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Micro-optimize the heck out of LEB128 reading and writing. #69050

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merged 1 commit into from
Feb 13, 2020
Merged

Micro-optimize the heck out of LEB128 reading and writing. #69050

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64 changes: 14 additions & 50 deletions src/libserialize/leb128.rs
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Original file line number Diff line number Diff line change
@@ -1,46 +1,14 @@
#[inline]
pub fn write_to_vec(vec: &mut Vec<u8>, byte: u8) {
vec.push(byte);
}

#[cfg(target_pointer_width = "32")]
const USIZE_LEB128_SIZE: usize = 5;
#[cfg(target_pointer_width = "64")]
const USIZE_LEB128_SIZE: usize = 10;

macro_rules! leb128_size {
(u16) => {
3
};
(u32) => {
5
};
(u64) => {
10
};
(u128) => {
19
};
(usize) => {
USIZE_LEB128_SIZE
};
}

macro_rules! impl_write_unsigned_leb128 {
($fn_name:ident, $int_ty:ident) => {
#[inline]
pub fn $fn_name(out: &mut Vec<u8>, mut value: $int_ty) {
for _ in 0..leb128_size!($int_ty) {
let mut byte = (value & 0x7F) as u8;
value >>= 7;
if value != 0 {
byte |= 0x80;
}

write_to_vec(out, byte);

if value == 0 {
loop {
if value < 0x80 {
out.push(value as u8);
break;
} else {
out.push(((value & 0x7f) | 0x80) as u8);
value >>= 7;
}
}
}
Expand All @@ -57,24 +25,20 @@ macro_rules! impl_read_unsigned_leb128 {
($fn_name:ident, $int_ty:ident) => {
#[inline]
pub fn $fn_name(slice: &[u8]) -> ($int_ty, usize) {
let mut result: $int_ty = 0;
let mut result = 0;
let mut shift = 0;
let mut position = 0;

for _ in 0..leb128_size!($int_ty) {
let byte = unsafe { *slice.get_unchecked(position) };
loop {
let byte = slice[position];
position += 1;
result |= ((byte & 0x7F) as $int_ty) << shift;
if (byte & 0x80) == 0 {
break;
result |= (byte as $int_ty) << shift;
return (result, position);
} else {
result |= ((byte & 0x7F) as $int_ty) << shift;
}
shift += 7;
}

// Do a single bounds check at the end instead of for every byte.
assert!(position <= slice.len());

(result, position)
}
};
}
Expand Down Expand Up @@ -116,7 +80,7 @@ where

#[inline]
pub fn write_signed_leb128(out: &mut Vec<u8>, value: i128) {
write_signed_leb128_to(value, |v| write_to_vec(out, v))
write_signed_leb128_to(value, |v| out.push(v))
}

#[inline]
Expand Down