` are UTF-8 char boundaries.
unsafe {
let index = ptr.offset_from(bytes.as_ptr()) as usize;
let line = self.text.get_unchecked(consumed..index);
// Set `consumed` to after `\n`
consumed = index + 1;
self.text = self.text.get_unchecked(consumed..);
return Some(line);
}
}
}
b'\r' => {
// SAFETY: Byte at `index` is `\r`, so `index` is on a UTF-8 char boundary
let line = unsafe { self.text.get_unchecked(..index) };
// If the next byte is `\n`, consume it as well
let skip_bytes =
if self.text.as_bytes().get(index + 1) == Some(&b'\n') { 2 } else { 1 };
// SAFETY: `index + skip_bytes` is after `\r` or `\n`, so on a UTF-8 char boundary.
// Therefore slice from `index + skip_bytes` is a valid `&str`.
self.text = unsafe { self.text.get_unchecked(index + skip_bytes..) };
return Some(line);
}
LS_OR_PS_FIRST_BYTE => {
let next2: [u8; 2] = {
// SAFETY: 0xE2 is always the start of a 3-byte Unicode character,
// so there must be 2 more bytes available to consume
let next2 =
unsafe { self.text.as_bytes().get_unchecked(index + 1..index + 3) };
next2.try_into().unwrap()
};
// If this is LS or PS, treat it as a line terminator
if matches!(next2, LS_LAST_2_BYTES | PS_LAST_2_BYTES) {
// SAFETY: `index` is the start of a 3-byte Unicode character,
// so `index` and `index + 3` are both UTF-8 char boundaries.
// Therefore, slices up to `index` and from `index + 3` are both valid `&str`s.
b'\r' => {
// SAFETY: `consumed` is initially 0, and only updated to valid UTF-8 boundaries.
// `index` is on `\r`, so `index` is a UTF-8 char boundary.
unsafe {
let line = self.text.get_unchecked(..index);
self.text = self.text.get_unchecked(index + 3..);
let index = ptr.offset_from(bytes.as_ptr()) as usize;
let line = self.text.get_unchecked(consumed..index);
// Check if next byte is `\n` and consume it as well
let skip_bytes =
if bytes.get(index + 1) == Some(&b'\n') { 2 } else { 1 };
// Set `consumed` to after `\r` or `\r\n`
consumed = index + skip_bytes;
self.text = self.text.get_unchecked(consumed..);
return Some(line);
}
}
LS_OR_PS_FIRST_BYTE => {
let next2: [u8; 2] = {
// SAFETY: We ensure `last_ptr` is at least 3 bytes before end,
// so safe to read 2 more bytes after `0xE2`
let next2 = unsafe {
let slice_ptr = ptr.add(1);
std::slice::from_raw_parts(slice_ptr, 2)
};
[next2[0], next2[1]]
};
// If this is LS or PS, treat it as a line terminator
if matches!(next2, LS_LAST_2_BYTES | PS_LAST_2_BYTES) {
// SAFETY: `consumed` is initially 0, and only updated to valid UTF-8 boundaries.
// `index` is start of 3-byte Unicode char, so `index` and `index + 3` are UTF-8 boundaries.
unsafe {
let index = ptr.offset_from(bytes.as_ptr()) as usize;
let line = self.text.get_unchecked(consumed..index);
// Set `consumed` to after the 3-byte LS/PS character
consumed = index + 3;
self.text = self.text.get_unchecked(consumed..);
return Some(line);
}
}
}
_ => {}
}

if ptr == last_ptr {
break;
}
// SAFETY: `ptr` is less than `last_ptr`, which is in bounds, so safe to increment `ptr`
ptr = unsafe { ptr.add(1) };
}
None
};

// Search string in chunks of 16 bytes
let mut chunks = bytes.chunks_exact(16);
for (chunk_index, chunk) in chunks.by_ref().enumerate() {
#[expect(clippy::missing_panics_doc, reason = "infallible")]
let chunk: &[u8; 16] = chunk.try_into().unwrap();

// Compiler vectorizes this loop to a few SIMD ops
let mut contains_line_terminator = false;
for &byte in chunk {
if matches!(byte, b'\r' | b'\n' | LS_OR_PS_FIRST_BYTE) {
contains_line_terminator = true;
break;
}
_ => {}
}
Comment on lines +136 to +143
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@overlookmotel overlookmotel Sep 8, 2025 •
edited
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Compiler does not vectorize this. The match is too complicated, and it uses byte-by-byte search.

https://godbolt.org/z/o3hP4cEMv

(for context, if it was using SIMD, it'd be about ~8 instructions, and you'd see instructions using xmm registers)

It's really hard to get the compiler to auto-vectorize cases like this.

The premise of this PR is that it's using SIMD for better perf. But as it's not actually using SIMD, unfortunately I doubt this PR has much value. It may even regress perf as it's searching bytes twice instead of once.

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This works better: https://godbolt.org/z/r795Gsq3T (but still not quite ideal - it should be possible to use only 1 pmovmskb instruction, rather than 3).


if contains_line_terminator {
// Chunk contains at least one line terminator.
// Find them and process.
//
// SAFETY: `index` is byte index of start of chunk.
// We search bytes starting with first byte of chunk, and ending with last byte of chunk.
// i.e. `index` to `index + 15` (inclusive).
// If this chunk is towards the end of the string, reduce the range of bytes searched
// so the last byte searched has at least 2 further bytes after it for LS/PS detection.
// i.e. safe to read 3 bytes at `last_ptr`.
return crate::str::cold_branch(|| unsafe {
let index = chunk_index * 16;
let remaining_bytes = bytes.len() - index;
let last_offset = if remaining_bytes >= 3 {
std::cmp::min(remaining_bytes - 3, 15)
} else {
// Not enough bytes for LS/PS, but still check for \r and \n
if remaining_bytes > 0 { remaining_bytes - 1 } else { 0 }
};
let ptr = bytes.as_ptr().add(index);
let last_ptr = ptr.add(last_offset);
search_bytes(ptr, last_ptr)
});
}
}

// Search last chunk byte-by-byte.
// Skip LS/PS checks if less than 3 bytes remaining.
let last_chunk = chunks.remainder();
if !last_chunk.is_empty() {
let ptr = last_chunk.as_ptr();
let last_offset = if last_chunk.len() >= 3 {
last_chunk.len() - 3
} else {
// Not enough bytes for LS/PS, but still check for \r and \n
if last_chunk.len() > 0 { last_chunk.len() - 1 } else { 0 }
};
// SAFETY: `last_offset` is calculated to be in bounds of `last_chunk`.
let last_ptr = unsafe { ptr.add(last_offset) };
if let Some(line) = search_bytes(ptr, last_ptr) {
return Some(line);
}
}

// No line break found - return the remaining text. Next call will return `None`.
let line = self.text;
// SAFETY: `consumed` is either 0 or set to valid UTF-8 boundaries from previous processing.
let line = unsafe { self.text.get_unchecked(consumed..) };
self.text = "";
Some(line)
}
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