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Modify the buffer position directly when reading leb128 values.

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It's a small but clear performance win.
This commit is contained in:
Nicholas Nethercote 2022-01-07 10:11:49 +11:00
parent ddabe0775c
commit 5f549d9b49
3 changed files with 11 additions and 19 deletions
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18
compiler/rustc_serialize/src/leb128.rs
View file

@ -53,16 +53,15 @@ impl_write_unsigned_leb128!(write_usize_leb128, usize);
macro_rules! impl_read_unsigned_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(slice: &[u8]) -> ($int_ty, usize) {
pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
let mut result = 0;
let mut shift = 0;
let mut position = 0;
loop {
let byte = slice[position];
position += 1;
let byte = slice[*position];
*position += 1;
if (byte & 0x80) == 0 {
result |= (byte as $int_ty) << shift;
return (result, position);
return result;
} else {
result |= ((byte & 0x7F) as $int_ty) << shift;
}
@ -122,15 +121,14 @@ impl_write_signed_leb128!(write_isize_leb128, isize);
macro_rules! impl_read_signed_leb128 {
($fn_name:ident, $int_ty:ty) => {
#[inline]
pub fn $fn_name(slice: &[u8]) -> ($int_ty, usize) {
pub fn $fn_name(slice: &[u8], position: &mut usize) -> $int_ty {
let mut result = 0;
let mut shift = 0;
let mut position = 0;
let mut byte;
loop {
byte = slice[position];
position += 1;
byte = slice[*position];
*position += 1;
result |= <$int_ty>::from(byte & 0x7F) << shift;
shift += 7;
@ -144,7 +142,7 @@ macro_rules! impl_read_signed_leb128 {
result |= (!0 << shift);
}
(result, position)
result
}
};
}

6
compiler/rustc_serialize/src/opaque.rs
View file

@ -559,11 +559,7 @@ impl<'a> Decoder<'a> {
}
macro_rules! read_leb128 {
($dec:expr, $fun:ident) => {{
let (value, bytes_read) = leb128::$fun(&$dec.data[$dec.position..]);
$dec.position += bytes_read;
Ok(value)
}};
($dec:expr, $fun:ident) => {{ Ok(leb128::$fun($dec.data, &mut $dec.position)) }};
}
impl<'a> serialize::Decoder for Decoder<'a> {

6
compiler/rustc_serialize/tests/leb128.rs
View file

@ -30,9 +30,8 @@ macro_rules! impl_test_unsigned_leb128 {
let mut position = 0;
for &expected in &values {
let (actual, bytes_read) = $read_fn_name(&stream[position..]);
let actual = $read_fn_name(&stream, &mut position);
assert_eq!(expected, actual);
position += bytes_read;
}
assert_eq!(stream.len(), position);
}
@ -77,9 +76,8 @@ macro_rules! impl_test_signed_leb128 {
let mut position = 0;
for &expected in &values {
let (actual, bytes_read) = $read_fn_name(&stream[position..]);
let actual = $read_fn_name(&stream, &mut position);
assert_eq!(expected, actual);
position += bytes_read;
}
assert_eq!(stream.len(), position);
}