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perf(es/codegen): Remove needless allocations (#9890)
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**Description:**

`get_quoted_utf16` allocates too much
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@kdy1
kdy1 authored Jan 17, 2025
1 parent ef29ef6 commit f230ff2
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Showing 2 changed files with 70 additions and 77 deletions.
6 changes: 6 additions & 0 deletions .changeset/mean-boxes-design.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
---
swc_core: minor
swc_ecma_codegen: minor
---

perf(es/codegen): Remove needless allocations
141 changes: 64 additions & 77 deletions crates/swc_ecma_codegen/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4126,12 +4126,35 @@ fn get_ascii_only_ident(sym: &str, may_need_quote: bool, target: EsVersion) -> C
}

fn get_quoted_utf16(v: &str, ascii_only: bool, target: EsVersion) -> String {
let mut buf = String::with_capacity(v.len() + 2);
let mut iter = v.chars().peekable();
// Count quotes first to determine which quote character to use
let (mut single_quote_count, mut double_quote_count) = (0, 0);
for c in v.chars() {
match c {
'\'' => single_quote_count += 1,
'"' => double_quote_count += 1,
_ => {}
}
}

// Pre-calculate capacity to avoid reallocations
let quote_char = if double_quote_count > single_quote_count {
'\''
} else {
'"'
};
let escape_char = if quote_char == '\'' { '\'' } else { '"' };
let escape_count = if quote_char == '\'' {
single_quote_count
} else {
double_quote_count
};

let mut single_quote_count = 0;
let mut double_quote_count = 0;
// Add 2 for quotes, and 1 for each escaped quote
let capacity = v.len() + 2 + escape_count;
let mut buf = String::with_capacity(capacity);
buf.push(quote_char);

let mut iter = v.chars().peekable();
while let Some(c) = iter.next() {
match c {
'\x00' => {
Expand All @@ -4149,29 +4172,24 @@ fn get_quoted_utf16(v: &str, ascii_only: bool, target: EsVersion) -> String {
'\t' => buf.push('\t'),
'\\' => {
let next = iter.peek();

match next {
// TODO fix me - workaround for surrogate pairs
Some('u') => {
let mut inner_iter = iter.clone();

inner_iter.next();

let mut is_curly = false;
let mut next = inner_iter.peek();

if next == Some(&'{') {
is_curly = true;

inner_iter.next();
next = inner_iter.peek();
} else if next != Some(&'D') && next != Some(&'d') {
buf.push('\\');
}

if let Some(c @ 'D' | c @ 'd') = next {
let mut inner_buf = String::new();

let mut inner_buf = String::with_capacity(8);
inner_buf.push('\\');
inner_buf.push('u');

Expand All @@ -4180,21 +4198,17 @@ fn get_quoted_utf16(v: &str, ascii_only: bool, target: EsVersion) -> String {
}

inner_buf.push(*c);

inner_iter.next();

let mut is_valid = true;

for _ in 0..3 {
let c = inner_iter.next();

match c {
Some('0'..='9') | Some('a'..='f') | Some('A'..='F') => {
inner_buf.push(c.unwrap());
match inner_iter.next() {
Some(c @ '0'..='9') | Some(c @ 'a'..='f')
| Some(c @ 'A'..='F') => {
inner_buf.push(c);
}
_ => {
is_valid = false;

break;
}
}
Expand All @@ -4212,108 +4226,81 @@ fn get_quoted_utf16(v: &str, ascii_only: bool, target: EsVersion) -> String {

if is_valid {
let val_str = &inner_buf[range];

let v = u32::from_str_radix(val_str, 16).unwrap_or_else(|err| {
unreachable!(
"failed to parse {} as a hex value: {:?}",
val_str, err
)
});

if v > 0xffff {
buf.push_str(&inner_buf);

let end = if is_curly { 7 } else { 5 };

for _ in 0..end {
iter.next();
if let Ok(v) = u32::from_str_radix(val_str, 16) {
if v > 0xffff {
buf.push_str(&inner_buf);
let end = if is_curly { 7 } else { 5 };
for _ in 0..end {
iter.next();
}
} else if (0xd800..=0xdfff).contains(&v) {
buf.push('\\');
} else {
buf.push_str("\\\\");
}
} else if (0xd800..=0xdfff).contains(&v) {
buf.push('\\');
} else {
buf.push_str("\\\\");
}
} else {
buf.push_str("\\\\")
buf.push_str("\\\\");
}
} else if is_curly {
buf.push_str("\\\\");
} else {
buf.push('\\');
}
}
_ => {
buf.push_str("\\\\");
}
_ => buf.push_str("\\\\"),
}
}
'\'' => {
single_quote_count += 1;
buf.push('\'');
}
'"' => {
double_quote_count += 1;
buf.push('"');
c if c == escape_char => {
buf.push('\\');
buf.push(c);
}
'\x01'..='\x0f' => {
let _ = write!(buf, "\\x0{:x}", c as u8);
buf.push_str("\\x0");
write!(&mut buf, "{:x}", c as u8).unwrap();
}
'\x10'..='\x1f' => {
let _ = write!(buf, "\\x{:x}", c as u8);
}
'\x20'..='\x7e' => {
buf.push(c);
buf.push_str("\\x");
write!(&mut buf, "{:x}", c as u8).unwrap();
}
'\x20'..='\x7e' => buf.push(c),
'\u{7f}'..='\u{ff}' => {
if ascii_only || target <= EsVersion::Es5 {
let _ = write!(buf, "\\x{:x}", c as u8);
buf.push_str("\\x");
write!(&mut buf, "{:x}", c as u8).unwrap();
} else {
buf.push(c);
}
}
'\u{2028}' => {
buf.push_str("\\u2028");
}
'\u{2029}' => {
buf.push_str("\\u2029");
}
'\u{FEFF}' => {
buf.push_str("\\uFEFF");
}
_ => {
'\u{2028}' => buf.push_str("\\u2028"),
'\u{2029}' => buf.push_str("\\u2029"),
'\u{FEFF}' => buf.push_str("\\uFEFF"),
c => {
if c.is_ascii() {
buf.push(c);
} else if c > '\u{FFFF}' {
// if we've got this far the char isn't reserved and if the callee has specified
// we should output unicode for non-ascii chars then we have
// to make sure we output unicode that is safe for the target
// Es5 does not support code point escapes and so surrograte formula must be
// used
if target <= EsVersion::Es5 {
// https://mathiasbynens.be/notes/javascript-encoding#surrogate-formulae
let h = ((c as u32 - 0x10000) / 0x400) + 0xd800;
let l = (c as u32 - 0x10000) % 0x400 + 0xdc00;

let _ = write!(buf, "\\u{:04X}\\u{:04X}", h, l);
write!(&mut buf, "\\u{:04X}\\u{:04X}", h, l).unwrap();
} else if ascii_only {
let _ = write!(buf, "\\u{{{:04X}}}", c as u32);
write!(&mut buf, "\\u{{{:04X}}}", c as u32).unwrap();
} else {
buf.push(c);
}
} else if ascii_only {
let _ = write!(buf, "\\u{:04X}", c as u16);
write!(&mut buf, "\\u{:04X}", c as u16).unwrap();
} else {
buf.push(c);
}
}
}
}

if double_quote_count > single_quote_count {
format!("'{}'", buf.replace('\'', "\\'"))
} else {
format!("\"{}\"", buf.replace('"', "\\\""))
}
buf.push(quote_char);
buf
}

fn handle_invalid_unicodes(s: &str) -> Cow<str> {
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