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Test LEB128 loop unrolling #71

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Test LEB128 loop unrolling #71

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61 changes: 57 additions & 4 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -355,24 +355,77 @@ fn decode_dag_cbor_multi<'py>(py: Python<'py>, data: &[u8]) -> PyResult<Bound<'p

#[inline]
fn read_u64_leb128<R: Read>(r: &mut R) -> Result<u64> {
let mut result = 0;
let mut shift = 0;
// Optimized version with unrolled first few iterations
// Most LEB128 values are 1-2 bytes, so we handle these cases directly

let mut buf = [0];
if let Err(_) = r.read_exact(&mut buf) {
return Err(anyhow!("Unexpected EOF while reading ULEB128 number."));
}

let byte = buf[0] as u64;
if (byte & 0x80) == 0 {
// Single byte case (most common)
return Ok(byte);
}

// Two byte case (second most common)
let mut result = byte & 0x7F;
if let Err(_) = r.read_exact(&mut buf) {
return Err(anyhow!("Unexpected EOF while reading ULEB128 number."));
}

let byte = buf[0] as u64;
if (byte & 0x80) == 0 {
result |= byte << 7;
return Ok(result);
}

// Three byte case
result |= (byte & 0x7F) << 7;
if let Err(_) = r.read_exact(&mut buf) {
return Err(anyhow!("Unexpected EOF while reading ULEB128 number."));
}

let byte = buf[0] as u64;
if (byte & 0x80) == 0 {
result |= byte << 14;
return Ok(result);
}

// Four byte case
result |= (byte & 0x7F) << 14;
if let Err(_) = r.read_exact(&mut buf) {
return Err(anyhow!("Unexpected EOF while reading ULEB128 number."));
}

let byte = buf[0] as u64;
if (byte & 0x80) == 0 {
result |= byte << 21;
return Ok(result);
}

// Fall back to loop for longer encodings (rare case)
result |= (byte & 0x7F) << 21;
let mut shift = 28;

loop {
let mut buf = [0];
if let Err(_) = r.read_exact(&mut buf) {
return Err(anyhow!("Unexpected EOF while reading ULEB128 number."));
}

let byte = buf[0] as u64;
if (byte & 0x80) == 0 {
result |= (byte) << shift;
result |= byte << shift;
return Ok(result);
} else {
result |= (byte & 0x7F) << shift;
}

shift += 7;
if shift >= 64 {
return Err(anyhow!("LEB128 value too large for u64"));
}
}
}

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