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perf(parser): skip redundant source cursor updates (#28409)

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Skip the cursor store and source-byte reload before token dispatch unless a space is consumed.

Validated parser conformance, token output, allocation snapshots, and Rust/Node suites. Current-base timing confirmation is pending due to concurrent workloads.

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This commit is contained in:
Boshen committed 2026-10-29 11:54:59 +00:00
1 parent 7941ca966b
commit 563d92a830
1 file changed
+20 -23
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crates/oxc_parser/src/lexer/mod.rs
+20 -23
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@@ -506,28 +506,24 @@ impl<'a, C: Config> Lexer<'a, C> {
let end_pos = self.source.end();
loop {
// Single spaces between tokens are common, so consume a space before processing the next token.
// Do this without a branch. This produces more instructions, but avoids an unpredictable branch.
// Can only do this if there are at least 2 bytes left in source.
// If there aren't 2 bytes left, delegate to `read_next_token_at_end` (cold branch).
let mut pos = self.source.position();
// SAFETY: `source.end()` is always equal to or after `source.position()`
let remaining_bytes = unsafe { end_pos.offset_from(pos) };
if remaining_bytes >= 2 {
// Read next byte.
// SAFETY: There are at least 2 bytes remaining in source.
let byte = unsafe { pos.read() };
let mut byte = unsafe { pos.read() };
// If next byte is a space, advance by 1 byte.
// Do this with maths, instead of a branch.
let is_space = byte == b' ';
// SAFETY: There are at least 2 bytes remaining in source, so advancing 1 byte cannot be out of bounds
pos = unsafe { pos.add(usize::from(is_space)) };
self.source.set_position(pos);
// Read next byte again, in case we skipped a space.
// SAFETY: We checked above that there were at least 2 bytes to read,
// and we skipped a maximum of 1 byte, so there's still at least 1 byte left to read.
let byte = unsafe { pos.read() };
// Only update the cursor and read again when consuming a space.
if byte == b' ' {
// SAFETY: At least 2 bytes remain, so advancing past one ASCII space
// leaves at least one byte to read.
pos = unsafe { pos.add(1) };
self.source.set_position(pos);
// SAFETY: At least one byte remains after the space.
byte = unsafe { pos.read() };
}
// Set token start
let offset = self.source.offset_of(pos);
@@ -578,16 +574,17 @@ impl<'a, C: Config> Lexer<'a, C> {
let remaining_bytes = unsafe { end_pos.offset_from(pos) };
if remaining_bytes >= 2 {
// SAFETY: There are at least 2 bytes remaining in source.
let byte = unsafe { pos.read() };
let mut byte = unsafe { pos.read() };
let is_space = byte == b' ';
// SAFETY: There are at least 2 bytes remaining in source, so advancing 1 byte cannot be out of bounds
pos = unsafe { pos.add(usize::from(is_space)) };
self.source.set_position(pos);
// SAFETY: We checked above that there were at least 2 bytes to read,
// and we skipped a maximum of 1 byte, so there's still at least 1 byte left to read.
let byte = unsafe { pos.read() };
// Only update the cursor and read again when consuming a space.
if byte == b' ' {
// SAFETY: At least 2 bytes remain, so advancing past one ASCII space
// leaves at least one byte to read.
pos = unsafe { pos.add(1) };
self.source.set_position(pos);
// SAFETY: At least one byte remains after the space.
byte = unsafe { pos.read() };
}
let offset = self.source.offset_of(pos);
self.token.set_start(offset);