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Merge pull request #5860 from paritytech/minor-modexp-optimization
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minor optimizations of the modexp builtin
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@rphmeier
rphmeier authored Jun 18, 2017
2 parents ed20fa4 + ca27234 commit a99d4c3
Showing 1 changed file with 35 additions and 34 deletions.
69 changes: 35 additions & 34 deletions ethcore/src/builtin.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -111,7 +111,7 @@ impl Builtin {
pub fn cost(&self, input: &[u8]) -> U256 { self.pricer.cost(input) }

/// Simple forwarder for execute.
pub fn execute(&self, input: &[u8], output: &mut BytesRef) -> Result<(), Error> {
pub fn execute(&self, input: &[u8], output: &mut BytesRef) -> Result<(), Error> {
self.native.execute(input, output)
}

Expand Down Expand Up @@ -288,9 +288,11 @@ impl Impl for ModexpImpl {
let exp = read_num(exp_len);
let modulus = read_num(mod_len);

// calculate modexp: exponentiation by squaring.
// calculate modexp: exponentiation by squaring. the `num` crate has pow, but not modular.
fn modexp(mut base: BigUint, mut exp: BigUint, modulus: BigUint) -> BigUint {
match (base == BigUint::zero(), exp == BigUint::zero()) {
use num::Integer;

match (base.is_zero(), exp.is_zero()) {
(_, true) => return BigUint::one(), // n^0 % m
(true, false) => return BigUint::zero(), // 0^n % m, n>0
(false, false) if modulus <= BigUint::one() => return BigUint::zero(), // a^b % 1 = 0.
Expand All @@ -301,10 +303,9 @@ impl Impl for ModexpImpl {
base = base % &modulus;

// fast path for base divisible by modulus.
if base == BigUint::zero() { return result }
while exp != BigUint::zero() {
// exp has to be on the right here to avoid move.
if BigUint::one() & &exp == BigUint::one() {
if base.is_zero() { return result }
while !exp.is_zero() {
if exp.is_odd() {
result = (result * &base) % &modulus;
}

Expand All @@ -331,22 +332,22 @@ impl Impl for ModexpImpl {

fn read_fr(reader: &mut io::Chain<&[u8], io::Repeat>) -> Result<::bn::Fr, Error> {
let mut buf = [0u8; 32];

reader.read_exact(&mut buf[..]).expect("reading from zero-extended memory cannot fail; qed");
::bn::Fr::from_slice(&buf[0..32]).map_err(|_| Error::from("Invalid field element"))
}

fn read_point(reader: &mut io::Chain<&[u8], io::Repeat>) -> Result<::bn::G1, Error> {
use bn::{Fq, AffineG1, G1, Group};

let mut buf = [0u8; 32];

reader.read_exact(&mut buf[..]).expect("reading from zero-extended memory cannot fail; qed");
let px = Fq::from_slice(&buf[0..32]).map_err(|_| Error::from("Invalid point x coordinate"))?;

reader.read_exact(&mut buf[..]).expect("reading from zero-extended memory cannot fail; qed");
let py = Fq::from_slice(&buf[0..32]).map_err(|_| Error::from("Invalid point x coordinate"))?;

Ok(
if px == Fq::zero() && py == Fq::zero() {
G1::zero()
Expand Down Expand Up @@ -374,7 +375,7 @@ impl Impl for Bn128AddImpl {
output.write(0, &write_buf);

Ok(())
}
}
}

impl Impl for Bn128MulImpl {
Expand All @@ -394,7 +395,7 @@ impl Impl for Bn128MulImpl {
}
output.write(0, &write_buf);
Ok(())
}
}
}

mod bn128_gen {
Expand All @@ -420,13 +421,13 @@ mod bn128_gen {
.expect("a valid field element"),
Fq::from_str("4082367875863433681332203403145435568316851327593401208105741076214120093531")
.expect("a valid field element"),
),
),
).expect("the generator P2(10857046999023057135944570762232829481370756359578518086990519993285655852781 + 11559732032986387107991004021392285783925812861821192530917403151452391805634i, 8495653923123431417604973247489272438418190587263600148770280649306958101930 + 4082367875863433681332203403145435568316851327593401208105741076214120093531i) is a valid curve point"));
}
}

lazy_static! {
pub static ref P1_P2_PAIRING: Gt = pairing(P1.clone(), P2.clone());
}
}
}

