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Manually unroll x^7 for poseidon for a ~50% speedup [master] #2400

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Jul 9, 2024
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Manually unroll x^7 for poseidon for a ~50% speedup [master] #2400

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15 changes: 13 additions & 2 deletions poseidon/src/poseidon.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -24,8 +24,19 @@ pub trait Sponge<Input: Field, Digest> {
fn reset(&mut self);
}

pub fn sbox<F: Field, SC: SpongeConstants>(x: F) -> F {
x.pow([SC::PERM_SBOX as u64])
pub fn sbox<F: Field, SC: SpongeConstants>(mut x: F) -> F {
if SC::PERM_SBOX == 7 {
// This is much faster than using the generic `pow`. Hard-code to get the ~50% speed-up
// that it gives to hashing.
let mut square = x;
square.square_in_place();
x *= square;
square.square_in_place();
x *= square;
x
} else {
x.pow([SC::PERM_SBOX as u64])
}
}

#[derive(Clone, Debug)]
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