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Fix and refactor number encodings

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Brian Carroll 2021-10-25 12:15:04 +02:00
parent 3992850387
commit ddf66293e9
3 changed files with 71 additions and 43 deletions
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86
compiler/gen_wasm/src/lib.rs
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@ -194,41 +194,75 @@ pub fn debug_panic<E: std::fmt::Debug>(error: E) {
panic!("{:?}", error);
}
/// Write a u32 value as LEB-128 encoded bytes into the provided buffer, returning byte length
/// Write an unsigned value into the provided buffer in LEB-128 format, returning byte length
///
/// All integers in Wasm are variable-length encoded, which saves space for small values.
/// The most significant bit indicates "more bytes are coming", and the other 7 are payload.
pub fn encode_u32<'a>(buffer: &mut Vec<'a, u8>, value: u32) -> usize {
let mut x = value;
let start_len = buffer.len();
while x >= 0x80 {
buffer.push(0x80 | ((x & 0x7f) as u8));
x >>= 7;
}
buffer.push(x as u8);
buffer.len() - start_len
macro_rules! encode_uleb128 {
($name: ident, $ty: ty) => {
pub fn $name<'a>(buffer: &mut Vec<'a, u8>, value: $ty) -> usize {
let mut x = value;
let start_len = buffer.len();
while x >= 0x80 {
buffer.push(0x80 | ((x & 0x7f) as u8));
x >>= 7;
}
buffer.push(x as u8);
buffer.len() - start_len
}
};
}
/// Write a u64 value as LEB-128 encoded bytes, into the provided buffer, returning byte length
///
/// All integers in Wasm are variable-length encoded, which saves space for small values.
/// The most significant bit indicates "more bytes are coming", and the other 7 are payload.
pub fn encode_u64<'a>(buffer: &mut Vec<'a, u8>, value: u64) -> usize {
let mut x = value;
let start_len = buffer.len();
while x >= 0x80 {
buffer.push(0x80 | ((x & 0x7f) as u8));
x >>= 7;
}
buffer.push(x as u8);
buffer.len() - start_len
encode_uleb128!(encode_u32, u32);
encode_uleb128!(encode_u64, u64);
/// Write a *signed* value into the provided buffer in LEB-128 format, returning byte length
macro_rules! encode_sleb128 {
($name: ident, $ty: ty) => {
pub fn $name<'a>(buffer: &mut Vec<'a, u8>, value: $ty) -> usize {
let mut x = value;
let start_len = buffer.len();
loop {
let byte = (x & 0x7f) as u8;
x >>= 7;
let byte_is_negative = (byte & 0x40) != 0;
if ((x == 0 && !byte_is_negative) || (x == -1 && byte_is_negative)) {
buffer.push(byte);
break;
}
buffer.push(byte | 0x80);
}
buffer.len() - start_len
}
};
}
/// Overwrite a LEB-128 encoded u32 value that has been padded to maximum length (5 bytes)
encode_sleb128!(encode_i32, i32);
encode_sleb128!(encode_i64, i64);
/// No LEB encoding, and always little-endian regardless of compiler host.
macro_rules! encode_float {
($name: ident, $ty: ty) => {
pub fn $name<'a>(buffer: &mut Vec<'a, u8>, value: $ty) {
let mut x = value.to_bits();
let size = std::mem::size_of::<$ty>();
for _ in 0..size {
buffer.push((x & 0xff) as u8);
x >>= 8;
}
}
};
}
encode_float!(encode_f32, f32);
encode_float!(encode_f64, f64);
/// Overwrite a LEB-128 encoded u32 value, padded to maximum length (5 bytes)
///
/// We need some fixed length values so we can overwrite them without moving all following bytes.
/// For example, the code section is prefixed with its length, which we only know at the end.
/// We need some fixed-length values so we can overwrite them without moving all following bytes.
/// Many parts of the binary format are prefixed with their length, which we only know at the end.
/// And relocation values get updated during linking.
/// This can help us to avoid copies, which is good for speed, but there's a tradeoff with output size.
///
/// The value 3 is encoded as 0x83 0x80 0x80 0x80 0x00.
/// https://github.com/WebAssembly/tool-conventions/blob/main/Linking.md#relocation-sections