impl Impl for Bn128PairingImpl {
Expand All @@ -438,8 +439,8 @@ impl Impl for Bn128PairingImpl {
use bn::{AffineG1, AffineG2, Fq, Fq2, pairing, G1, G2, Gt};

let elements = input.len() / 192; // (a, b_a, b_b - each 64-byte affine coordinates)
if input.len() % 192 != 0 {
return Err("Invalid input length, must be multiple of 192 (3 * (32*2))".into())
if input.len() % 192 != 0 {
return Err("Invalid input length, must be multiple of 192 (3 * (32*2))".into())
}
let ret_val = if input.len() == 0 {
U256::one()
Expand All @@ -459,11 +460,11 @@ impl Impl for Bn128PairingImpl {
.map_err(|_| Error::from("Invalid b argument imaginary coeff y coordinate"))?;

let b_a_x = Fq::from_slice(&input[idx*192+128..idx*192+160])
.map_err(|_| Error::from("Invalid b argument real coeff x coordinate"))?;
.map_err(|_| Error::from("Invalid b argument real coeff x coordinate"))?;

let b_a_y = Fq::from_slice(&input[idx*192+160..idx*192+192])
.map_err(|_| Error::from("Invalid b argument real coeff y coordinate"))?;
.map_err(|_| Error::from("Invalid b argument real coeff y coordinate"))?;

vals.push((
G1::from(
AffineG1::new(a_x, a_y).map_err(|_| Error::from("Invalid a argument - not on curve"))?
Expand Down Expand Up @@ -719,7 +720,7 @@ mod tests {
pricer: Box::new(Linear { base: 0, word: 0 }),
native: ethereum_builtin("bn128_add"),
activate_at: 0,
};
};

// zero-points additions
{
Expand All @@ -738,7 +739,7 @@ mod tests {

f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..])).expect("Builtin should not fail");
assert_eq!(output, expected);
}
}


// no input, should not fail
Expand All @@ -754,7 +755,7 @@ mod tests {

f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..])).expect("Builtin should not fail");
assert_eq!(output, expected);
}
}

// should fail - point not on curve
{
Expand All @@ -769,7 +770,7 @@ mod tests {

let res = f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..]));
assert!(res.is_err(), "There should be built-in error here");
}
}
}


Expand All @@ -781,7 +782,7 @@ mod tests {
pricer: Box::new(Linear { base: 0, word: 0 }),
native: ethereum_builtin("bn128_mul"),
activate_at: 0,
};
};

// zero-point multiplication
{
Expand All @@ -799,7 +800,7 @@ mod tests {

f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..])).expect("Builtin should not fail");
assert_eq!(output, expected);
}
}

// should fail - point not on curve
{
Expand All @@ -813,7 +814,7 @@ mod tests {

let res = f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..]));
assert!(res.is_err(), "There should be built-in error here");
}
}
}

fn builtin_pairing() -> Builtin {
Expand All @@ -826,12 +827,12 @@ mod tests {

fn empty_test(f: Builtin, expected: Vec<u8>) {
let mut empty = [0u8; 0];
let input = BytesRef::Fixed(&mut empty);
let input = BytesRef::Fixed(&mut empty);

let mut output = vec![0u8; expected.len()];

f.execute(&input[..], &mut BytesRef::Fixed(&mut output[..])).expect("Builtin should not fail");
assert_eq!(output, expected);
assert_eq!(output, expected);
}

fn error_test(f: Builtin, input: &[u8], msg_contains: Option<&str>) {
Expand All @@ -851,12 +852,12 @@ mod tests {
fn bytes(s: &'static str) -> Vec<u8> {
FromHex::from_hex(s).expect("static str should contain valid hex bytes")
}

#[test]
fn bn128_pairing_empty() {
// should not fail, because empty input is a valid input of 0 elements
empty_test(
builtin_pairing(),
builtin_pairing(),
bytes("0000000000000000000000000000000000000000000000000000000000000001"),
);
}
Expand Down Expand Up @@ -890,7 +891,7 @@ mod tests {
),
Some("Invalid input length"),
);
}
}

#[test]
#[should_panic]
Expand Down Expand Up @@ -953,4 +954,4 @@ mod tests {
b.execute(&i[..], &mut BytesRef::Fixed(&mut o[..])).expect("Builtin should not fail");
assert_eq!(i, o);
}
}
}

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